Liver Physiology Flashcards
Structure of ferritin
Large spherical protein consisting of 24 noncovalently linked subunits
Subunits form a shell surrounding a central core
Core contains up to 5000 atoms of iron
Functions of the liver
Carbohydrate metabolism
Fat metabolism
Protein metabolism
Hormone metabolism
Toxin/drug metabolism and excretion
Storage
Bilirubin metabolism and excretion
Where is ferrritin found
In cytoplasm of cells and can also be found in the serum
What is the concentration of ferritin directly proportional to
Total iron stores in the body
Excess iron storage disorders
Hereditary haemochromatosis
Haemolytic anaemia
Sideroblastic anaemia
Multiple blood transfusions
Iron replacement therapy
Non-iron overload
Liver disease
Some malignancies
Significant tissue destruction
Acute phase response:
-Inflammation
-Infection
-Autoimmune disorders
Ferritin deficiency (iron deficiency)
Can result in anaemia
Ferritin less than 20ug/L
Depletion
Ferritin less than 12ug/L
Complete absence of stored iron
Average amount of iron absorbed each day
1-2 mg/day
Where is iron absorbed in the body
Duodenum
Where is iron stored
Liver parenchyma
Reticuloendothelial macrophages
Iron utilisation
Myoglobin in muscle
Haemoglobin
Which molecule stores iron
Ferritin
How many atoms of iron can a molecule of ferritin store
Up to 5000
Number of subunits in ferritin
24
Function of vitamins
Gene activators
Free-radical scavengers
Coenzymes or cofactors in metabolic reactions
Water soluble vitamin examples
Vitamin B and C
Fat soluble vitamin examples
Vitamins A, D, E and K
Vitamin A
Retinoids
Vertebrates ingest retinal directly from meat or produce retinal from carotenes
Sources of vitamin A
Retinols
Carotenoids
Sources of carotenoids
Tomatoes
Spinach
Carrots
Sweet potato
Sources of retinols
Dairy
Eggs
Cereal
Meat
Male daily requirement of vitamin A
0.6 mg/day
Female daily requirement of vitamin A
0.7 mg/day
Vitamin A functions
Vision
Reproduction
Growth
Stabilisation of cellular membranes
Vitamin A and vision
Used to form rhodopsin in the rod cells of the retina
Vitamin A and reproduction
Spermatogenesis in male
Prevention of fetal resorption of female
Vitamin A deficiency
Rare in affluent countries as vitamin A levels drop only when liver stores are severely depleted.
Deficiency may occur due to fat malabsorption
Clinical Features:
-Night blindness
-Xeropthalmia
-Blindness
Clinical features of vitamin A deficiency
Night blindness
Xerophthalmia- inability to produce tears
Blindness
What molecules does the liver store
Ferritin
Vitamins
Clotting factors
Acute vitamin A excess
Abdominal pain, nausea and vomiting
Severe headaches, dizziness, sluggishness and irritability
Desquamation of the skin
Chronic vitamin A deficiency
Joint and bone pain
Hair loss, dryness of the lips
Anorexia
Weight loss and hepatomegaly
Storage of water soluble vs fat soluble vitamins
Water soluble vitamins pass more readily through the body so require more regular intake than fat soluble vitamins
Fat soluble vitamins more readily stored
Carotenemia and vitamin A excess
Reversible yellowing of the skin
Does not cause toxicity
Vitamin D functions
Increased intestinal absorption of calcium
Resorption and formation of bone
Reduced renal excretion of calcium
Sources of vitamin D
Sunlight
Vitamin D3 = fish, meat
Vitamin D2= supplements
Vitamin D deficiency
Demineralisation of bone:
- rickets in children
- osteomalacia in adults
Where is vitamin E stored
Non-adipose cells such as liver and plasma - labile and fixed pool
Adipose cells- fixed pool
Function of vitamin E
Important antioxidant
Male daily vitamin E requirements
4 mg/day
Female vitamin E daily requirements
3 mg/day
Sources of vitamin E
Nuts
Oils
Avocado
Carrots
Spinach
Causes of vitamin E deficiency
Fat malabsorption eg cystic fibrosis
Premature infants
Rare congenital defects in fat metabolism eg abetalipoproteinaemia
Vitamin D3
Cholecalciferol
What does liver convert vitamin D3 to
25-hydroxyvitamin D3
Which molecule maintains calcium balance in the body
1,25-dihydroxyvitamin D3
Which organ converts 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D
Kidney
Clinical manifestations of vitamin E deficiency
Haemolytic anaemia
Myopathy
Retinopathy
Ataxia
Neuropathy
Vitamin E excess
Relatively safe
Fixed pool of vitamin E
Adipose cells
Labile and fixed pool of vitamin E
Non-adipose cells such as liver and plasma
Types of vitamin K
K1
K2
K3
K4
Vitamin K1
Phylloquinone
Synthesised by plants and present in food
Vitamin K2
Menaquinone
Synthesised in humans by intestinal bacteria
Synthetic vitamin Ks
K3
K4
Vitamin K3
Menadione
Synthetic
Vitamin K4
Menadiol
Synthetic
Vitamin K and the liver
Rapidly taken up by the liver
Transferred to very low density lipoproteins (VLDL) and low density lipoproteins (LDL) which carry it into the plasma
Functions of vitamin K
Activation of some blood clotting factors
Necessary for liver synthesis of plasma clotting factors II, VII, IX , X
How to measure vitamin K levels
Prothrombin time
Sources of vitamin K
Green leafy vegetables
Sunflower pil
Vitamin K deficiency
Haemorrhagic disease of the newborn: vitamin K injection given to newborn babies
Rare in adults unless on warfarin
Vitamin K excess
K1 is relatively safe
Synthetic forms are more toxic
Can result in oxidative damage, red cell fragility and formation of methaemoglobin.
Functions of vitamin C
Collagen synthesis
Antioxidant
Iron absorption
Adult daily requirement of vitamin C
40 mg/day
Sources of vitamin C
Fresh fruit
Vegetables
Vitamin C deficiency
Scurvy:
Easy bruising and bleeding
Teeth and gum disease
Hair loss
Treatment of vitamin C deficiency
Treated with vitamin C
Improves symptoms quickly
Joint pain gone within 48 hours
Full recovery within 2 weeks
Vitamin C excess
Doses > 1g/day can cause GI side effects
No evidence that increased vitamin C reduces the incidence or duration of colds.
Vitamin B12
Cobalamins
2 active forms of vitamin B12
Methylcobalamin
5-deoxyadenosylcobalamin
Where is vitamin B12 released from
Food by acid and enzymes in stomach
Transportation of vitamin B12
Binds to R protein to protect it from stomach acid
Released from R proteins by pancreatic polypeptide
Intrinsic factor produced by the stomach needed for absorption
IF-B12 complex absorbed in terminal ileum
Where is vitamin B12 stored
Liver
Where is intrinsic factor produced
Stomach
Which protein protects vitamin B12 from stomach acid
R protein
What molecule is needed for absorption of Vitamin B12
Intrinsic factor
Where is IF-B12 complex absorbed
Terminal ileum
Sources of Vitamin B12
Meat
Fish
Eggs
Milk
Which enzyme releases vitamin B12 from R protein
Pancreatic polypeptide
Causes of vitamin B12 deficiency
Pernicious anaemia – autoimmune destruction of IF-producing cells in stomach.
Malabsorption – lack of stomach acid, pancreatic disease, small bowel disease.
Veganism
Symptoms of vitamin B12 deficiency
Macrocytic anaemia
Peripheral neuropathy in prolonged deficiency
Folate
found in may foods fortified with folic acid.
Individuals have higher requirements in pregnancy.
Functions of folate
Coenzyme in methylation reactions
DNA synthesis
Synthesis of methionine from homocysteine
Causes of folate deficiency
Malabsorption
Drugs that interfere with folic acid metabolism (anticonvulsants, methotrexate)
Disease states that increase cell turnover (e.g. leukaemia, haemolytic anaemia, psoriasis)
Symptoms of folate deficiency
High homocysteine levels
Macrocytic anaemia
Foetal development abnormalities (neural tube defects)
Clotting factors
Intrinsic pathway activated by contact
Extrinsic pathway activated by FVII coming in contact with tissue factor
Initiates a cascade which ultimately results in fibrin clot formation
What is intrinsic clotting pathway activated by
Contact
What is extrinsic clotting pathway activated by
FVII coming in contact with tissue factor
Clotting factors produced by the liver
I (fibrinogen)
II (prothrombin)
IV
V
VI
VII
Performance of clotting pathways measured using
Prothrombin time (extrinsic pathway)
International normalised ratio
Activated partial thromboplastin time (aPTT) (intrinsic pathway)
What measures extrinsic clotting factor
Prothrombin time
What measures intrinsic clotting pathway
Activated partial thromboplastin time
Prolonged PT
May indicate a deficiency in the synthetic capacity of the liver
Causes of prolonged prothrombin time
Liver disease
DIC
severe GI bleeding
Some drugs
Vitamin K deficiency
Unwanted dietary components
Xenobiotics
How many phases are there of biotransformation reactions
2
Phase I biotransformation reactions
Functionalisation - non synthetic
Add or expose functional groups -OH, -SH, -NH2, -COOH
Phase II biotransformation reactions
Conjugation- biosynthetic
Conjugation with endogenous molecules: glucuronic acid, sulphate, glutathione
Covalent bonds formed
Purpose of xenobiotic biotransformation reactions
Make compounds non-toxic and water soluble
Xenobiotics
foreign substances that don’t have nutritional value. Xenobiotic compounds are mostly in the diet, but we also breathe in potential toxins, and importantly the body treats medications as xenobiotics.
Phase I and hydrophilicity
Small increase
Phase II and hydrophilicity
Large increase
Glucuronides
Polar and hydrophilic
Eg paracetamol
glucuronyl group
has a number of hydroxyl groups which make the molecule polar and facilitate excretion in the urine.
Where does detoxification take place
Most in liver
some takes place in the lungs & small intestine before compounds are absorbed into the bloodstream.
Detoxification in liver
- inactivation and facilitated elimination of drugs and other xenobiotics
- active metabolites formed, with similar or occasionally enhanced activity
- activation of pro-drugs
- toxification of less toxic xenobiotics
Where does biotransformation occur in liver cells
Smooth endoplasmic reticulum
Coding for cytochrome P450 enzymes
Encoded by a superfamily of more than 50 different genes in humans
Common features of cytochrome P450 enzymes
All present in sER- microsomal enzymes
All oxidise the substrate and reduce oxygen
All have a cytochrome reductase subunit which uses NADPH
Are inducible- enzyme activity may be increased by certain drugs, some dietary components and some environmental toxins eg smoking
Generate a reactive free radical compound
What can cytochrome P450 enzymes be induced by
Certain drugs, some dietary components, some environmental toxins eg smoking
What does the cytochrome reductase subunit use
NADPH
Phase I reactions- oxidation
Hydroxylation (addition of -OH groups)
N- and O- dealkylation (removal of -CH side chains)
Deamination (removal of -NH side chain)
Epoxidation (formation of epoxides)
Phase I reactions - reduction
Hydrogen addition (unsaturated—>saturated)
Donor molecules include GSH, FAD, NAD(P)H
Phase I reactions - hydrolysis
Splitting of C-N-C (amide) and C-O-C (ester) bonds
Reason for biotransformation reactions
To be filtered and excreted in the urine a molecule needs to be polar (thus more hydrophilic) increasing its solubility
Can occur via 2 methods
Phase II reactions
Glycoside conjugation - glucuronidation (most common)
Sulphate - sulphation
Glutathione (GSH)
Example of molecule that can go straight to phase II biotransformation reactions
Morphine
Pharmacokinetics
A = absorption
D - distribution
M = metabolism
E = elimination/excretion
Effect of xenobiotics
Damage proteins, lipids and can bind to DNA (carcinogens)
Mechanism of Xenobiotics
React with O2 and release free radicals
Why do most medications require 2 phase biotransformation
Tend to be lipophilic, non-polar and non-ionised at physiological pH to allow pharmaceutical action
Location of microsomal enzymes
Smooth ER of liver, kidneys and intestinal mucosa
Microsomal enzymes
Mono-oxygenases (CYPs, FMOs)
Reaction of microsomal enzymes
Majority of drug biotransformation
Are microsomal enzymes inducible
Yes by diet and drugs
Location of non-microsomal enzymes
Cytoplasm and mitochondria of hepatocytes
Non-microsomal enzymes
Protein oxidases, esterases etc
Reaction of non-microsomal enzymes
Non-specific enzymes for conjugation
Are non-microsomal enzymes inducible
No but have polymorphisms
Which organ excretes drugs and metabolites
Kidney
When are cytochrome-P450 enzymes required
Phase I biotransformation
Which cytochrome-P450 enzyme is in highest concentration
CYP3A4
Responsible for 2/3 all known drugs
How many main groups of cytochrome-P450 are there
At least 10
Enzyme induction of cytochrome-P450
Molecule binds to an intracellular receptor within the cytoplasm
This molecule-receptor complex migrates to the nucleus
Increases transcription of mRNA for cytochrome-P450s
Increases the effect of the CYP
One substance can induce a number of enzymes
cytochrome-P450 mechanism of action
Contain a haem component which is capable of oxidising molecules (-OH addition) by becoming reduced themselves
The reductase use NADPH to become active
It reduces CYPs allowing the oxidation of the foreign molecule
Reaction forms water and has an intermediate of a haem free radical
Overall the addition of the -OH group increases the solubility of the molecule
What 3 things can metabolism of compounds results in
Complete inactivation and elimination
Formation of another active compound
Activation of pro-drugs
Toxification of less toxic Xenobiotics
Active drug to reactive intermediates
Phase III
Removal of drugs/ metabolites by transporter-mediated elimination via the liver gut kidney and lung
Where does phase III occur
Liver
Gut
Kidney
Lung
Complete inactivation and elimination
Eg phenobarbital (barbiturate derivative)
Metabolised using phase I and II
distributed into fat and bound to plasma proteins so metabolism is slow
Formation of another active component
Can have similar or new activity
Eg codeine breaking down into morphine
Eg diazepam into oxazepam
Activation of pro-drugs
Eg hetacillin converted into ampicillin
Eg into-glycerine into nitric oxide
Active drug to reactive intermediates
Eg benzopyrene in cigarette smoke
Bind to DNA and induce CYPs which increase epoxide levels
CYP and smoking
CYP1A2 can be induced by smoking
This increased activity has effects on metabolism of other molecules
Eg clozapine- dose has to be tightly controlled depending on how much the patient smokes
CYP and grapefruit
Grapefruit juice has effects on medications eg statins
Contains products that inhibit CYPs
Statins become more potent
Grapefruit is contraindicated
CYP2E1 and paracetamol
Usually disposed of safely by glucuronidation (50%) as glucuronide has a lot of -OH groups, also by sulfation (40%)
Harmful intermediate NAPQI is created from CYP2E1 (10%)
If normal pathways are overwhelmed, CYP2E1 becomes more significant and more NAPQI created- normally metabolised by glutathione-S-transferase but this is overwhelmed by high levels
Hepatocytes then become damaged
CYP2E1 and ethanol
Uses the same enzyme as paracetamol so has a dual effect when taken together
Enzymes are induced by 2 factors
More likely to go done the harmful route- enzymes overwhelmed
Treatment for paracetamol overdose
Acetylcysteine
Acetaminophen——> conjungation ——> elimination
Glucuronidation
Sulfation
Acetaminophen——> NAPQI
CYP450
NAPQI ——> conjugation ——> elimination
GSH
NAPQI ——> adducts NO-, O2 nitration peroxidation ——> cell death
GSH-depletion
Ethanol and alcohol dehydrogenase
Alcohol is polar but also slightly lipid soluble
Can be excreted (2-10%) but is more commonly used as fuel
Metabolised in acetaldehyde (tocis)
Further metabolism to acetate by ALDH which can be used in Kreb’s cycle
This second metabolism becomes overwhelmed and acetaldehyde builds up
What percentage of ethanol is excreted
2-10%
First metabolism reaction of ethanol
Acetaldehyde
Acetaldehyde—> acetate
ALDH
What is acetaldehyde converted to
Acetate
Ethanol and microsomal system- uses CYP2E1
Microsomal ethanol oxidising system (MEOS) also produces acetaldehyde
Chronic alcohol use increases CYP2E1 levels 5-10 fold therefore alcohol is metabolised quicker
Acetaldehyde is produced quicker and in larger quantities- more toxic
Results in liver damage from the production of free radicals
2 processes that produce acetaldehyde from ethanol
Microsomal ethanol oxidising system
Alcohol dehydrogenase
Inhibition cytochrome P450 enzymes
can result in increased blood concentrations of certain medications (less breakdown).
Suitable molecule that can bind to nuclear hormone receptor
phenobarbital
Vitamin D produced in the body
Cholecalciferol
Vitamin D found in food
Ergocalcaiferol
Which vitamin protects vitamin A
Vitamin E
How much vitamin B12 is stored in the body
2-5 mg
What percentage of vitamin B12 is stored in the liver
50%
Vitamin metabolism
Liver is important in metabolic activation of vitamin D
3 types of jaundice
pre-hepatic, hepatic or post-hepatic
Pre-hepatic jaundice
caused by increased haemolysis- results in increased presence of unconjugated bilirubin in the blood as the liver is unable to conjugate it all at the same rate
• Any bilirubin that manages to become conjugated will be excreted normally, yet it is the unconjugated bilirubin that remains in the blood stream to cause the jaundice.
Causes of pre-hepatic jaundice
• Tropical disease eg malaria, yellow fever
• Genetic disorders eg sickle-cell anaemia, Gilbert’s syndrome
• Haemolytic anaemias
Hepatic jaundice
caused by liver impairment- causes the decreased ability of the liver to conjugate bilirubin, resulting in the presence of conjugated and unconjugated bilirubin in the blood
• The liver loses the ability to conjugate bilirubin, but in cases where it also may become cirrhotic, it compresses the intra-hepatic portions of the biliary tree to cause a degree of obstruction. This leads to both unconjugated and conjugated bilirubin in the blood
Causes of hepatic jaundice
• Viral hepatitis
• Hepatotoxic drugs eg paracetamol overdose, alcohol abuse
Post-hepatic jaundice
caused by the blockage of bile ducts- results in back-flow of conjugated bilirubin into the blood as it cannot move past the obstruction
• bilirubin that is not excreted will have been conjugated by the liver, hence the result is a conjugated hyperbilirubinaemia.
Causes of post-hepatic jaundice
Gallstones
• Hepatic tumours
• Pancreatic tumours
Gilbert’s syndrome
• inherited disorder where there is hyperbilirubinaemia (excess bilirubin in bloodstream) due to a fault in the UGT1A1 gene leading to a deficiency in UPD-gluconoryltransferase.
• Results in slower conjugation of bilirubin in the liver so it builds up in the bloodstream
• When well, patients are usually asymptomatic and have normal bilirubin levels
• Under physiological stressors such as illness, alcohol abuse and extreme exercise, patients can become markedly jaundiced
Why does jaundice give you dark urine
excessive conjugated bilirubin excreted through the kidneys
Why does jaundice give you pale stool
reduced levels of stercobilin entering the GI tract. Obstructive or post-hepatic liver cause as normal faeces get their colour from bile pigments
Hepatobiliary system
Between adjacent hepatocytes, grooves in the cell membranes provide room for bile canaliculi: these are small ducts that accumulate the bile produced by hepatocytes. Bile flows first into bile ductules and then into bile ducts which unite to form the larger left and right hepatic ducts. These merge and exit the liver as the common hepatic duct. The common hepatic duct then joins with the cystic duct (from the gallbladder), forming the common bile duct which flows into the duodenum.
Bilirubin
yellow bile pigment produced through the breakdown of red blood cells (haemolysis). It is metabolised prior to excretion through the faeces and urine.
Bilirubin exists in 2 forms: conjugated and unconjugated. Unconjugated bilirubin is insoluble in water so can only travel in the bloodstream if bound to albumin and cannot be directly excreted from the body. Whereas, conjugated bilirubin is water soluble so it can travel in the bloodstream and excreted out of the body.
2 forms of bilirubin
Conjugated and unconjugated
Unconjugated bilirubin
insoluble in water so can only travel in the bloodstream if bound to albumin and cannot be directly excreted from the body
Conjugated bilirubin
water soluble so it can travel in the bloodstream and excreted out of the body.
Creation of bilirubin
reticuloendothelial cells are macrophages which are responsible for the maintenance of blood through destruction of old or abnormal cells. They take up red blood cells and metabolise the haemoglobin present into its individual components: haem and globin. Globin is further broken down into amino acids which are recycled. Haem is broken down into iron and biliverdin, catalysed by haem oxygenase. The iron is recycled and the biliverdin is reduced to form unconjugated bilirubin.
Bilirubin conjugation
In the bloodstream, unconjugated bilirubin binds to albumin to facilitate its transport to the liver. In the liver, glucuronic acid is added to unconjugated bilirubin by the enzyme glucuronyl transferase. This forms conjugated bilirubin, which is soluble, allowing it to be excreted into the duodenum in bile.
Bilirubin excretion
Once in the colon, colonic bacteria deconjugate bilirubin and convert it into urobilinogen. Around 80% is further oxidised by intestinal bacteria and converted to stercobilin and then excreted through faeces (giving it its colour). Around 20% of the urobilinogen is reabsorbed into the bloodstream as part of enterohepatic circulation. It is carried to the liver where some is recycled for bile production, and a small percentage reaches the kidneys. In the kidneys, it is further oxidised to urobilin and then excreted into the urine.
Which cells destroy old or damaged red blood cells (haemolysis)
reticuloendothelial cells
What is haemoglobin metabolised into
Haem and globin
What is globin broken into
Amino acids which are recycled
What is haem broken down into
Iron and biliverdin
Which enzyme breaks down haem
Haem oxygenase
What happens to the iron produced by haemolysis
It is recyled
What happens to biliverdin
It is reduced to form unconjugated bilirubin
What must unconjugated bilirubin bind to in order to be transported in blood
Albumin
Where is unconjugated bilirubin conjugated
Liver
Which enzyme conjugates bilirubin
glucuronyl transferase
What is added to unconjugated bilirubin to form conjugated bilirubin
Glucuronic acid
How is conjugate bilirubin excreted from the liver
In the bile into the duodenum
What happens to conjugated bilirubin in the colon
Colonic bacteria deconjugates it forming urobilinogen
What is formed in the colon from conjugated bilirubin
Urobilinogen
What happens to 80% of the urobilinogen
further oxidised by intestinal bacteria and converted to stercobilin and then excreted through faeces (giving it its colour)
What gives faeces its colour
Stercobilin
What happens to 20% of the urobilinogen formed
reabsorbed into the bloodstream as part of enterohepatic circulation. It is carried to the liver where some is recycled for bile production, and a small percentage reaches the kidneys. In the kidneys, it is further oxidised to urobilin and then excreted into the urine.
Where is urobilinogen metabolised before excretion in the urine
Kidneys
What is urobilinogen oxidised to in the kidneys
Urobilin
What is urobilinogen oxidised to by intestinal bacteria
Stercobilin
Scheme of principal blood flow through the liver
Heart —> abdominal aorta —> hepatic artery proper —> liver —> hepatic veins —> inferior vena cava —> heart
Microanatomy of liver
Organised in lobules with a central hepatic wein
Hexagon- portal triads in the corners
Number of functions of the liver
Approximately 500
Main functions of liver
Detoxification - filters and cleans blood of waste products (drugs, hormones)
Immune functions - fights infections and diseases (RE system)
Involved in synthesis of clotting factors, proteins, enzymes, glycogen and fats
Production of bile and breakdown of bilirubin
Energy storage- glycogen and fats
Regulation of fat metabolism
Ability to regenerate
Metabolic role of the liver
Maintains a continuous supply of energy for the body by controlling the metabolism of CHO and fats
What is the liver regulated by
Endocrine glands eg pancreas, adrenal, thyroid
Nerves
Lipid definition
Esters of fatty acids and glycerol or other compounds (cholesterol)
Large and diverse group of naturally occurring organic compounds that are insoluble in water
Variety of structure and functions
Tri(acyl)glycerides TG, TAG
1 glycerol molecule esterified to 3 fatty acids bonded at carboxylate head
Function of triglyceride
Storage form of fat in our body
-adipocytes
-hepatocytes
-elsewhere
Saturated fatty acid
Lining up close together
Esters are solid at room temperature
Solid ‘fats’
Unsaturated fatty acids
Needs more space due to kink in chain
Less tightly packed
MUFA, PUFA
Esters are liquid at room temperature
‘Oils’
Lipid functions
Energy reserve
Structural and other functions
Hormone metabolism
Lipid functions: structural and other functions
Part of cell membranes
Integral to form and functions of cells
Inflammatory cascade (LC-PUFAs precursors to eicosanoids, eg prostaglandins)
Lipid functions: inflammatory cascade
LC-PUFAs precursors to eicosanoids e.g. prostaglandins
Lipid functions: hormone metabolism
Cholesterol is backbone of adrenocorticoid and sex hormones
Vitamin D
Lipids yield how much energy per gram
9-10 kcal
Lipid reserve 100000 kcal can last how long
30-40 days
Lipid transport
Often transported as TGs or FAs bound to albumin or within lipoproteins
Triglycerides cannot diffuse through cell membrane
Fatty acids are released through lipases to facilitate transport into cells- in the cell re-esterified to triglycerides
Fatty acid uptake
Diffusion through the lipid bilayer of the cell membrane
Facilitated transport
Facilitated transport of fatty acids
Increases if increased substrate or increase in receptor molecules
Several transporter systems
Transporter systems for facilitated diffusion of fatty acids
FA binding protein = mitochondrial AST - induction to increased expression may result in increased uptake of fatty acid in hepatocytes
FAT -fatty acid translocase
FATP- fatty acid transport polypeptide
Trans fats
Hardly kinked
Hard to metabolise
What percentage of dietary fats of triglycerides
90%
Proportion of western diet that are dietary fats
1/3
What inhibits absorption of cholesterol in small intestine
Ezetimibe
Bile acids
polar derivates of of cholesterol - aid diestestion of fats and fat soluble vitamins
Ba are amphipathic and emulsify fat globules into smaller miclelles -= hence higher surface area for lipid hydrolysing emzymes – they act as detergents (washing up liquid)
Secretion of bile acids and cholesterol
Secretion of BA and cholesterol through bile is the only excretion mechansim of cholesterol
Enterohepatic circulation
Reabsorption of bile acids and cholesterol in the ileum
What can disrupt reabsorption of cholesterol in ileum
Resins- like cholestyramine
Diet- oat, bran fibre and fruit pectins
Chylomicrons
From the gut are transported through the lymphatic system to be delivered to muscle and adipose tissue, bypassing the liver- protects the liver from a large fatty acid influx
5 ways lipids accumulate in the liver
Excess intake - triglycerides in Chylomicrons remnants reach liver
Increased de novo synthesis
Increased fatty acid influx from lipolysis in adipocytes
Reduced export
Reduced oxidation in liver
Action of insulin
Fat storage in adipocytes
Stimulates LPL (lipoprotein lipase)—> breakdown of triglycerides, releases free fatty acids to store )in form of TG) in adipocytes
Reduced activity of hormone sensitive lipase (HSL) leading to reduced fatty acid export from adipocytes
Insulin resistance
Increased lipolysis in adipocytes leading to increased triglyceride in circulation
Increased ‘offer’ of fatty acids to hepatocytes leading to increased uptake
Increased glucose level leads to less demand for lipids to be used as energy source
Normal fatty acid uptake into hepatocytes
Lipoprotein lipase —> free fatty acids —> facilitated diffusion into adipocytes—> triglycerides—> hormone sensitive lipase —> free fatty acid -> hepatic lipase —> facilitated diffusion into hepatocytes
Enzymes involved in uptake of fatty acids
Lipoprotein lipase
Hormone sensitive lipase
Hepatic lipase
What is De novo lipogenesis in the liver dependent on
Insulin concentration and sensitivity
Purpose of hepatic de novo lipogenesis
Export in lipoproteins as energy source and structural components for membranes
De novo lipogenesis in the liver
Sequential extension of alkanoic chain staring from Acetyl-CoA via serial decarboxylative condensation reactions
Control of de novo lipogenesis in the liver
Nutrition and hormones
High KH diet increases hepatic lipogenesis- KH are burnt to generate ATP, surplus glucose fills glycogen stored and further surplus is converted to fatty acids
Fasting and fat feeding inhibit it
De novo lipogenesis in adipocytes
Long term energy storage
SREBP-1c
Activated fatty acid synthase
ChREBP
Carbohydrate response element binding protein
Dietary protein intake
0.75g/kg/day
Loss of nitrogen
Faecal loss- 10g/day
Renal excretion- 70g/day in form of urea
Skin/hair/sweat loss
Positive nitrogen balance examples
Pregnancy
Lactation
Bodybuilder and anabolic steroids
Recovery ohase
Negative nitrogen balance examples
Protein malnutrition
Severe illness/sepsis/trauma/burns
Corticosteroids
Cahexia: malignancy/heart failure/uraemia
Essential amino acid deficiency
Nitrogen balance
concept that compares the amount of nitrogen (overwhelmingly in the form of dietary proteins) that enters the body compared to that which is excreted from the body. Nitrogen is said to be in balance when the two are roughly equal (+/- 4g/day)
Positive nitrogen balance
Intake of nitrogen greater than excretion
Anabolic- gain of protein
Negative nitrogen balance
Excretion is greater than intake
Catanolism- loss of protein
3 main fates of amino acids
they can be incorporated ‘as is’ with other amino acids to form peptides and proteins
they can be modified to form other biomolecules such as nucleotide bases and neurotransmitters
they can have their nitrogenous amino group removed (to be excreted as urea) and have their ‘carbon backbone’ reutilised for energy, either via the Citric acid cycle or through the formation of glucose via gluconeogenesis.
No amino acid is stored.
Kwashiorkor
Adequate calories but inadequate proteins
Protein-energy metabolism
Features: oedema , fatty liver, dermatoses
Marasmus
Both protein and calories insufficient
Amino acid metabolism: fed state
Amino acids are absorbed from the gut and enter the portal circulation where they travel to the liver, which is the centre for the majority of metabolic processes. Accordingly, it is also the site where most of the humoral, that is to say, blood-bourne proteins are formed.
They may be utilised here in protein formation, or be used to form other nitrogen-containing compounds.
Some enter the systemic circulation to supply the builiding blocks of protein to different bodily tissues.
Those which cannot be used in such a fashion have their amino groups removed and their carbon backbones used as metabolic substrates, helping to form carbohydrates like glucose from gluconeogenesis or fatty acids to form triacylglycerols.
GI proteolysis
Dietary protein —> denatured protein —> oligopeptides and AAS —> amino acids in blood stream (active transport)
What converts dietary protein to denatured protein
HCl and pepsin in stomach
What converts denatured protein to oligopeptides and AAs
Chymotrypsin
Trypsin
Aminopeptidase
In small intestine
What converts oligopeptides and AAs to amino acids in bloodstream
Enterocyte peptidases
In enterocytes
Transportation of free amino acid into enterocyte
first stage is via a cell surface channel where it is cotransported with a sodium ion.
Then via the basal membrane is is actively transported into the portal veinous circulation by an ATP consuming process.
From there it travels into the liver.
Glucogenic amino acids
Carbon backbone produces gluconeogenic/TCA cycle intermediates
Ketogenic amino acids
Carbon backbone produces Acetyl-CoA/ Acetoacetyl-CoA
Which amino acids are solely ketogenic
Leucine
Lysine
Essential amino acids
Phenylalanine
Valine
Leucine
Isoleucine
Tryptophan
Methionine
Threonine
Histidine
What are essential amino acids
Cannot be synthesised de novo in vivo
Important hepatic proteins
Albumin
Coagulation factors
IGF-1
C-reactive protein
Carrier proteins eg caeruloplasmin
Apolipoproteins
Importance of albumin
Maintains oncotic blood pressure
Important carrier protein eg for sex hormones, magnesium, calcium and drugs
Glycine derivatives
Heme
Creatinine
Purine bases
Biosynthetic pathways for nitrogen from amino acids
Can produce non-peptide molecules eg neurotransmitters, nitric oxide and nucleotides
Aspartate derivatives
Purine and pyrimidine bases
Arginine derivatives
Nitric oxide
Tryptophan derivatives
Serotonin
Melotonin
Tyrosine derivatives
Dopamine
Catecholamines (enable fight or flight response)
Thyroid hormones
Melanin
Alpha-ketoacid
Carbon backbone after R group of amino acid cleaved
Used in kreb’s cycle eg alanine —>pyruvate
Transamination enzyme
Aminotransferase (with PLP group)
Transamination of alanine
Alanine + Alpha-ketoglutatate <—> pyruvate + glutamate
Alanine aminotransferase (ALT)
Transamination
In most transamination reactions, a-ketoglutarate and glutamate form one of the a-ketoacid/amino acid pairs.
This means a-ketoglutarate is a receiver of nitrogen (the amino group), which is transferred to glutamate.
Fate of nitrogen after degradation of amino acids
Forms ammonia (NH4+) which is combined with bicarbonate to form carbamyl phosphate
Enters urea cycle to produce urea which is excreted
What enzyme converts glutamate to alpha-ketoglutarate
Glutamate dehydrogenase
Other products when glutamate converted to alpha-ketoglutarate
NADP + H20 ——> NADPH + NH4+ (ammonium)
What two molecules form carbamyl phosphate
Ammonium and bicarbonate
Causes of Protein degradation
Faulty/aging/obsolete proteins
Signal transduction
Flexible system to meet protein/energy requirements of environment
Main means of protein degradation
Proteasome (ubiquitin-dependent)
Lysosome
Ubiquitin
Mark of death
Small protein
Carboxyl group forms isopeptide bond with multiple lysine residues
Formation of ubiquitin chains (stronger signal, especially >4)
3 Enzymes involved with ubiquitin
Ubiquitin-activating enzyme
Ubiquitin-conjugating enzyme
Ubiquitin-protein ligase
Proteasome
The executioner
N-terminal rule
N-terminal residues determine protein half-life
PEST sequences (proline, glutamate, serine, threonine)
Cyclin destruction box
Lysosomes
Proteolytic enzymes within lysosome separated from cytosolic components
4 types of lysosomal mechanisms
Macroautography
Microautography
Chaperone-mediated autographs
Endocytosis/phagocytosis
Macroautography
Non-selective
ER derived autophagisomes engulf cytosolic proteins/aggregates organelles
Lysosome fuses with this to initiate proteolysis
Microautophagy
Non-selective
Invaginations of lysosomal membrane engulf proteins/organelles
Chaperone-mediated autographs
Selective
Chaperone protein hsc70 in cytosolic and intralysomal accompany specific cytosolic proteins in response to stressors (fasting/oxidative stress etc)
Endocytosis/phagocytosis
Extracellular substances
Cystinosis
Autosomal recessive condition
1 in 200000
Defect in transporter leads to cystine accumulation in tissue lysosomes
Eye and kidney problems
Cortisol activates
Proteolysis
Gluconeogenesis
Cortisol inhibits
Protein synthesis
Alanine - amino acid catabolism
Glucose-alanine cycle transports nitrogen from amino acid breakdown from the tissues to the liver, whilst recycling a carbon backbone that can be converted to glucose for energy
Glutamine - amino acid catabolism
Formed from BCAA degradation in the tissues
In the fasting state, it is an important metabolic fuel for the kidney and gut and provides ammonia to buffer proton diuretics in metabolic acidosis states
Branched chain amino acids - amino acid catabolism
Isoleucine/valine/leucine
Major amino acids that can be oxidised in tissues other than the liver, especially skeletal muscle
Glucagon - amino acid catabolism
Stimulates:
Glycogenolysis
Gluconeogenesis
Amino acid degradation
Ureagenesis
Entry of amino acids to liver
What percentage of cholesterol is endogenous
90%
What percentage of cholesterol is dietary
10%
Excretion of cholesterol
Through bile
Enterohepatic circulation
Cholesterol is esterified intracellularly in lipoprotein by
Acyl-CoA or by lecithin by cholesterol acyltransferase
Which enzymes esterifies cholesterol
Cholesterol acyltransferase
What do lipoproteins consist of
A core containing triglycerides and cholesterol-esters
A surface monolayer of phospholipids cholesterol and specific proteins (eg apoproteins)
What determine the density of lipoproteins
Protein to lipid ratio
Types of lipoprotein
Chylomicrons
VLDL
LDL
HDL
Chylomicrons lipoproteins
Largest lowest density
High lipid to protein ratio
Highest triglyceride content
VLDL
Very low density lipoprotein
2nd highest in triglycerides as percentage of weight
LDL
Low density lipoprotein
High cholesterol ester as percentage of weight
Raised by saturated fats and trans fatty acids
HDL
High density lipoprotein
Low lipid to protein ratio
Chylomicrons remnant
Taken up by the liver via receptor-mediated Endocytosis
Recognition of ApoE by hepatocyte surface receptors
Where is Apoprotein B synthesised
Rough ER
Fatty acid export
Microsomal TAG transfer protein adds the lipid components to ApoB
Transported in vesicle to Golgi apparatus where ApoB is glycosylates
This buds off the Golgi and migrate the sinusoidal membrane of the hepatocytes
Vesicle fuse with the membranes and VLDL is released
What is fatty acid export rate limiting for
VLDL production
What combines the lipid components to ApoB
Microsomal TAG transfer protein
Where is ApoB glycosylated
Golgi apparatus
Rate limiting steps of lipogenesis
Acetyl-CoA —> Malonyl-CoA
Catalysed by Acetyl-CoA carboxylase
Rate is also related to FAS-FA synthetase
What converts acetyl-CoA to malonyl-CoA
Acetyl-CoA carboxylase
Lipogenesis and fasting
Reduced in fasting
Hepatic glycogen stores depleted
Triglycerides broken down in adipocytes and more free fatty acids released
Oxidised in the liver as an energy source
Inflow into the liver of lipids from
Portal vein
Hepatic artery
Lymphatics
Lipids enter liver in form of
Triglycerides
Lipoproteins
Chylomicron remnants
HDL
(Often transported as triglycerides of fatty acids bound to albumin or within lipoproteins)
Lipids from adipocytes to hepatocytes
Hormone sensitive lipase release free fatty acids
Hepatic lipase enables the uptake into hepatocytes
Release of lipids from liver
Release controlled by hormones
Released as VLDLs, energy substrates and detoxified substrates
Storage capacity of far higher than demand
Lipids bypassing the liver
Chylomicrons can bypass the liver as transported by lymphatic system
Protects liver from large fatty acid influx
3 locations of lipid oxidation in liver
Peroxysomal beta-oxidation
Mitochondrial beta-oxidation
ER Microsomal Ω -oxidation (CYP4a catalysed)
Lipid oxidation in the liver
Fatty acid oxidation is proportional plasma levels of free fatty acids released from adipocytes
Peripheral fatty acid mobilisation when increased glucagon and decreased insukin
What causes fatty acid mobilisation
Increased glucagon
Decreased insulin
What is fatty acid synthetase activated by
Insulin
Substrate (citrate, isocitrate)
What is FA synthetase inactivated by
Catecholamines
Glucagon
FAS
FA synthetase
Negative feedback- high FAS in hepatocytes inhibit FAS
Related to de novo lipogenesis in the liver
Mitochondrial beta-oxidation
Primarily involved in oxidation of fatty acids of various chain length
Multistep process
Progressive shortening into acetyl -CoA subunits
- condensed into ketone bodies providing oxidisable energy to cells
- enter tricarboxyl acid cycle- resulting in H20 and CO2
What is mitochondrial beta-oxidation regulated by
CPT (carnitine palmitosyl transferase), carnitine concentration and malonyl-CoA (which inhibits CPT)
What leads to hepatic steatosis
Genetic disorders inhibiting mitochondrial oxidation, certain drugs eg alcohol and toxins
Number of carbons - short chain fatty acid
<8
Number of carbons - medium chain fatty acid
8-12
Number of carbons - long chain fatty acid
12-20
Dicarbolic acids
Very toxic
Inhibits mitochondrial fatty acid oxidation
Peroxisomal beta-oxidation
4 step process is repeatedly performed to shorten chain length
Each step can be carried out by at least 2 enzymes
Disruption of Peroxisomal beta-oxidation
Leads to micro-vesicular steatosis
What are enzymes of Peroxisomal beta-oxidation induced by
PPARά
Main role of Peroxisomal beta-oxidation
detoxification of
-very long chain fatty acids (>C 20)
-2-methyl-branched FAs
-Dicarbolic acids – very toxic – inhibiting mitochondrial fatty acid oxidation
-Prostanoids
-C-27 bile acid intermediaries
Microsomal Ω -oxidation
Normally a minor metabolic pathway but in fat overload increases
CYP4A enzymes oxidise saturated and unsaturated fatty acids
Ω-hydroxylation in the ER, followed by decarboxylation of the Ω-hydroxy fatty acid in the cytosol – in turn enter the β-oxidation pathway
Dicarboxyl FA act as ligands to PPARά – induction of the oxidation systems
PPARά
Lipid sensor- gene transcription
Its activity determines whether fatty acids are stored as triglycerides in hepatocytes or oxidised for energy
How do fatty acids regulate gene expression
Control activity of key transcription factors
Function of transcription factors
Integration of signals from diverse pathways
Co-ordination of the metabolic machinery for fatty acid metabolism
Transcription factors controlled by fatty acids
Peroxisome proliferator-activated receptors
(PPAR ά, β and γ)
Retinoid X receptor (RXR)
Sterol regulator element binding protein (SREBP)
Peroxisome proliferator activated receptor (PPAR)
All PPARs (ά, γ,β/ δ) are involved in lipid homeostasis
PPAR ά and β/ δ facilitate energy combustion
PPAR γ facilitates energy storage
PPAR ά is a lipid sensor – gene transcription
Reduced PPAR ά sensing/activity leads to steatosis, possible by induction of CYP2E1, proinflammatory cytokines and TFN ά
Role of PPAR ά and β/ δ
Facilitate energy combustion
PPAR γ
Facilitates energy storage
Reduced PPAR ά
sensing/activity leads to steatosis, possible by induction of CYP2E1, proinflammatory cytokines and TFN ά
Adiposities
Increased energy storage
Developing fatty liver
Increase plasma in fatty acids (mainly in TG)
Excess dietary fat intake
Excess dietary caloric intake overall
But also increased flux of FAs
Increased release of FAs from adipocytes
Increased FA uptake in hepatocytes
Decreased FA oxidation
Decreased demand for lipids for fuel leading to
increased storage
Resulting in bland steatosis
Insulin resistance augments the process
Non-alcoholic fatty liver
Overstorage of unmetabolised energy exceeding the energy combustion capability of the PPAR a mediated system
Hepatic steatosis
Fat content exceeding 5-10% of the weight of the liver
Incidence of NAFL - diabetic patients
50%
Incidence of NAFL - obese patients
75%
Incidence of NAFL - morbidly obese patients
98%
Steatohepatitis
Increased steatosis (bland)
Apoptosis of fat-laden hepatocyte releases TG and toxic FAs
FAs induce CYP2E1 & FA oxidation systems →generation of reactive oxygen species (ROS) resulting in oxidative stress
Oxidative stress induces release of proinflammatory cytokines from Kupffer cells (hepatitis) and ROS (and ethanol) activates stellate cells (fibrogenesis)
Lipidperoxidation products develop immunogenic properties causing inflammation
Gut derived products (e.g. endotoxins) also activate Kupffer cells
causes of change from NAFLD—>NASH
Adipose tissue inflammation
Gut microbiota
Oxidative stress
Hepatocyte apoptosis
Hepatic inflammation
Stages of NAFLD
1 simple fatty liver (steatosis)- a largely harmless build-up of fat in the liver cells that may only be diagnosed during tests carried out for another reason. Benign and no liver damage. Reversible
2 non-alcoholic steatohepatitis (NASH)- a more serious form of NAFLD, where the liver has become inflamed- estimated to affect up to 5% of the UK population
3 fibrosis - where persistent inflammationcauses scar tissue around the liver and nearbybloodvessels, but the liver is still able to function normally
4 cirrhosis- the most severe stage, occurring afteryears of inflammation, where the liver shrinks and becomes scarred and lumpy; this damage is permanent and canlead toliver failure andliver cancer
Stage 1 of NAFLD
simple fatty liver (steatosis)- a largely harmless build-up of fat in the liver cells that may only be diagnosed during tests carried out for another reason. Benign and no liver damage
Reversible
Stage 2 of NAFLD
non-alcoholic steatohepatitis (NASH)- a more serious form of NAFLD, where the liver has become inflamed- estimated to affect up to 5% of the UK population
Stage 3 of NAFLD
fibrosis - where persistent inflammationcauses scar tissue around the liver and nearbybloodvessels, but the liver is still able to function normally
Stage 4 of NAFLD
cirrhosis- the most severe stage, occurring afteryears of inflammation, where the liver shrinks and becomes scarred and lumpy; this damage is permanent and canlead toliver failure andliver cancer
Management of fatty liver disease
Diet and exercise
↓ Supply - Reduced intake of calories
↑ Demand – increased consumption
The body will get energy from the “compartment” least in demand e.g.
Exercise – fat burning
Illness – muscle protein utilisation
Alcohol and liver fat
High caloric load – energy load
Metabolised in the liver – increased load leads to:
Impairment and inhibition of PPARά and SREBP
PPARά ↓ - ↓ fat oxidation
SREBP ↓ - ↑ FAS - lipogenesis
Damage to cell organelles – mitochondria, ER – reduced fat oxidation
Apoptotic hepatocytes release TG and very-long chain FA – augment liver injury
Stellate cell activation –leading to increased fibrogenesis
Conditionally essential amino acid
Under certain circumstances they may be needed to be consumed in the diet eg dependent on consumption of other amino acids
Examples of conditionally essential amino acids
Arginine
Cysteine
Glycine
Glutamine
Proline
Tyrosine
Universal acceptor of amine groups
Alpha-ketoglutarate
Transamination
Turning an amino acid into an intermediate in the TCA cycle
Catalysed an amino transferase eg pyridoxal phosphate (PLP) dervived from vitamin B6
Taking an amine group from an amino acid to an alpha-ketoacid
Turns it into an amino acid and becomes and alpha-ketoacid itself
What is pyridoxal phosphate derived from
Vitamin B6
Deamination
Glutamate is converted back to alpha-ketoglutamate by glutamate dehydrogenase
Produces ammonia
Remove via the urea cycle
Molecular weight of albumin
66 kDa
g/day of albumin produced by the liver
9-12
Can increase to 36
Transcapillary escape rate
Rate of movement of albumin between vessels
How does albumin leave circulation
Interstitium
How is albumin returned to circulation
Thoracic duct (lymphatics)
Functions of albumin
Binding and transport
Maintenance of colloid osmotic pressure
Free radicals
Anticoagulant effects
Fed (anabolic) state
Amino acid surplus to requirement for protein synthesis can be metabolised to non-nitrogenous substances
Eg glucose , glycogen or fatty acids
Can be oxidised to generate ATP
Fasting (catabolic) state
Alanine transported to hepatocytes in large quantities
Alanine aminotransferase transaminates the amino group from glutamate- producing pyruvate
Pyruvate substrate in TCA cycle to form glucose
Glutamate recycled back and urea is formed
Glucose taken up by muscle cells and used in glycolysis
Pyruvate produced and lactate released as a by-product
Pyruvate then converted back to alanine
Glucose-alanine cycle - removal
Moves carbon atoms of pyruvate
Moves excess ammonia from muscle to liver as alanine
In liver, alanine yields pyruvate- starting block for gluconeogenesis
Releases ammonia for conversion into urea]
Energetic burden of gluconeogenesis imposed on liver rather than muscle (muscle ATP devoted to muscle contraction)
Pyruvate is the metabolic precursor for
Alanine
Oxaloacetate is the metabolic precursor for
Aspartate
Asparagine
Alpha-ketoglutarate is the metabolic precursor for
Glutamate
Glutamine
Proline
Arginine
3-phosphoglycerate is the metabolic precursor for
Serine
Cysteine
Glycine
Phosphoenolpyruvate and erythrose-4-P is the metabolic precursor for
Tyrosine
Which hormone drives all metabolic pathways in fed state
Insulin
Pathways of increased glucose in liver in fed state
- Glycogenesis
- Pentose phosphate pathway
- Formation of GA3P and DHAP to then form 2 pyruvate
- Fatty acid synthesis
Pentose phosphate pathway
Forms ribose-5-phosphate from glucose
Generates NADPH
If high levels of Acetyl-CoA in the liver
Kreb’s cycle inhibited
Citrate is broken down into oxaloacetate and acetyl-coa by citrate ligase
Acety-CoA is converted to malonyl-CoA—-> fatty acids
Intermediate between acetyl-CoA and fatty acids
Malonyl-CoA
Where does the urea cycle take place
Partly in the cytosol and partly in the mitochondria
Control of the urea cycle
Via up/down regulation of the enzymes responsible
Long-term changes in level of dietary protein can result in 20-fold up regulation - could be seen in both high protein diets and starvation (protein breakdown)
The urea cycle overview
- Ammonia enters cycle by a Transamination reaction to form glutamate (recycle by deamination)
- NH3 moves into the mitochondria and reacts with HCO3- (with ATP) to form carbamoyl phosphate catalysed by carbamoyl phosphate synthase
- Carbmoyl phosphate reacts with ornithine to form citrulline catalysed by ornithine transcarbamylase
- Moves out into cytoplasm through transporter
- Reacts with aspartate and ATP to form arginine succinate catalysed by argininosuccinate synthase
- This breaks down into arginine and releases demarcate catalysed by argininosuccinate lyase
- Arginine reacts with water to produce ornithine releasing urea catalysed by arginase/ornithine aminotransferase
- Ornithine then re-enters the mitochondria to continue the cycle
Major proteins synthesised in the liver
Albumin
CRP
Hormone binding globulins
Apolipoproteins
Other transport proteins eg caeruloplasmin, ferritin
Factors in the complement cascade
Inhibitors of clotting
Fibrinolysis
Inhibitors of fibrinolysis
Complement
Which proteins does the liver not synthesise
Immunoglobulins
What charge does albumin have
Negative
Causes of hypoalbuminaemia
Inflammation
Liver disease
Renal disease
Burns/trauma
Sepsis
Malnutrition
Consequences of hypoalbuminaemia
Oedema
Effusions
Carrier proteins
Albumin calculations
Exudates vs Transudates
Adjusting for electrolytes – esp Ca2+
Adjusting for hormone levels – eg free testosterone
Renal disease
Chronic liver disease and bleeding
Reduced synthesis of clotting factors
-Hepatic dysfunction
-Vitamin K deficiency/malabsorption
Reduced synthesis of inhibitors
Production of abnormal/dysfunctional proteins
Enhanced fibrolytic activity
-Reduced clearance of activators of fibrinolysis
-Reduced production of inhibitors
Reduced hepatic clearance of clotting factors
Disseminated intravascular coagulation
-Multifactorial – includes endotoxaemia
Platelet abnormalities
-Number
-Function
Development of varices
Which vitamin is required for the liver to produce clotting factors
Vitamin K
How is NH4+ produced via catabolism
Amino acid split into alpha-keto acid and NH4+
Hypoalbuminaemia
Low blood albumin
Urea cycles treatment
Avoidance of catabolism, glucose polymers when unwell
Induction of anabolism – give dextrose 10% 2ml/kg/hr -> insulin!
Low dietary protein, arginine, benzoate, phenylbutyrate
Haemofiltration
Liver transplantation, umbilical vein hepatocyte transfusion
Gene therapy: NIH NGVL UPenn trial stopped after death (adenovirus E1 E4 del., fever, multi-organ failure)
What is the only anabolic hormone
Insulin
Energy requirements of 1 cycle of urea cycle
Consumes 3 ATP molecules
4 high energy nucleotide PO4-
How is the energy consumed by urea production generated
Production of the cycle intermediates
Products of urea cycle
Urea is only compound generated
Other components are all recycled
How does ammonia enter urea cycle
Transamination reaction to form glutamate
How is glutamate recycled
Deaminatiom
What does NH3 react with to form carbamoyl phosphate in urea cycle
HCO3- with ATP
NH3 and HCO3- forms
carbamoyl phosphate
Which enzyme catalyses production of carbamoyl phosphate
carbamoyl phosphate synthase
What does carbamoyl phosphate react with to form citrulline
Ornithine
carbamoyl phosphate and ornithine forms
Citrulline
Which enzymes catalyses formation of citrulline
Ornithine transcarbamylase
At what stage does the urea cycle go from mitochondria to cytoplasm
Citrulline moves out of mitochondria via a transporter
What does citrulline react with to form arginine succinate
Aspartate and ATP
Fate of NH4+
Biosynthesis of amino acids, nucleotides and biological amines
Excretion
Citrulline and aspartate forms
Arginine succinate
Which enzyme catalyses production of arginine succinate
Argininosuccinate synthase
What does arginine succinate break down into
Arginine and fumarate
Which enzyme catalyses the break down of arginine succinate
Argininosuccinate lyase
What does arginine react with to produce ornithine
Water
Arginine and water form
Ornithine and releases urea
Which enzyme catalyses production ornithine and urea
Arginase/ornithine aminotransferase
How are the urea cycle and TCA cycle linked
Through the aspartate-argininosuccinate shunt of the TCA cycle
Which molecule produced in the urea cycle enters the TCA cycle
Fumarate
Urea and TCA cycle
Fumarate produced by argininosuccinate lyase enters the TCA cycle
Converted to oxaloacetate
Oxaloacetate accepts an amino group from glutamate (transamination)
Forms aspartate - leaves mitochondria
Donates its amino group to the urea cycle in the argininosuccinate synthetase reaction
Intermediates in the citric acid cycle are boxed
What is added to oxaloacetate to form aspartate
Amino group from glutamate (transamination)
Main problem with high ammonia
Neurotoxicity
Ammonia crosses the blood-brain barrier readily. Once inside it is converted to glutamate via glutamate dehydrogenase and so depletes the brain of α ketoglutarate. As ketoglutarate falls, so does oxaloacetate and ultimately citric acid cycle activity stops, leading to irreparable cell damage and neural cell death
Early feature of hyperammonaemia
Respiratory alkalosis
Bile acid function
Removal of lipid soluble xenobiotics/drug metabolites/heavy metals
Induce bile flow and secretion of biliary lipids
Digestion of dietary fat
Facilitates protein absorption
Cholesterol homeostasis
Anti microbial
Induce bile flow and solubilise cholesterol
Prevents calcium gallstones and oxalate renal stones
Bile acid function- digestion of dietary fats
By solubilising lipids and lipid digestion products as mixed micelles facilitating aqueous diffusion across intestinal mucosa
Bile acid function- facilitates protein absorption
Accelerating hydrolysis by pancreatic proteases
Bile acid function- cholesterol homeostasis
Facilitates dietary absorption/elimination as bile acids are water soluble end products of cholesterol catabolism
Bile acid function- induce bile flow and solubilise cholesterol
Enabling movement from hepatocyte to intestinal lumen
Bile acid function- anti microbial
Physicochemical and inducing anti-microbial genes
Bile acid composition
Water
Inorganic electrolytes
Organic solutes- bile acids, phospholipids, cholesterol, pigment
Faecal bile acids (secondary)
2/3 deoxycholic
1/2 lithocholic
Hepatic/gallbladder bile
2/3 bile acids (primary)
-cholic- 1/3
-chenodeoxycholic 1/3
Deoxycholic 1/3
-lithocholic and ursodeoxycholic
1/4 phospholipids
Small amounts of cholesterol, bilirubin, proteins
Bile production per day
500-600 ml
Primary Bile acid production
Synthesised from cholesterol in hepatocytes
Converted into cholic acid and chenodeoxycholic acid
Enzyme = CYP7A1
Conjugated with glycine/taurine before secretion into bile canaliculi
Coagulation effects on bile salts
Increases hydrophilicity
Increases acidic strength of the side chain
Decreases passive diffusion of bile across cell membranes (keeps it intraluminal)
Secondary bile acid production
Presence of intestinal bacteria converts primary to secondary
Enzyme = 7 alpha-dehydroxylase
Which enzyme converts primary bile salts to secondary bile salts
7 alpha-dehydroxylase
Formation of bile salts is dependent on
Hepatic synthesis and canalicular secretion of bile acids (major organic anion in bile)
Which enzyme converts cholesterol into cholic acid and chenodeoxycholic acid
CYP7A1
Two main acids cholesterol is converted into to form bile acids
Cholic acid
Chenodeoxycholic acid
Number of enterohepatic circulation cycles per meal
2-3
Regulation of bile acid secretion- fasted state
Bile acids travel down biliary tract to the gallbladder where it is concentrated 10-fold
Regulation of bile acid secretion- fed state
CCK released from duodenal mucosa
Effects of CCK on bile acid secretion
Relaxes sphincter of Oddi
Contracts gallbladder
Releasing concentrated solution of mixed micelles (bile acid, phospholipids, cholesterol)
Reabsorption of bile acids
Conjugated bile acids remain intraluminal
Some reabsorbed passively in Jejunum-ileum
Actively transported via the apical sodium bile acid transporter in the ileum
Re-enters liver via portal circulation
Bile acids take up by hepatocytes and re-conjugated secreted into biliary canaliculi
Negative feedback mechanism- bile acids
Too much bile acid
Ligand for farnisoid X receptor in ileum
Results in synthesis of FGF-19 (endocrine polypeptide molecule)
Inhibition of CYP7A1 (cholesterol 7 alpha hydroxylase) - first step in converting cholesterol into bile acids
What are bile acids a ligand for
Farnisoid X receptor in ileum
Activation of farnisoid X receptor synthesises
FGF-19
What does FGF-19 inhibit
CYP7A1
What inhibits formation of cholesterol
Statins- inhibit HMG CoA reductase
What reduces absorption of cholesterol
Ezetimibe- stops protein mediated transport across enterocyte membrane
Excess amino acids —> fatty acids
Amino acids —> Acetyl-CoA—> citrate—> acetyl-CoA —> malonyl CoA—> fatty acids
3 fates of cholesterol in the liver
- Bile acids - CYP7A1
- Cholesterol esters for storage - ACAT
- Combine with triglycerides to form VLDLs
What is produced in skeletal or adipose tissue from VLDL
IDL (intermediate density lipoprotein)
Cells use triglycerides and cholesterol
IDL —> LDL
Loss of cholesterol
Main hormones for fasting state metabolism
Glucagon
Cortisol
Growth hormone
Adrenaline
Noradrenaline
Thyroid hormone
Bile acids are amphipathic
Reduce surface tension
Aid emulsification
Ileal resection or disease
unabsorbed bile acids enter colon where inhibit water absorption / induce secretion resulting in ‘bile salt diarrhoea’
Cholecystectomy
daily bile acid secretion is not altered much. Bile is ‘stored’ in proximal small intestine – likely big ‘shift’ to distal small intestine ‘overwhelms’ transport mechanism or feedback mechanism
Biliary obstruction
CBD stone, pancreatic carcinoma – intestinal malabsorption of fat soluble vitamins and fat resulting in steatorrhoea and develop jaundice
Disruption of enterohepatic circulation may be due to
bacterial overgrowth- deconjugation of bile acids
cholecystectomy
ileal resection may result in diarrhoea / steatorrhoea
malabsorption of fat soluble vitamins
Small intestinal bacterial overgrowth
Alteration of number/composition of bacteria in small intestine
Bloating, diarrhoea, abdominal pain
Treat with antibiotics
Which cells store vitamin C
None
What clotting factors are vitamin K dependent
Factor II, VII, IX X
Which hormone acts on the gallbladder to cause pain when suffering from gallstones
Cholecystokinin CCK- causes contraction of gallbladder but flow of bile is obstructed
What is the relationship between bile acid synthesis in the liver and bile acid reabsorption in the small intestine
Inversely proportional
More bile acid reabsorbed— > bile acids recycled and re-secreted via enterohepatic cycle so less bile acids produced
What stimulates bile secretion
Presence of partially digested fats and proteins in duodenum
Cholecystokinin and secretin released
CCK acts on gallbladder to secrete bile
Secretin stimulates biliary duct cells to secrete bicarbonate and water - expanding volume of bile
How does glucose-6-dehydrogenase deficiency cause jaundice
Erythrocytes more susceptible to to oxidative stress
Increased haemolysis
Where is iron stored in the body
Liver
Spleen
Bone marrow
Gilbert’s syndrome
Non-functional glucuronyl transferase
Why does jaundice cause itchy skin
Build up of bile salts
Serum concentration of bilirubin to be able to see yellow skin cause of jaundice
50 mmol/L
Serum concentration of bilirubin- jaundice
> 21 mmol/L
Bilirubin is the by-product of Haemoglobin breakdown.
Microsomal enzyme uridine diphosphoglucoronosyl transferase (glucuronyl transferase) catalyses the formation of what?
Conjugated bilirubin
A 53 year old patient is admitted with jaundice.
Which of the following causes an increased serum unconjugated (free) bilirubin and increased faecal urobilinogen?
Pre-hepatic
Which coagulation factors does the liver produce
1972:
- 10
-9
-7
-2
Which cells store iron in the form of ferritin
Kupffer cells
Haemoglobin is the chemical responsible for carrying oxygen around the body, the products of its breakdown give urine and faeces their distinct colours. In one stage of the breakdown of haemoglobin, unconjugated bilirubin is converted to conjugated bilirubin. This process occurs in the liver. What enzyme is responsible for converting unconjugated bilirubin to conjugated bilirubin?
Glucuronyl transferase
Carbamoyl phosphate synthetase deficiency I is an autosomal recessive disorder which causes toxic ammonia to accumulate in the body. Babies born with this disorder have a deficiency of the enzyme carbamoyl phosphate synthetase. In severe cases, this leads to respiratory distress, seizures and coma. What stage of the urea cycle is the enzyme carbamoyl phosphate synthetase important for?
Conversion of ornithine to citrulline
Albumin, which is produced in the liver, is a vital plasma protein that has many functions in the body. Which of the following is NOT a function of albumin?
Bind to and transport iron
