Case 5 - Hepatitis Flashcards
What is stored in the liver?
Glycogen, B12, Vitamin A
What does the liver detoxify?
Xenobiotics (ammonia, drugs etc.)
Steroids
Thyroid hormone
Metabolites
What does the liver synthesise?
Albumin, clotting factors, binding proteins, non-essential amino acids
What cell is responsible for phagocytosis in the liver?
Kupffer cells
What is the blood supply of the liver? Include the flow rate
➢75% from portal vein: rich in absorbed nutrients, recycled bile acids/ salts, about 1300 ml/min
➢ 25% from hepatic artery: regular systemic arterial blood, about 500ml/min
What cells line the sinusoids?
Fenestrated endothelial cells
What are the functions of zone I (periportal) of the liver?
Close to portal venule and oxygenated blood, so getting most oxygen to the cells. Functions:
* Amino acid catabolism
* Gluconeogenesis
* Cholesterol synthesis
What are the functions of zone II and III (pericentral)?
Zone III = functions that require the least oxygen so:
* Lipid synthesis
* Ketogenesis
* Glutamine synthesis
* Drug metabolism
Zone II = mix of zone I and III
With liver fibrosis, where are fibrotic changes most likely to occur?
Around the central veins (where there is least oxygen) as the cells there are less able to regenerate.
What type cells are hepatocytes?
polarised epithelial cells
What are the modifications of the epithelium to allow substances to pass from the blood to the interstitial fluid?
The endothelia is fenestratred, i.e. there are holes in the cell to allow substances to pass (‘Space of Disse’ in the liver).
Are the sinusoidal and canalicular membanes apical or basolateral?
Sinusoidal = apical
Canalicular = basolateral
How much bile is secreted per day from the liver? How much reaches the duodenum?
1000ml/day is secreted by the liver, however only around 500ml/day reaches the duodenum (as it is concentrated by the gall bladder)
What does bile include?
- Bilirubin (conjugated)
- Bile acids / salts (+ phospholipids and cholesterol)
- Metabolites of hormones and drugs
- Heavy metal ions
- HCO3- to neutralise acid and water
Compare the structure of bile acids and bile salts
Bile salts = conjugated, i.e. with taurine, glycine, sulphate, glucuronate (water-soluble)
Bile acids = unconjugated (BA-), proton attached. pKa around 5
What are secondary bile acids?
Bile acids that have been modified by the terminal ileum and colon. Can be conjugated to lower the dissociation constant
What type transporters are responsible for importing/exporting bile salts? How do they work?
APC transporters (ATP-binding cassete) - utilise ATP hydrolysis to pump
Give 2 examples of APC transporters for bile and one for cholesterol
Bile:
- BSEP (bile salt export pump)
- MRP2 (multidrug resistance-associated protein 2)
Cholesterol = ABCA1
How is unconjugated bile reabsorbed?
Most bile is conjugated but becomes unconjugated as it moves through the intestine. It is then reabsorbed passively
How is conjugated bile reabsorbed?
Mainly reabsorbed in the terminal ileum via ASBT (sodium bile salt cotransporter, active uptake), once it has entered the cell it then leaves via OST (organic solute transporter)
What is the average daily excretion of bile acids? What is it compensated by?
600mg/d, compensated by the synthesis of new bile acids
After recycled bile salts arrive at the liver, what are the 3 routes of uptake?
- Simple diffusion of unconjugated neutral BAH
- NTCP = Co-transport with Na+
- OATP = exchange with Cl-
Give some examples of organic ions that may be excreted by the liver into the bile
Thyroid and steroid hormones, prostaglandins, drugs (statins), toxins
How are organic ions excreted by the liver?
- OATP; basolateral uptake with exchange with Cl-
- Conjugation with glucuronate or sulphate
- Apical secretion via MRP2
Give some examples of organic cations excreted by the liver
Cytotoxic drugs, local anaesthetics, antibiotics
How are organic cations excreted by the liver?
Small cations = via facilitated diffusion of OCT1/3, then exchanged with H+ via MATE1
Bigger cations = OATP then MDR1
What transporter secretes cholesterol?
ABCG5/8
What produces bilirubin? Where is it transported to after it is produced?
RBC broken down by phagocytosis into haem and globin. Haem is then broken down into unconjugated bilirubin + iron. It is then bound to albumin and transported to the liver
How is bilirubin taken up by the liver? Where does it go after this?
After arriving with albumin, it is taken up by OATP. It is then conjugated with glucuronate in the ER (to become water soluble) and secreted by MRP2 into the bile = excreted
Why are faeces brown and urine yellow?
Bilirubin is deconjugated by bacteira in the SI, which converts it to urobilinogen. - Some of this is converted to stercobilin which is brown, hence faeces
- Some urobilinogen is reabsorbed into the blood and excreted as urobilin by the kidneys (yellow)
What is ketogenesis? What is it stimulated by?
Using acetyl-CoA to produce emergency fuel.
Promoted by surplus of mobilised FA in starvation or T1DM (as ketogenesis is suppressed by insulin)
What can surplus ammonia arrive to the liver as?
glutamate, glutamine or alanine (then de- or transaminated to dispose N as urea)
Which GLUT transporters are present in the adipose tissue and liver?
GLUT4 = adipose (target of insulin)
GLUT2 = liver
What traps glucose in a cell?
Phosphorylation (as the ionic phosphate cannot cross the membrane spontaneously) via hexokinase (liver) or glucokinase (other tissues). Uses ATP
What is the rate limiting step of glycolysis?
Phosphofructokinase-1 (PFK1): phosphorylates F6P to F1,6, biphosphate (this step requires ATP, very slow)
How much energy does glycolysis produce?
2 ATP per molecule glucose
What are the 2 main storages of glycogen in the body? How much (g) can they store?
Skeletal muscle = 400g
Liver = 100g
Why is glycogen branched?
More free ends mean it is more mobile / efficient to use as fuel
How does glucagon lead to its effects on glycogen metabolism?
1: glucagon increases cAMP which activates protein kinase A
2: PKA phosphorylates glycogen synthase directly and glycogen phosphorylase indirectly (via phosphorylase kinase)
3: Leads to glycogen breakdown (glycogenolysis) and inhibits glycogen formation (inhibits glycogenesis)
How does glucagon regulate gluconeogenesis?
- represses pyruvate kinase = increases PEP availability by preventing pyruvate formation
- Increases expression of PEP carboxykinase, promoting oxaloacetate to be converted into PEP
- Represses formation of F2, 6BP
How do catecholamines affect glucose production?
increase glucose production by cAMP activation of glycogen phosphorylase and gluconeogenesis.
What regulates phosphofructokinase-1 (PFK1)?
Activated by= allosterically by AMP (low levels in the cell) and F2,6BP
Inhibited by = ATP and citrate
How is F2, 6BP activated? What regulates this?
By PFK2 enzyme, which is inhibited by glucagon (when blood sugar is low) and stimulated by insulin (well fed state)
What is the role of pyruvate dehydrogenase complex (PDC)?
Contains E1, E2 and E3 for oxidative carboxylation of pyruvate into acetyl CoA, so then acetyl CoA can enter the TCA cycle
Why is oxygen required for the TCA cycle?
For oxidation (recycling) of the reduced cofactors (NADHH+ and FADH2)
How much ATP does the TCA cycle produce?
28 ATP per glucose (and 2 GTP)
What regulates lipogenesis?
- Availability of substrate: i.e. carbohydrate rich meals provide pyruvate/ acetyl-CoA and NADPH
- Insulin: stimulates
What is responsible for catalysing acetyl CoA –> Malonyl-CoA?
Acetyl-Coa-decarboxylase (ACC)
What regulates ACC enzyme?
ACC is inhibited by:
- Fatty acyl CoA (negative feedback)
- AMP
- Glucagon
- AMPK
ACC is stimulated by:
- Citrate
- Insulin
How much energy does beta-oxidation of FA produce?
Long chain FA are broken down 2 carbon atoms at a time, per 2-carbon unit it produces:
- one FADH2 = 2 ATP
- one NADH = 3 ATP
- one acetyl CoA = 12 ATP
What causes the ‘fruity’ breath noticed in T1DM patients? What is this a sign of?
Sign of serious ketoacidosis. Caused by spontaneous production of acetone (volatile) from acetoacetate
How can type 1 diabetes lead to ketoacidosis?
1: lack of insulin = loss of repression of hormone-sensitive lipase in adipocytes = TG is hydrolysed and FA is released
2: limited ability of liver to oxidise FA, so liver produces ketone bodies
3: leads to increased ketone bodies in blood (ketonemia) and in urine (ketouria)
4: Acidity of ketone bodies lowers blood pH = ketoacidosis
How can untreated T1D lead to hyperglycaemia?
Glucagon promotes gluconeogenesis in the liver, raising blood glucose levels
What is the reaction that catabolism of amino acids begins with?
Transaminase reaction, i.e. removal of a-amino group (transferred to a-ketoglurate)
What 2 liver transaminases are measured in a liver function test? Which is more sensitive and specific?
- Alanine transaminase (ALT) = more specific (only found in hepatocytes)
- Aspartate transaminase (AST) = more sensitive (as is in higher amounts, but is also present in heart, muscle and RBCs)
What is the role of AST?
Catalyzes the conversion of aspartate and a-ketoglutarate to oxaloacetate and glutamate
What are the 2 routes for how ammonia can enter the liver?
1- as glutamine, then releases 2 ammonia molecules by glutaminase and then glutamate decarboxylase
2- ammonia may be delivered by alanine-glucose shuttle; alanine delivers ammonia via ALT and resulting pyruvate goes into gluconeogenesis = glucose
How much ATP does it cost for ammonia and CO2 to be converted into urea?
4 ATP