Liver Biochemistry Lecture (TEST 2) Flashcards
Structure of the Liver
- LARGEST SOLID ORGAN in the body
- Weighs ~1500 g
- Consists of 2 LOBES each which is Subdivided unto MULTIPLE LOBES and SINUSOIDS
- Liver is covered by a Capsule of Connective Tissue
- Blood Flow:
a) 75% supplied by PORTAL VEIN
b) 25% by HEPATIC ARTERY - Biliary Component made of BILE DUCTS and GALL BLADDER
Liver Cell Types
- Hepatocytes
- Endothelial Cells
- Kupffer Cells —-> Macrophages
- Hepatic Stellate Cells —> Vitamin A and RA
- Pit Cells (lymphocytes)—-> NK Cells
Functions of the Liver
- Primary Receiving, Distribution and Recycling Center
- Lipid Biosynthesis and Management (Triacylglycerol, Phospholipids, Steroids)
- Protein Synthesis: Albumin, IgG, Lipid Transport Proteins (Apoproteins), Blood Coagulation Proteins (Fibrinogen, Prothrombin), C Reactive Protein
- Nitrogen Metabolism: Urea Cycle
- Waste Management: Xenobiotic Reactions
- Bilirubin Metabolism
- Fuel Management
Central Receiving, Distribution and Recycling Center
- Major Role of Liver is MONITORING, SYNTHESIZING, RECYCLING, DISTRIBUTING, and MODIFYING METABOLITES!!!!!!!!
- Any portion of Ingested Material that may be useful to the body is RETRIEVED BY THE LIVER and converted into a Useful Form
- Any HARMFUL PRODUCT that is ingested or produced in the body is CONVERTED INTO A SAFE PRODUCT and Excreted
Structural Adaptations
- Liver receives Blood from the Enteric Circulation (VIS PORTAL VEIN) and from Periphery (VIA HEPATIC ARTERY)
STRUCTURAL FEATURES:
a) Lack of BASEMENT MEMBRANE between Endothelial Cells and Hepatocytes
b) Gaps between Endothelial Cells
c) Fenestrations in Endothelial Cells
d) Low Portal Blood Pressure
* * These features ALLOW GREATER ACCESS AND INCREASED CONTACT BETWEEN LIVER AND BLOOD*
- HEPATOCYTES: Well Developed Plasma Membrane with ENDOCYTIC and EXOCYTIC System, well developed ER (Smooth and Rough), Metabolically Active Cells, lots of Mitochondria, lots of Lysosomes
Features of Isoprenoids
- THREE ACETYL CoA used to generate ISOPENTYL PYROPHOSPHATE (IPP)
- This Five Carbon IPP serves as the Building block for the Synthesis of all Isoprenoids, including Steroids and Lipid-Soluble Vitamins
Sources of Acetyl CoA
- Generated in MITOCHONDRIA form Various Pathways
a) OXIDATIVE DECARBOXYLATION of PYRUVATE
b) BETA OXIDATION of FATTY ACIDS
c) Breakdown of AMINO ACIDS
- Transported into Cytoplasm via CITRATE SHUTTLE
Sterane
- SIX UNITS of IPP form TETRACYCLIC (4 Rign) STERANE!!!!!!!!!!!!!!!!!!
- BACKBONE of most STEROIDS!!!!!
Structure of Cholesterol
- ALLICYCLIC COMPOUND made of 4 Fused Rings
- Molecular Weight: 386 kDa
- Has 27 CARBONS!!!!!!!!
STERANE:
- Sterane Ring has 17 Carbons
- Side Chain is 8 members Hydrocarbon Chain attached to C17!!!!!!!
- TWO METYL Groups at C10 and C13
- ONE HYDROXYL Group at C3
- ONE DOUBLE BOND between C5 and C6
Cholesterol
- Most ABUNDANT Sterol (~0.05% of Total Body Weight)
- Component of Plasma Membranes and Precursor of Biologically Active Compounds:
a) BILE ACIDS and BILE SALTS
b) VITAMIN D
c) STEROID HORMONES (Progesterone, Aldosterone, Cortisol, Testosterone, Estradiol)
Synthesis of Cholesterol
18 AcetylCoA + 18 ATP + 16NADPH + 16(H+) + 4O2 —————-> Cholesterol + 16 NADP+ + 18 ADP + 18 Pi
Cholesterol Synthesis Phase 1 and 2
PHASE 1:
- Starts with Acetyl CoA
- Ends with ISOPENTYL PYROPHOSPHATE (IPP)
- **Insulin and Thyroxin = Stimulate
- **Glucagon, Sterols, High AMP = Inhibits
PHASE 2:
- Starts with ISOPENTYL PYROPHOSPHATE (IPP)
- Ends with Cholesterol
- **Azoles and Squalestatins = Inhibits
Synthesis of Mevalonate from Acetyl CoA
1) Acetyl CoA ——> Acetoacetyl CoA
2) Acetoacetyl CoA —-(HMG-CoA Synthase)—-> HMG-CoA
3) **HMG-CoA —- ( HMG-CoA REDUCTASE!!!!!!!!) —-> Mevalonate*******
Compounds derived from Intermediates in Cholesterol Synthesis
1) QUINOL Form of UBIQUINONE (CoQ10)
2) HEME A (in Cytochromes)
Fate of Cholesterol
All Tissue:
- Cholesterol incorporated into Cellular Membranes
Liver:
- Cholesterol used to Synthesize BILE ACIDS
Adrenal Glands, Ovaries, TEstes:
- Cholesterol used to Synthesize Steroid Hormones
Skin:
- Cholesterol used to Synthesize Vitamin D
***PACKAGED into VLDL and released into Circulation
Lipid Rafts Contents
- Cholesterol Enriched Microdomains
Contains:
- GPI (Glycosylphosphatidylinositol)
- TM1 (Transmembrane)
Lipid Rafts
- PM Microdomains enriched in CHOLESTEROL, SPHINGOLIPIDS, and GANGLIOSIDES
- DETERGENT INSOLUBLE, LOW BUOYANT Density
- Local Centers for Signal Transduction Processes
- Sites for ABNORMAL PROCESSING of PROTEINS in NEURODEGENERATIVE DISORDERS!!!!!!!!!!!!
Regulation of Cholesterol Synthesis
- Regulated via effects on HMG CoA REDUCTASE (Rate Limiting Enzyme)
a) Direct Inhibition
b) Covalent Modification
c) Transcriptional Control
d) Translation Control
e) Post Translation Control- Protein Turnover
HMG-CoA Reductase
Target for Regulation of Cholesterol Synthesis*!!!!!!!!!!!
Sites of Ubiquitination:
- Lysine 89
- Lysine 248
Direct Inhibition
- HMG CoA REDUTASE inhibited by FREE FATTY Acids, Bile Acids, and Oxysterols
- Also Inhibited by STATINS, Competitive Inhibitors of the ENZYME. Compete with HMG CoA for binding to ACTIVE SITE!!!!!!!!!
(Ex: Simvastatin binds to the Active Site of the HMG CoA Reductase)
Competitive Inhibition of HMGR by Statins
1) HMGR-Inhibitor Complex
- Substrate, HMG CoA cannot bind to Enzyme-Inhibitor Complex
2) Comparative Structure sof HMG COA and Active Forms of Some Statins
Covalent Modification
- HMG CoA Reductase
a) ACTIVE when DEPHOSPHORYLATED
b) INACTIVE when PHOSPHORYLATED - Conditions of Low Energy, Characterized by HIGH AMP levels, stimulate AMP-Activated Kinase (AMPK) which Phosphorylates and Inactivates it
- Glucagon INHIBITS Enzyme by Preventing DEPHOSPHORYLATION
- Insulin ACTIVATES Enzyme by Promoting DEPHOSPHORYLATION
Regulation of HMGR Acivity via Phosphorylation/ Dephosphorylation:
a) HIGH [AMP], GLUCAGON, STEROLS = INACTIVE
b) INSULIN = ACTIVE
Transcriptional Control
- HMG CoA Reductase gene ahas a STEROL REGULATORY ELEMENT (SRE) in its Promoter
- Consensus sequence that binds Transcription Factors called Sterol Regulatory Element Binding Protiens (SREBP)
- Inactive Precursor for SREBP Interacts with a Protein SREBP cleavage activating Protein (SCAP)
- In presence of Cholesterol or Oxysterols SREBP-SCAP Complex retained in ER due to Binding to INSIG!!!!!!!!!!!!!!
- Under these Conditions rate of TRANSCRIPTION SLOW
- Low Sterol promotes release of SREBP-SCAP Complex from ER to Golgi to where SREBP undergoes PROTEOLYSIS to release the Mature form of SREBP which Dimerizes and Translocates to Nucleus!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
- Bidns to SRE and Promotes Transcription of HMG CoA Reductase and several other genes in the Pathway
Translation and Post Translational Control
- TRANSLATOIN REDUCED by GAMMA-TOCOTRIENOL (Vitamin E Family) and OXYLANOSTEROLS
- Protein Turnover
- In presence of Cholesterol and/or Oxysterols, HMG CoA interacts with INSIG which promotes Polyubiquitination of HMG CoA Reductase
- Leads to removal from ER and Degradation by the 26S PROTEASOME!!!!!
- Degradation Enhanced by STEROLS, METHYLATED STEROLS, OXYSTEROLS, TOCOTRIENOLS, MEVALONATE DERIVATIVE, and BISPHOSPHONATE SR-12813
Role of Liver in Cholesterol Homeostasis
- Liver plays key role in Cholesterol Metabolism and Homeostasis
- Cholesterol Biosynthesis happens in Liver
- Liver packages it into VLDL…. released into Blood…… Metabolized to LDL by Peripheral Tissues
- Dietary Cholesterol delivered to Liver via CHYLOMICRON REMNANTS
- Liver is Major source of NASCENT HDLs
- Helps to clear to BULK Lipoproteins from Blood
- Synthesizes Bile Acids and Eliminates CHOLESTEROL into Bile
Inhibitors of HMG CoA Reductase: STATINS
- Statins are Cholesterol Lowering Drugs
- Ex: Lovastatin, Simvastatin, Pravastatin, Atorvastatin
- Reduce levels Significantly (20 to 60%), prevent Cardiovascular Disease
- Strong Competitive inhibitors of HMG CoA Reductase (Rate Limiting Reaction in Cholesterol Biosynthesis)
- Km for HMG CoA 4 microM, Ki for Statins 5 to 45 nanoM
- HYPOCHOLESTEROLEMIC Action due to INCREASE in SREBP Maturation which leads to Transcription of LDL Receptor and subsequent enhanced clearance of Cholesterol via LDL-Recpetor Mediated Endocytosis
- Overproduction of HMG CoA Reductase Protein
- MYOTOXIC SIDE EFFECTS: Decreased formation of UBIQUINONE and PRENYLATED PROTEINS
Cytochrome P450 and Drug Interactions
- Cytochrome P450 (CYP) Enzymes convert LINEAR ISOPRENOID SQUALENE into CHOLESTEROL
- CYP also DETOXIFY XENOBIOTICS and Pharmacological Agents (Including Statins)
- Agents that Inhibit CYP will cause INCREASE in STATIN Levels leading to Toxic Side Effects (Myopathy and Rhabdomyolysis)
- Such Inhibitor include: Itraconozole, Clarithromycin, and Cyclosporin. Citrus Juices, Grapefruit Juice
- Agents that include CYP (Ex: Rifampicin, Carbamazepine and St John’s wort)- Decrease Levels of Statin in Plasma
Elimination of Cholesterol
- No Enzyme can degrade Sterane Ring of Cholesterol
- Cholesterol converted to BILE ACIDS and stored in Bile
- Some Cholesterol and Bile excreted in FECES
Bile Acids and Bile Salts
- Hepatic Cholesterol is a PRECURSOR of BILE ACIDS and BILE SALTS
- STRONG DETERGENTS: Amphipathic, which Polar and Non-polar Regions
- BILE made in Hepatocytes, stored and concentrated in GALLBLADDER
- Bile made of Bile Acids, Cholesterol, Phospholipids, Fatty Acids, Proteins, Bile Pigments, and Inorganic Salts
- Discharged into Duodenum in response to Food
- LIPID EMULSIFYING Mixture, AIDS in Lipid Digestion by forming Micelles which INCREASE Surface Are of Lipids exposed to LIPASES
Synthesis of Bile Acids
Cholesterol ————> 7 alpha Hydroxycholesterol
Enzyme: 7 Alpha Hydroxylase (Needs Cytochrome P450 F3+)
Synthesis of Bile Acids
1) Cholesterol —- (7Alpha Hydroxylase) —> 7 alpha Hydoxycholesterol
* **CLEAVAGE AT THE SIDE CHAIN!!!!**
2a) 7 Alpha Hydroxycholesterol —–> CHENOCHOLIC ACID (2 OH)
2b) 7 Alpha Hydroxycholesterol —–> CHOLIC ACID (3 OH)
Conjugation of Bile Acids
1) Cholic Acid —–> CHOLYL CoA (Pka 6)
2a) Cholyl CoA ——> Taurine
3a) Taurine ——-> TAUROCHOLIC ACID
2b) Cholyl CoA ——-> Glycine
3b) Glycine ——-> GLYCOCHOLIC ACID
Bile Acid vs Bile Salt
Bile Acid = COOH
Bile Salt = COO-
Mixed Micelle
- Contain BILE SALTS, Cholesterol, and Phospholipids
Bile Metabolism in the Liver, Gallbladder, and Small Intestine
- Starts out as Cholesterol in the Liver and gets converted to Bile that is then stored in the Gallbladder
Primary Bile Salts can be used for two things:
1) Primary Bile Salts are used in the DUODENUM to EMULSIFY DIETARY LIPIDS to aid in their Digestion and Absorption
2) Bateria DECONJUGATE and DEHYDROXYLATE Primary Bile Salts into Primary and Secondary Bile Acids, which are ABSORBED by the ILEUM and are then:
a) EXCRETED in Feces (5%)
b) Recycled in the Liver via Enterohepatic Circulation (95%)
Overview of Bile Salt Metabolism
1) Liver (Synthesizes 0.2 to 0.6 g.day and recycles > 95%). Secondary Bile Salts are Reconjugated
2) Bile Salts reabsorbed (12 to 32 g/day) and returned to Liver for Recycling. > 95% Efficiency
3) Pool of Bile Salts = 2 to 4 grams (Recycles 6 to 8 times/day). Bacteria in Gut DECONJUGATE and DEHYDROXYLATE Bile Salts
Gallstones
- Crystal made up of Bile SUPERSATURATED with CHOLESTEROL
- CHOLELITHIASIS: Insufficient Secretion of Bile Salts or Phospholipids or Excess Cholesterol Secretion
- Chronic Disturbance in Bile Salt Metabolism leads to MAL-ABSOPRTION Syndromes (Steatorrhea), and deficiency in Fat Soluble Vitamins
- Oral administration of URSODEOXYCHOLIC ACID (A Secondary Bile Acid) reduces Cholesterol Secretion into Bile. USED TO DISSOLVE SMALL/ MEDIUM SIZED STONES!!!!!!!!!!!!
Assessment of Liver Function
- Albumin
- Transaminases: ALT and AST
- Alkaline Phosphatase
- Lactate Dehydrogenase
- Gamma- Glutamyltransferase
- Urea (BUN)
- Ammonia
- Prothrombin Time (PT)
- Triglyceride Levels
- Cholesterol Levels
- Bilirubin Levels
