Lecture 15: Liver Biochemistry Flashcards
Liver recieves blood supply from 2 sources, what are they, and what is the major source?
1) 75% from portal vein
2) 25% from hepatic artery
List the important functions of the liver
1) CHO metabolis
2) Lipid metabolism
3) Nucleotide biosynthesis
4) AA metabolism, ammonia and urea cycle
5) Synthesis of blood proteins
6) Bilirubin metabolism
7) Waste management - inactivation and detox of metabolites and xenobiotics
What is the major role of the liver?
Monitoring, synthesizing, recycling, distributing, and modifying metabolites
What is unique about the circulation of the liver; why is it important?
- Liver receives blood from enteric circulation (via portal vein) and from periphery (via hepatic artery)
- Low portal blood pressure allows for hepatocytes and endothelial cells to have increased contact w/ the blood
What are some of the structural features of the liver that facilitate its function?
1) Low portal blood pressure
2) Lack of basement membrane and tight junctions
3) Gaps between endothelial cells
4) Fenestrations (pores) in endothelial cells
List the important functional characteristics of Hepatocytes?
- Well developed plasma membrane w/ endocytic and exocytic system
- Well developed ER (both rough and smooth)
- Metabolically very active
- Large # of endosomes, mitochondria, and lysosomes
What is needed to generate isopentenyl pyrophosphate (IPP)?
Three acetyl CoA molecules make a 5 carbon IPP
Why is IPP important?
Serves as a building block for the synthesis of all isoprenoids, including steroids, lipid soluble vitamins (ADEK), and prenyl groups that attach proteins to the plasma membrane
What are the sources of acetyl CoA; where is it generated?
- Generated in the mitochondria
- Oxidative decarboxylation of pyruvate
- Beta-oxidation of FA’s
- Catabolism of several AA’s
How is the acetyl CoA transported in the cytoplasm?
The Citrate Shuttle
How is a sterane made and its functional importance?
- 6 units of IPP form a tetracyclic (4-ring) sterane
- Backbone of most steroids
What is the structure of Cholesterol?
- Allicyclic compound made of 4 fused rings
- A sterane ring + hydrocaron chain
- One hydroxyl group at C3
Why is cholesterol important?
- Component of plasma membranes and precurso of biologically active compounds:
- Bile acids and bile salts
- Vit D
- Steroid hormones (progesterone, aldosteone, cortisol, testosterone…)
How is the biosynthesis of cholesterol related to dietary intake?
Biosynthesis is inversely proportional to dietary intake
What is the equation for the cholesterol synthesis pathway?
18 Acetyl CoA + 18 ATP + 16 NADPH + 16H+ + 4O2 —> Cholesterol + 16 NADP+ + 18 ADP + 18Pi
*Just focus on the fact that it takes a lot of energy to synthesize cholesterol
What is phase I of cholesterol synthesis (key steps and enzymes)?
- Acetyl Coa —> Acetoacetyl CoA
- Acetoacetyl CoA —(HMG CoA synthetase) –> HMG CoA
- HMG CoA —(HMG CoA reductase)–> Mevalonate
- Mevalonate —> IPP
What is the rate limiting enzyme of phase I of cholesterol synthesis; what are its activators and inhibitors?
- Hydroxymethylglutaryl CoA reductase
Activators: Insulin and Thyroxine
Inhibitors: Glucagon, sterols, high [AMP], Vit E, and statins
What are the rxns of Phase II of cholesterol synthesis?
- 6 IPP’s to Squalene to Lanosterol to Cholesterol
*Remember I-S-L-C (I Surely Love Cholesterol!)
What are Antimycotics (i.e., miconazole, ketoconazole) and why are they relevant to cholesterol synthesis
- Used to treat fungal infections (anti-fungals) by inhibiting formation of ergosterol (needed to maintain plasma membrane of fungal cells
- At high concentrations they block the enzymes that catalyze the rxn converting Lanosterol –> Cholesterol
What are some of the important side products generated during the synthesis of cholestrol?
- Prenylated proteins (i.e., Ras)
- Heme A
- Dolichol
- Ubiquinone (CoQ10)
What are statins and how do they work?
- Cholesterol lowering drugs
- Strong Competitive inhibitors of HMG CoA reductase
- Increase in SREBP maturation which leads to transcription of LDL receptor and subsequent ehanced clearance of cholesterol via LDL-receptor mediated endocytosis
What are some of the myotoxic side effects of statins?
Decreased formation of ubiquinon (CoQ10) and prenylated proteins
What is the fate of cholesterol once its made and what happens specifically in the liver?
- Packaged into VLDL and released into the blood (CP1)
- In liver: cholesterol used to synthesize bile acids
How is cholesterol synthesis regulated?
- Direct inhibition - by FFA’s, sterols, and statins
- Covalent modification - inactive in phosphorylated form, active in dephospho form. Mediated by energy status and hormones such as insulin, thyroxine, and glucagon
- Transcriptional control - binding of transcription factor to promoter (sterol response element (SRE)) on the gene alters mRNA level
- Translational control - protein synthesis level
- Post-translational control - protein turnover/degradation
How do antiestrogens (triparanol, progesterone, tamoxifen) inhbit the formation of cholesterol?
Prevent the conversion of desmosterol to cholesterol
How do Antiepileptic drugs (U18666A) inhibit cholesterol synthesis?
Inhibits the conversion of squalene to lanosterol AND impairs cholesterol trafficking
How does Ketoconazole affect cholesterol?
Inhibits 7-alpha hydroxylase that converts cholesterol to bile acids
Which enzymes can degrade the sterane ring of cholesterol?
- NO enzymes can
- Cholestrol is converted to bile acids and stored in bile
What is the function of bile?
Lipid emulsifying mixture, that aids in lipid digestion by fomring micelles which increase surface area of lipids thus exposing them to lipases
Bile acids and bile salts are made from; act as?
- Made from hepatic cholesterol
- Strong detergents - amphipathic, w/ polar and non-polar regions
Explain the synthesis of bile salts from cholesterol?
- Cholesterol is converted to 7α-Hydroxycholesterol by 7α-Hydroxylase, which adds another hydroxyl group to C-7
- Side chain is cut and a COOH group is added giving you Chenodeoxycholic acid (bild acid)
- If a third -OH group is added you get Cholic acid (bile acid)
How are bile acids conjugated, describe both steps.
- Bile acid is made active by adding CoA, which primes the molecule and gives you Cholyl CoA
- The CoA is removed and either a glycine or taurine is added
Why are bile acids conjugated; which conjugated bile acid is most likely to be ionized?
- Significantly lowers the pKa, which increases the chance the molecule will be in an ionized form, making it a MORE efficient detergent
- Taurocholic acid has the lowest pKa = 2, so has greatest chance of being ionized.
What is the difference between a bile acid and bile salt?
- Acid = Protonated form
- Salt = De-protonated form
What are the 2 fates of bile once released into the duodenum?
- Aid in the digestion and absorption of lipids
- Are metabolized by intestinal bacteria which deconjugate and dehydoxylate them into secondary bile acids, which are absorbed by the ileum and then..
- Excreted in feces (5%)
- Recycled to the liver via enterohepatic circulation (95%)
What are gallstones, and what do they lead to?
- Cyrstals made up of bile supersaturated w/ cholesterol
- Cholelithiasis: insufficient secretion of biles salts or phospholipids into gall bladder or excess cholesterol secretion into bile
What do chronic disturbances in bile salt metabolism lead to?
Malabsorption syndrome (steatorrhea), and deficiency in fat soluble vitamins (A, D, E, K)
What is commonly used to dissolve small/medium sized gallstones?
Oral administration of Ursodeoxycholic acid (secondary bile acid) reduces cholesterol secretion into bile
What do non-absorbable bile-acid binding resins such as cholestyramine cause?
- Large increase in the excretion of bile acids
- As a result, rate of bile acid synthesis is stimulated by the induction of cholesterol 7α-hydroxylase.
- Depletion of the liver pool leads to an increase in hepatic uptake of LDL cholesterol, which lowers plasma cholesterol levels
Liver is the primary site for the degradation of what 2 things?
- Metabolites - compounds made in body
- Xenobiotics - compounds ingested from outside w/ no nutritional value and potentially toxic (i.e drugs and food additives)
Explain the 2 phased of Xenobiotic detoxification?
Phase I - reduction, oxidation, hydroxylation or hydrolysis of xenobiotic to a more polar primary metabolite
Phase II - primary metabolite is conjugated, sulfated, methylated, or glucuronidated to a secondary metabolite suitable for excretion
What are Cytochrome P450 enzymes (co-localize with, metabolize what compounds)?
- Superfamily of proteins containing Heme
- Co-localize w/ cytochrome P450 reductase
- Active in the metabolism of multiple hydrophobic compounds (i.e., steroids, eicodanoids, Vit D, and xenobiotics
- Operate via an electron transfer system
What are the cytochrome P450 enzymes inducible by and what are the CYP’s responsible for drug metabolism?
- Inducible by their substrate
- CYP1, CYP2, and CYP 3
What is the rxn sequence of Cytochrome P450?
- P450 w/ Fe3+ attaches to drug and working with CYP450 reductase, utilizes a pair of electrons from NADPH to incorporate one atom of oxygen into a substrate (drug), while reducing the second oxygen atom to water.
- The drug is now more polar and ready to be conjugated into secondary metabolite suitable for excretion
How do CYP’s affect pharmacological agents such as statins; what occurs if an agent inhibits CYP?
- Detoxify and increase metabolism of pharmacological agents
- Inhibiting CYP will cause increase in statin levels
- May lead to toxic side effects (myopathy and rhabdomyolysis)
What are some of the CYP inhibitors?
Itraconozole, clarithromyocin, cyclosporine, and citrus juices like grapefruit juice
What are some of the agents that induce CYP?
- Rifampicin, carbamazepine and St. John’s Wort
- Decrease in statin levels in plasma
How do diseases of the liver affect hepatocytes?
Impair the free exchange of material between hepatocytes and blood
What compounds are used in the assessment of liver function?
- Albumin (made in liver, pt’s w/ liver disease will have edema of limbs)
- Transaminases: ALT and AST (should be in liver, not in plasma)
- Alkaline Phosphatase
- Lactate dehydrogenase
- Urea (BUN)
- Ammonia
- Prothrombin Time (PT)
- TAG levels
- Cholesterol levels (total and VLDL, LDL, and HDL)
- Bilirubin level (unconjugated and conjugated)
- Serum glucose (measures ability of liver to make glucose via gluconeogenesis)
What is one way that alcohol has a toxic effect on the liver in regards to the detoxification of substances?
Accelerates the detoxification of drugs such as acetaminophen, and when taken in high doses the acetaminophen will be metabolized to a hepatotoxic metabolite
