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Questions and Answers
What effect do plant sterols have on dietary cholesterol absorption?
What effect do plant sterols have on dietary cholesterol absorption?
Where is most plasma cholesterol found?
Where is most plasma cholesterol found?
What is the source of carbon atoms in cholesterol synthesis?
What is the source of carbon atoms in cholesterol synthesis?
Which tissue contributes the most to the body's cholesterol pool?
Which tissue contributes the most to the body's cholesterol pool?
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What drives the endergonic reaction of cholesterol synthesis?
What drives the endergonic reaction of cholesterol synthesis?
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How is HMG CoA reductase significant in cholesterol biosynthesis?
How is HMG CoA reductase significant in cholesterol biosynthesis?
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Which of the following best describes cholesteryl esters?
Which of the following best describes cholesteryl esters?
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What are the essential functions of cholesterol in the body?
What are the essential functions of cholesterol in the body?
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What is the impact of an imbalance in cholesterol regulation in the body?
What is the impact of an imbalance in cholesterol regulation in the body?
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How does cholesterol primarily enter the liver's cholesterol pool?
How does cholesterol primarily enter the liver's cholesterol pool?
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What is a potential consequence of an imbalance in cholesterol influx and efflux?
What is a potential consequence of an imbalance in cholesterol influx and efflux?
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What characterizes the structure of cholesterol?
What characterizes the structure of cholesterol?
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What happens to cholesterol that is eliminated from the liver?
What happens to cholesterol that is eliminated from the liver?
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What type of sterols are poorly absorbed by humans?
What type of sterols are poorly absorbed by humans?
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What component of blood is cholesterol sometimes part of?
What component of blood is cholesterol sometimes part of?
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What causes the narrowing of blood vessels associated with atherosclerosis?
What causes the narrowing of blood vessels associated with atherosclerosis?
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What is produced when a hydroxyl group is removed from cholic acid?
What is produced when a hydroxyl group is removed from cholic acid?
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What is the primary site for the absorption of bile acids and salts?
What is the primary site for the absorption of bile acids and salts?
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What percentage of bile salts secreted into the duodenum is typically lost in feces each day?
What percentage of bile salts secreted into the duodenum is typically lost in feces each day?
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What role do bile acid sequestrants like cholestyramine play in the treatment of hypercholesterolemia?
What role do bile acid sequestrants like cholestyramine play in the treatment of hypercholesterolemia?
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In enterohepatic circulation, how are bile acids transported in the portal blood?
In enterohepatic circulation, how are bile acids transported in the portal blood?
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How much bile acid is synthesized from cholesterol in the liver daily to replace lost bile acids?
How much bile acid is synthesized from cholesterol in the liver daily to replace lost bile acids?
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What is the primary function of plasma lipoproteins?
What is the primary function of plasma lipoproteins?
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What happens to bile acids when dietary fiber is consumed?
What happens to bile acids when dietary fiber is consumed?
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Which lipoprotein is primarily responsible for transporting dietary lipids?
Which lipoprotein is primarily responsible for transporting dietary lipids?
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What mechanism do lipoproteins use to keep lipids soluble in plasma?
What mechanism do lipoproteins use to keep lipids soluble in plasma?
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Which of the following factors contributes to the narrowing of blood vessels in humans?
Which of the following factors contributes to the narrowing of blood vessels in humans?
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What is a known side effect of cholesterol deposition in tissues?
What is a known side effect of cholesterol deposition in tissues?
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Which of the following statements is true regarding statins?
Which of the following statements is true regarding statins?
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Which statement correctly describes how cholesterol can be excreted from the liver?
Which statement correctly describes how cholesterol can be excreted from the liver?
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What role does cholesterol play in hormone production?
What role does cholesterol play in hormone production?
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Which of the following lipoproteins is considered the most dense?
Which of the following lipoproteins is considered the most dense?
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Which hormones are associated with the up-regulation of HMG CoA reductase expression?
Which hormones are associated with the up-regulation of HMG CoA reductase expression?
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What is the mechanism of action of statin drugs on HMG CoA reductase?
What is the mechanism of action of statin drugs on HMG CoA reductase?
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How does cholesterol degradation primarily occur in humans?
How does cholesterol degradation primarily occur in humans?
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Which organic compounds are the most important components of bile?
Which organic compounds are the most important components of bile?
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Where is bile produced before it is transported to the intestine?
Where is bile produced before it is transported to the intestine?
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What occurs to bile salts in the intestine?
What occurs to bile salts in the intestine?
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Which effect do glucagon and glucocorticoids have on HMG CoA reductase expression?
Which effect do glucagon and glucocorticoids have on HMG CoA reductase expression?
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How is cholesterol primarily eliminated from the body?
How is cholesterol primarily eliminated from the body?
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Which method is NOT a process for eliminating cholesterol from the body?
Which method is NOT a process for eliminating cholesterol from the body?
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Between how many grams and how many grams of bile salts are secreted from the liver into the duodenum daily?
Between how many grams and how many grams of bile salts are secreted from the liver into the duodenum daily?
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What is the structural difference between cholic acid and chenodeoxycholic acid?
What is the structural difference between cholic acid and chenodeoxycholic acid?
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Which of the following is NOT a lipoprotein particle in the human body?
Which of the following is NOT a lipoprotein particle in the human body?
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What is the generic name for Altoprev 20 mg?
What is the generic name for Altoprev 20 mg?
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Which statin should be taken with the evening meal?
Which statin should be taken with the evening meal?
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What is the correct mechanism of action of cholestyramine?
What is the correct mechanism of action of cholestyramine?
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What is the correct mechanism of action of statins?
What is the correct mechanism of action of statins?
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Study Notes
Lipid Metabolism III
- Learning Objectives: Describe and apply lipids metabolism part III-Part I (cholesterol)
- Cholesterol and Steroid Metabolism: Cholesterol is a crucial steroid alcohol in animal tissues, essential for cell membranes, bile acids, steroid hormones, and vitamin D.
- Liver's Role: The liver regulates cholesterol homeostasis, absorbing dietary cholesterol and synthesizing its own. Cholesterol is also eliminated as unmodified cholesterol in bile or converted to bile salts, and can also be a component of plasma lipoproteins.
- Cholesterol Influx and Efflux: The balance between cholesterol influx and efflux is usually imprecise, leading to gradual cholesterol deposition in tissues, especially endothelial linings of blood vessels. This process, atherosclerosis, causes narrowing of blood vessels, increasing the risk of cardiovascular, cerebrovascular, and peripheral vascular diseases.
- Major Sources of Liver Cholesterol: Dietary cholesterol, cholesterol from extrahepatic tissues (chylomicron remnants), and de novo synthesis in the liver contribute to the liver's cholesterol pool.
- Routes of Cholesterol Leaving the Liver: Free cholesterol secretion into the bile, conversion to bile acids (and bile salts), and secretion of VLDL are major pathways by which cholesterol leaves the liver.
Structure of Cholesterol
- Hydrophobic Compound: Cholesterol is a hydrophobic compound comprised of four fused hydrocarbon rings and an eight-carbon branched hydrocarbon chain.
- Steroids: Steroids with eight or ten carbon atoms and a hydroxyl group at carbon 3 are designated as sterols. Cholesterol is a key sterol in animal tissues.
- Plant Sterols: Plant sterols, such as β-sitosterol, are not absorbed as efficiently as cholesterol.
Cholesterol Esters
- Esterified Form: Most plasma cholesterol is in an esterified form (with fatty acid at carbon 3), making it more hydrophobic than free cholesterol.
- Membrane Absence: Cholesteryl esters are not found in membranes and are generally present in low levels in most cells.
Synthesis of Cholesterol
- Tissue Contributions: Although all tissues synthesize cholesterol, the liver, intestine, adrenal cortex, and reproductive tissues (ovaries, testes, and placenta) are the major contributors.
- Carbon Source: Carbon atoms in cholesterol come from acetate and the reducing equivalents are provided by NADPH.
- Endergonic Pathway: The synthesis pathway (requiring energy) involves hydrolysis of high-energy bonds of acetyl CoA and ATP.
- ER Involvement: Cholesterol synthesis requires enzymes located in both the cytosol and the membranes of the smooth endoplasmic reticulum (ER).
- Regulatory Mechanisms: The synthesis rate is responsive to cholesterol concentration, with regulatory systems balancing synthesis with excretion to maintain levels.
Inhibition by Drugs (Statins)
- Structural Analogs: Statin drugs are structural analogs of HMG-CoA and are (or are metabolized to) reversible, competitive inhibitors of HMG-CoA reductase.
- Inhibiting Synthesis: Statins inhibit HMG-CoA reductase, which stops the conversion of HMG-CoA to mevalonate (a key step in cholesterol synthesis).
Degradation of Cholesterol
- Non-Metabolism: The ring structure of cholesterol isn't broken down to CO2 and H2O in humans.
- Bile Acid and Salts: The liver converts cholesterol into bile acids and bile salts to be excreted in feces.
- Elimination via Bile: Cholesterol is eliminated via secretion in the bile, which transports it to the intestine.
Enterohepatic Circulation
- Reabsorption and Reuse: Bile salts secreted into the intestine are reabsorbed (over 95%), reused, and continuously circulated.
- Conversion to Bile Salts: The liver converts primary and secondary bile acids to bile salts through conjugation with glycine or taurine.
- Ileal Absorption: Bile acids and bile salts are absorbed in the ileum via a sodium-dependent cotransporter.
- Hepatocyte Uptake: The hepatocytes take up these metabolites through a cotransporter.
- Enterohepatic Circulation Summary: Secretion of bile salts into bile, passage through the duodenum, uptake in the ileum, and return to the liver comprise the enterohepatic circulation.
- Significant Excretion Differences Although continuous secretion and absorption are involved, daily loss in feces is usually minimal compared to the circulated amounts.
- Synthesis of New Bile Acids: A small amount (less than 3%) of bile acids is lost daily in the feces and is replaced by cholesterol synthesis in the liver.
Other Drugs and Therapies
- Additional Therapies Various non-statin drugs may be used when statins are not effective or cause adverse effects. These include ezetimibe, PCSK9 monoclonal antibodies (mAbs), bempedoic acid, and inclisiran.
- Fish Oils: Fish oils like icosapent ethyl (Vascepa) are used to lower triglycerides, in conjunction with a statin for some patients, but their mechanism is not fully understood.
Quiz Questions for Lipid Metabolism
- Multiple Choice Questions Various questions covering the mechanism, components, and related topics of lipid metabolism are presented.
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Description
Test your knowledge on cholesterol's role in the body, its synthesis, absorption, and regulation. This quiz covers various aspects, including the functions of cholesterol, its metabolism in the liver, and the implications of cholesterol imbalance. Ideal for students studying biochemistry or related fields.