Cholesterol Biology and Metabolism Quiz

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Questions and Answers

What effect do plant sterols have on dietary cholesterol absorption?

They increase absorption of dietary cholesterol.

They have no effect on cholesterol absorption.

They reduce the absorption of dietary cholesterol. (correct)

They completely block cholesterol absorption.

Where is most plasma cholesterol found?

In the cytosol as free cholesterol.

In free form within the cell membranes.

In the liver only.

In esterified form, as cholesteryl esters. (correct)

What is the source of carbon atoms in cholesterol synthesis?

Fatty acids.

Sodium bicarbonate.

Glucose.

Acetate. (correct)

Which tissue contributes the most to the body's cholesterol pool?

Liver. (D) Signup and view all the answers

What drives the endergonic reaction of cholesterol synthesis?

Hydrolysis of acetyl coenzyme A thioester bond. (A) Signup and view all the answers

How is HMG CoA reductase significant in cholesterol biosynthesis?

It is the rate-limiting enzyme for cholesterol biosynthesis. (C) Signup and view all the answers

Which of the following best describes cholesteryl esters?

They contain a fatty acid attached at carbon 3. (A) Signup and view all the answers

What are the essential functions of cholesterol in the body?

Precursor for bile acids and steroid hormones (A), Modulating fluidity of cell membranes (D) Signup and view all the answers

What is the impact of an imbalance in cholesterol regulation in the body?

Elevated levels of circulating plasma cholesterol. (C) Signup and view all the answers

How does cholesterol primarily enter the liver's cholesterol pool?

From both dietary sources and synthesis by extrahepatic tissues (D) Signup and view all the answers

What is a potential consequence of an imbalance in cholesterol influx and efflux?

Gradual deposition of cholesterol in tissues (C) Signup and view all the answers

What characterizes the structure of cholesterol?

It consists of four fused hydrocarbon rings and an eight-carbon branched chain (A) Signup and view all the answers

What happens to cholesterol that is eliminated from the liver?

It is excreted in unmodified form or as bile salts (D) Signup and view all the answers

What type of sterols are poorly absorbed by humans?

Plant sterols such as β-sitosterol (A) Signup and view all the answers

What component of blood is cholesterol sometimes part of?

Plasma lipoproteins (A) Signup and view all the answers

What causes the narrowing of blood vessels associated with atherosclerosis?

Gradual deposition of cholesterol leading to plaque formation (D) Signup and view all the answers

What is produced when a hydroxyl group is removed from cholic acid?

Deoxycholic acid (A) Signup and view all the answers

What is the primary site for the absorption of bile acids and salts?

Ileum (B) Signup and view all the answers

What percentage of bile salts secreted into the duodenum is typically lost in feces each day?

3% (B) Signup and view all the answers

What role do bile acid sequestrants like cholestyramine play in the treatment of hypercholesterolemia?

They promote bile acid excretion (B) Signup and view all the answers

In enterohepatic circulation, how are bile acids transported in the portal blood?

In a noncovalent complex with albumin (B) Signup and view all the answers

How much bile acid is synthesized from cholesterol in the liver daily to replace lost bile acids?

0.5 g (D) Signup and view all the answers

What is the primary function of plasma lipoproteins?

To form macromolecular complexes with lipids and proteins (C) Signup and view all the answers

What happens to bile acids when dietary fiber is consumed?

They are bound and excreted in higher amounts (B) Signup and view all the answers

Which lipoprotein is primarily responsible for transporting dietary lipids?

Chylomicrons (CM) (D) Signup and view all the answers

What mechanism do lipoproteins use to keep lipids soluble in plasma?

By providing an efficient transport system (D) Signup and view all the answers

Which of the following factors contributes to the narrowing of blood vessels in humans?

Gradual lipid deposition in tissues (A) Signup and view all the answers

What is a known side effect of cholesterol deposition in tissues?

Atherosclerosis (A) Signup and view all the answers

Which of the following statements is true regarding statins?

Atorvastatin and simvastatin are CYP3A4 substrates (A) Signup and view all the answers

Which statement correctly describes how cholesterol can be excreted from the liver?

Cholesterol can be secreted as bile salts or as unmodified cholesterol in bile (B) Signup and view all the answers

What role does cholesterol play in hormone production?

It serves as a precursor for steroid hormones (B) Signup and view all the answers

Which of the following lipoproteins is considered the most dense?

High-density lipoproteins (HDL) (A) Signup and view all the answers

Which hormones are associated with the up-regulation of HMG CoA reductase expression?

Insulin and thyroxine (A) Signup and view all the answers

What is the mechanism of action of statin drugs on HMG CoA reductase?

Competitive inhibition of HMG CoA reductase (C) Signup and view all the answers

How does cholesterol degradation primarily occur in humans?

Conversion to bile acids and bile salts (D) Signup and view all the answers

Which organic compounds are the most important components of bile?

Phosphatidylcholine and bile salts (B) Signup and view all the answers

Where is bile produced before it is transported to the intestine?

Liver (A) Signup and view all the answers

What occurs to bile salts in the intestine?

Glycine and taurine are removed, regenerating bile acids (A) Signup and view all the answers

Which effect do glucagon and glucocorticoids have on HMG CoA reductase expression?

They inhibit expression (D) Signup and view all the answers

How is cholesterol primarily eliminated from the body?

Secretion into the bile for intestinal elimination (D) Signup and view all the answers

Which method is NOT a process for eliminating cholesterol from the body?

Reabsorbed into the liver (D) Signup and view all the answers

Between how many grams and how many grams of bile salts are secreted from the liver into the duodenum daily?

0.5, 30 (D) Signup and view all the answers

What is the structural difference between cholic acid and chenodeoxycholic acid?

Hydroxyl group removal at C-4 (A) Signup and view all the answers

Which of the following is NOT a lipoprotein particle in the human body?

Bile acids (D) Signup and view all the answers

What is the generic name for Altoprev 20 mg?

Lovastatin (A) Signup and view all the answers

Which statin should be taken with the evening meal?

Lovastatin immediate-release (B) Signup and view all the answers

What is the correct mechanism of action of cholestyramine?

It binds to bile acids in the gut. (A) Signup and view all the answers

What is the correct mechanism of action of statins?

Statins inhibit the enzyme HMG-CoA reductase. (D) Signup and view all the answers

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.