35. Conversion of Cholesterol to Specialised Products: Bile Acids. Cholesterol Excretion. Enterohepatic Circulation of Bile Acids. Clinical Aspects of Cholesterol Metabolism.
Explains how the liver converts cholesterol into cholic and chenodeoxycholic acids, the rate-limiting role of 7α-hydroxylase, and how bile acids are conjugated into bile salts that emulsify fats. Covers cholesterol excretion as free cholesterol and bile acids, including secondary bile acids formed by gut bacteria, then the enterohepatic circulation and how interrupting it lowers blood cholesterol. Closes with clinical links to gallstones, liver disease, ileal disease and hypercholesterolaemia, with its treatments.
Topic 35 of 76 in the MU Pleven syllabus for Biochemistry (Medicine, Year 2). The free AceMed note covers it in 6 sections, about 337 words (3 min of reading), with 10 flashcards.
In this note
Conversion of Cholesterol to Specialised Products: Bile Acids. Cholesterol Excretion.
Conversion of Cholesterol to Specialized Products
Bile Acids and Bile Salts
Cholesterol Excretion
Enterohepatic Circulation
Clinical Aspects of Cholesterol and Bile Acid Metabolism
Conversion of Cholesterol to Specialized Products
1. Overview
Cholesterol is a precursor for:
Bile acids and bile salts
Steroid hormones
Vitamin D
Cell membrane cholesterol
The liver is the main site for conversion of cholesterol into bile acids, which is the primary route for cholesterol elimination (since cholesterol cannot be degraded to CO₂ and H₂O).
Bile Acids and Bile Salts
2. Synthesis of Bile Acids (in the liver)
Cholesterol → Cholic acid and Chenodeoxycholic acid (primary bile acids).
Requires:
Oxygen
NADPH
Cytochrome P450 enzymes
Rate-limiting enzyme:
7α-hydroxylase
Inhibited by high bile acid levels (negative feedback).
3. Conversion to Bile Salts
Primary bile acids are conjugated with:
Glycine or Taurine
Forms bile salts, which are:
More water-soluble
More effective detergents
4. Function of Bile Salts
Emulsify dietary fats → increases surface area for lipase action.
Essential for digestion and absorption of:
Lipids
Fat-soluble vitamins (A, D, E, K)
Prevent cholesterol precipitation in bile.
Cholesterol Excretion
5. Only Significant Way to Remove Cholesterol
Cholesterol is eliminated as:
Free cholesterol in bile
Bile acids
Some bile acids are converted by intestinal bacteria to secondary bile acids (e.g., deoxycholic acid, lithocholic acid).
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