Structure and Main Functions
Bile acids are amphiphilic molecules possessing surfactant properties. Their chemical structure features a hydrophobic moiety—a massive steroid nucleus (cyclopentanoperhydrophenanthrene)—and a hydrophilic moiety represented by a side chain with a carboxyl or ionized group, along with hydroxyl radicals.
They perform three main functions in the body:
- Digestive: Facilitates lipid emulsification in the intestine. Acid molecules surround a fat droplet, embedding their hydrophobic regions inward while their hydrophilic regions face the aqueous phase. This lowers surface tension, prevents droplet coalescence, and makes fats accessible to enzymes.
- Solubilizing: Maintains cholesterol in a dissolved state within bile, preventing precipitation and gallstone formation.
- Excretory: Elimination of bile acids in feces (1.0–1.3 g/day) represents the final step of catabolism and the primary pathway for cholesterol removal from the body (accounting for over 90% of total losses).
Biosynthesis and Classification
Acid synthesis occurs within the endoplasmic reticulum membranes of hepatocytes. The process is catalyzed by hydroxylase enzymes (including cytochrome $P_{450}$), which require oxygen and the coenzyme NADPH + $H^+$. During chemical conversion, hydroxyl groups are added to the steroid nucleus (at the $7\alpha$ and $12\alpha$ positions), the side chain is shortened, and its terminal end is oxidized to a carboxyl group.
Based on their site of formation, bile acids are divided into two groups:
- Primary bile acids: Synthesized directly in the liver from cholesterol. The main representatives are cholic acid (three hydroxyl groups) and chenodeoxycholic acid (two hydroxyl groups).
- Secondary bile acids: Formed in the intestine from primary acids through the action of bacterial microflora, which remove the $7\alpha$-hydroxyl group. Cholic acid is converted into deoxycholic acid, and chenodeoxycholic acid yields lithocholic acid.
Conjugation and Regulation Mechanisms
Before entering bile, primary acids undergo conjugation. They are activated by coenzyme A (forming cholyl-CoA and chenodeoxycholyl-CoA) and subsequently linked to either the amino acid glycine or the cysteine derivative taurine. The glycine-to-taurine conjugate ratio is approximately 3:1. This step is vital: conjugation makes the molecules more polar, dramatically enhancing their emulsifying properties.
Synthesis is tightly regulated at the level of the rate-limiting enzyme, $7\alpha$-hydroxylase:
- Upregulation: The substrate cholesterol induces the enzyme. Thyroid hormones also exert a stimulatory effect.
- Downregulation: Bile acids themselves inhibit gene transcription via negative feedback. Estrogens act as corepressors.
Enterohepatic Circulation
The body continuously maintains a cycle of bile acids between the liver and the intestine. The release of bile stored in the gallbladder is stimulated by the hormone cholecystokinin, which is secreted in response to dietary fat intake.
After fulfilling their emulsifying role in the small intestine and being partially converted into secondary acids by bacteria, approximately 95% of the bile acid pool is reabsorbed in the ileum. Via the portal vein, they return to the liver, where they are reconjugated and resecreted. Lithocholic acid, being the most hydrophobic, is reabsorbed the least and rapidly exits the body with feces.