VPB 112: General Veterinary Biochemistry (1+1)

BILE SALTS

• Cholesterol is not degraded to yield energy as there is no enzyme to degrade the ring structure of cholesterol.
• They are eliminated from the body mostly in the form of bile acids, in which the ring structure is intact.

Reactions

• The first step and the rate limiting step is the catabolism of cholesterol is the conversion of cholesterol to 7α-hydroxycholesterol by the enzyme 7-α-hydroxylase. This reaction needs NADPH,O₂ and Vit-C.
• In the subsequent step, the double bond is reduced; the side chain is oxidized and converted to a five carbon branch possessing a carboxyl group at the end, followed by hydroxylation to form two compounds. One has hydroxyl group at positions 3 and 7 and the other at positions 3,7 & 12, namely chenodeoxycholic acid and cholic acid respectively. These compounds are called primary bile acids ( they are produced in the liver).
• They are then conjugated to a molecule of glycine or taurine by an amide bond. These new structures are called as bile salts and they are
  • Glycocholic acid
  • Glycochenodeoxycholic acid
  • Taurocholic acid 
  • Taurochenodeoxycholic acid.
• The bile salts are then transported to the intestine via the bile.
• Bacteria in the intestine can remove glycine and taurine from the bile salts.
• Some of the primary acids are also converted into secondary bile acids by removing a hydroxyl group attached at carbon 7, producing deoxycholic acid from cholic acid and lithocholic acid from chenodeoxycholic acid.
• About 94% of the bile salts are reabsorbed and returns to liver by enterohepatic circulation and 6% is lost in the feces.
• Bile salts lower surface tension and are  required for digestion and absorption of lipids.