Chemical species of lipids in bile

Bile salts, phospholipids and sterols are the major lipid species in bile. The common bile salts possess a steroid nucleus of four fused hydrocarbon rings with polar hydroxyl functions and an aliphatic side chain conjugated in amide linkage with glycine or taurine. Since the ionized carboxylate or sulfonate group on the side chain renders bile salts water soluble, they are formally classified as soluble amphiphiles. The common bile salts differ in the number and orientation of the hydroxyl groups on the steroid nucleus. Bile salts synthesized from cholesterol in the liver are called primary bile salts, which in humans are cholate, with three hydroxyl groups, and chenodeoxycholate, with two hydroxyl groups. Secondary bile salts are created by the action of intestinal bacteria on primary bile salts; deoxycholate, with two hydroxyl groups, and lithocholate, with a single hydroxyl group, are formed from cholate and chenodeoxycholate, respectively. "Tertiary" bile salts are the result of modification of secondary bile salts by intestinal flora or hepatocytes; in humans these are the sulfate ester of lithocholate and ursodeoxycholate, the 7 beta-epimer of chenodeoxycholate. Lecithin (diacylphosphatidylcholine), the major phospholipid in bile, is an insoluble, swelling amphiphile with a hydrophilic, zwitterionic phosphocholine head group and hydrophobic tails comprised of two long fatty acyl chains. Biliary lecithin is derived from the least hydrophobic hepatic lecithins and typically contains a saturated C-16 acyl chain in the sn-1 position and an unsaturated C-18 or C-20 acyl chain in the sn-2 position. Cholesterol, present solely as the nonesterified free alcohol, accounts for more than 90% to 95% of the sterols in bile.(ABSTRACT TRUNCATED AT 250 WORDS)