Structure and origin of bile acids: An overview

An overview of the structure and the origin of naturally occurring bile acids is given. Most naturally occurring bile acids belong to the 5β-series, with hydroxyl groups in the A, B, and C ring of the steroid system. Hydroxyl groups are mostly found at the C3, C6, C7, C12 and C23 positions and are α- rather than β-oriented. In most bile acids, the A/B ring junction iscis (5β-series). However, the A ring can be usually present in the more stable (chair) or less stable (boat) conformation. Both B/C and C/D ring junction aretrans. With respect to the angular C9-methyl group, the hydrogen atoms at C5 and C8 are m-oriented whereas those at C9 and C14 aretrans-oriented. The archetypal bile acid is 5β-cholanic acid (3) from which all other CM bile acids can be derived. In addition to the bile acids with 24 carbons, some naturally occurring C27 bile acids have been identified including di-, tri- and tetra-hydroxy derivatives of coprostanic acid isolated from bile of several reptile species. The most dominant bile acids and their natural sources are given and a selection of naturally occurring bile acids with unusual structures which have been mostly isolated from the bile of reptiles and amphibians is described.