Illustration of 5α bile salts. The 5α series of C27 bile alcohol, C27 bile acid, and C24 bile acid structures are illustrated with examples of animals (if any) whose major bile salts are in these classes. The structures are the stem structures for each of the three major classes of bile salts. Multiple enzymes are involved in the transition indicated by arrow A. The enzymes responsible for 5α-reduction of bile salt precursors are currently unknown. If 5α-bile salt pathways are homologous to 5β pathways, the reaction indicated by arrow B would be carried out by CYP27A1. Side-chain cleavage (arrow C) is likely a peroxisomal reaction in most or all animals. Arrow D covers all the possible additional modifications to stem bile salt structures (e.g., additional hydroxylation of the nucleus or side-chain, introduction of oxo groups, unsaturation of the side-chain, etc.).
Hagey et al. BMC Evolutionary Biology 2010 10:133 doi:10.1186/1471-2148-10-133