Figure 1.

Representative bile salts and their structures. (A) Simplified version of the bile salt synthetic pathway showing the three major classes of compounds that can serve as primary bile salts (C27 bile alcohols, C27 bile acids, and C24 bile acids). (B) All bile salts are derived from cholesterol (topmost structure), illustrated with the carbon atoms numbered and the steroid rings labelled A, B, C, and D. Iguanian lizards utilize 5α C24 bile acids such as 5α-cholic acid (allocholic acid) that have an A/B ring juncture that is trans, resulting in an overall planar and extended structure of the steroid rings (see representation of A, B, and C rings on the right side). Crocodylians utilize trihydroxy 5β C27 bile acids with hydroxylation at C-12 in addition to the default 3α, 7α-dihydroxylation. One of the two most common primary salts in mammals is chenodeoxycholic acid (CDCA), the stem C24 bile acid that has the default default 3α, 7α-dihydroxylation.

Hagey et al. BMC Evolutionary Biology 2010 10:133   doi:10.1186/1471-2148-10-133
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