Species differences in bile acid synthesis. (IMAGE)
Caption
(A) The classic bile acid synthesis pathway: cholesterol 7α-hydroxylase (CYP7A1) converts cholesterol to 7α-hydroxycholesterol (7α-HOC). The sterol 12α-hydroxylase (CYP8B1) converts the intermediate 7α-hydroxy-4 cholesten-3-one (C4) to 7α, 12α-dihydroxy-4-cholesten-3-one, eventually leading to synthesis of cholic acid (CA). C4 can also be eventually converted to chenodeoxycholic acid (CDCA). The mitochondrial sterol 27-hydroxylase (CYP27A1) catalyses the steroid side-chain oxidation of CA and CDCA. The alternative bile acid synthesis pathway: CYP27A1 converts cholesterol to 27-hydroxycholesterol (27-HOC), which mainly leads to the synthesis of CDCA. In mouse liver, CYP2C70 converts CDCA to α-MCA, which can be epimerized to β-MCA. In small and large intestine, bacterial bile salt hydrolase (BSH) deconjugates bile acids. Bacterial 7-dehydroxylase dehydroxylates CA to produce deoxycholic acid (DCA) and CDCA to produce lithocholic acid (LCA). Bacterial enzymes also produce secondary bile acids, including ω-muricholic acid (ω-MCA) and ursodeoxycholic acid (UDCA). (B–D) Structure of primary and secondary bile acids.
Credit
By Tiangang Li, Mohammad Nazmul Hasan, Lijie Gu.
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CC BY-NC