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. 2021 Feb 16;11(2):292. doi: 10.3390/biom11020292

Figure 1.

Figure 1

Overview of bile acid metabolism. Bile acids (BAs) are synthesized via two major biosynthetic pathways. In the classic pathway, cholesterol is converted to 7α-hydroxycholesterol by the rate-limiting enzyme CYP7A1 (cholesterol 7 alpha-hydroxylase A1). 7α-hydroxycholesterol is ultimately converted into CA (cholic acid) by a sterol 12α-hydroxylase (CYP8B1) or CDCA (chenodeoxycholic acid) without 12α-hydroxylation by CYP8B1. In the alternative pathway, cholesterol is first converted to 27-hydroxycholesterol by CYP27A1 (cytochrome P450 Family 27 Subfamily A Member 1), which eventually is converted to CDCA. In the large intestine, bacterial 7α-dehydroxylase removes a hydroxyl group from C-7 and converts CA to DCA (deoxycholic acid) and CDCA to LCA (lithocholic acid). In mouse liver, most of CDCA is converted to α- and β-MCA. In the intestine, bacterial 7α-dehydroxylase activity converts CA and CDCA to DCA and LCA, respectively. CYP3A1 (cytochrome P450 3A1) and epimerase also convert CDCA to the secondary BAs, including THCA (taurohyocholic acid), TMDCA (tauromurideoxycholic acid), ω-MCA (ω-muricholic acid), THDCA (taurohyodeoxycholic acid), and TUDCA (tauroursodeoxycholic acid). LCA and ω-MCA are excreted into feces.