FIGURE 1.

Classic and alternative pathways of bile acids synthesis. In human liver, two primary bile acids, cholic acid (CA), and chenodeoxycholic acid (CDCA), are produced. Key regulatory enzymes in both pathways are shown. Microsomal cholesterol 7α-hydroxylase (CYP7A1) catalyzes the first the rate-limiting step in the classic pathway converting cholesterol into 7-hydroxycholesterol, whereas the mitochondrial sterol 27-hydroxylase (CYP27A1) initiates the alternative pathway by converting cholesterol into 27-hydroxycholesterol, which is then 7-hydroxylated by microsomal oxysterol 7-hydroxylase (CYP7B1). CYP8B1 regulates the cholic acid synthesis in the classic pathway.