FIGURE 8: Biosynthesis of bile acids. Bile acids are synthesized in liver through two pathways: the classic pathway and the alternative pathway. CYP7A1 initializes the classic pathway, while CYP27A1 initializes the alternative pathway. CYP8B1 regulates the CA synthesis in the classic pathway, and is the only enzyme catalyzing the CA synthesis. In the intestine, primary bile acid CA and CDCA are metabolized by the gut microbiota to produce the secondary bile acids, DCA and LCA, respectively.