Figure 2.

Enterohepatic circulation of BAs. BAs are synthesized from cholesterol in the liver through classic and alternative pathways. The classic pathway includes a series of reactions to produce diols and triols, which are carboxylated by mitochondrial CYP27A1 to produce the primary BAs CA and CDCA. These two BAs combine with taurine or glycine to form primary conjugated BAs, which are then secreted from the biliary tract into the intestinal cavity and metabolized by the intestinal flora to form secondary BAs (deoxycholic acid and lithocholic acid). Approximately 95% of BAs are reabsorbed in the terminal ileum and ascending colon, and approximately 5% are excreted in the feces. The alternative pathway includes a series of enzymatic reactions to form 5-cholesten-3β,7α,27-triol. When 5-cholesten-3β,7α-diol production disorders or secondary BA excretion disorders occur, they may lead to coronary heart disease, hypertension and other cardiovascular diseases. BAs, bile acids; CA, cholic acid; CDCA, chenodeoxycholic acid.