Figure 1.

Schematic representation of the classic, alternative, and neural cholesterol clearance pathways of bile acid synthesis. The classic pathway of bile acid synthesis is initiated from cholesterol by CYP7A1, which leads to the formation of 7α-hydroxycholesterol which, after some metabolic reactions, interacts with CYP8B1 or CYP27A1 to form cholic acid or chenodeoxycholic acid. The alternative pathway of bile acid synthesis begins with the metabolism of cholesterol to 27-hydroxycholesterol via CYP27A1. 27-Hydroxysterol is subsequently metabolized by CYP7B1 which, after a few metabolic steps leads to the synthesis of CDCA. The final pathway depicted is the neural cholesterol clearance pathway, which is initiated by the formation of 24(S)-hydroxycholesterol from cholesterol by CYP46A1. 24(S)-hydroxycholesterol subsequently exits the brain and enters the bloodstream where it is metabolized by CYP39A1 in the liver to continue bile acid synthesis.