Cytochrome P450 7A1 (CYP7A1)

Cholesterol 7 alpha-hydroxylase (CYP7A1) is the rate-limiting enzyme in the synthesis of bile acid from cholesterol, catalyzing the formation of 7-alpha-hydroxycholesterol. CYP7A1 is a cytochrome P450 heme enzyme that oxidizes cholesterol using molecular oxygen.

It is downregulated by Sterol regulatory element binding proteins (SREBP) when plasma cholesterol levels are low. It is upregulated by the nuclear receptor LXR (liver X receptor) when cholesterol (specifically - oxysterol) levels are high. The effect of this upregulation is to increase the production of bile acids and reduce the level of cholesterol in hepatocytes.