The synthesis of essential 7α-hydroxylated bile acids in the liver is mediated by two
pathways that involve distinct 7α-hydroxylases. One pathway is initiated in the endoplasmic
reticulum by cholesterol 7α-hydroxylase, a well studied cytochrome P450 enzyme. A second
pathway is initiated by a less well defined oxysterol 7α-hydroxylase. Here, we show that a
mouse hepatic oxysterol 7α-hydroxylase is encoded by Cyp7b1, a cytochrome P450 cDNA
originally isolated from the hippocampus. Expression of a Cyp7b1 cDNA in cultured cells …