Structure, evolution, and liver-specific expression of sterol 12 alpha-hydroxylase P450 (CYP8B)

The rat CYP8B cDNA encoding sterol 12 alpha-hydroxylase was cloned and sequ enced. The amino acid sequence of the heme-binding region of CYP8B was clos e to those of CYP7A (cholesterol 7 alpha-hydroxylase) and CYP7B (oxysterol 7 alpha-hydroxylase), Molecular phylogenetic analysis suggests that CYP8B a nd the CYP7 family derive from a common ancestor. The P450s of the CYP7 and CYP8 families, except for CYP8A (prostacyclin synthase), catalyze the oxyg enation of sterols from an alpha surface in the middle of the steroid skele ton. These facts suggest that CYP8B is a P450 closely linked to those of th e CYP7 family. CYP8B was expressed specifically in liver. Hepatic CYP8B mRN A level and the 12 alpha-hydroxylase activity were altered by cholestyramin e feeding, starvation, streptozotocin-induced diabetes mellitus, and admini stration of clofibrate, dexamethasone or thyroxin, indicating the pretransl ational regulation of CYP8B expression. The enhanced CYP8B mRNA expression in streptozotocin-induced diabetic rats was significantly decreased by insu lin within 3 h of its administration. These facts demonstrate a regulatory role of insulin in CYP8B expression as a suppressor.