Structure, Evolution, and Liver-Specific Expression of Sterol 12 -Hydroxylase P450 (CYP8B)

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Abstract
The rat CYP8B cDNA encoding sterol 12a-hydroxylase was cloned and sequenced. The amino acid sequence of the heme-binding region of CYP8B was close to those of CYP7A (cholesterol 7a-hydroxylase) and CYP7B (oxysterol 7a-hydroxylase). Molecular phylo-genetic analysis suggests that CYP8B and the CYP7 family derive from a common ancestor. The P460s of the CYP7 and CYP8 families, except for CYP8A (prostacyclin synthase), catalyze the oxygenation of sterols from an alpha surface in the middle of the steroid skeleton. These facts suggest that CYP8B is a P450 closely linked to those of the CYP7 family. CYP8B was expressed specifically in liver. Hepatic CYP8B mRNA level and the 12a-hydroxylase activity were altered by cholestyramine feeding, starvation, streptozoto-cin-induced diabetes mellitus, and administration of clofibrate, dexamethasone or thyrox-in, indicating the pretranslational regulation of CYP8B expression. The enhanced CYP8B mRNA expression in streptozotocin-induced diabetic rats was significantly decreased by insulin within 3 h of its administration. These facts demonstrate a regulatory role of insulin in CYP8B expression as a suppressor.

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