Liver mitochondrial P450 involved in cholesterol catabolism and vitamin D activation.

The isolation, purification, and cloning of the mitochondrial P450 enzyme catalyzing not only the 27-hydroxylation of 5 beta-cholestane-3 alpha, 7 alpha-diol and cholestane-3 alpha, 7 alpha, 12 alpha-triol, but also the 25-hydroxylation of vitamin D3 are reviewed. The sterol hydroxylase was shown to be present on the mitochondrial inner membrane-matrix, to be inactivated by carbon monoxide, and to be activated by 450 nm radiation, establishing its membership in the P450 class. Characterization of the reaction product indicated that hydroxylation occurred on the 27 methyl group; the enzyme also mediated 25-hydroxylation of vitamin D3. Cloning of the enzyme confirmed that it was of mitochondrial origin and that it catalyzed both sterol and vitamin D hydroxylation. Enzymatic activity was deficient in fibroblasts from patients with cerebrotendinous xanthomatosis. Genetic analysis of patients with cerebrotendinous xanthomatosis has shown several genetic defects: 1) two different point mutations in which arginine codons were replaced by cysteine codons; 2) deletion of thymidine in exon 4; and 3) a guanosine to adenosine substitution at the 3' splice acceptor site of intron 4 of the gene. The mitochondrial 27-hydroxylase is a key enzyme in bile acid biosynthesis and can be distinguished from microsomal hydroxylases that also catalyze hydroxylation of the side chain of cholesterol and intermediates in bile acid biosynthesis. In the rat, a microsomal enzyme catalyzes 26-hydroxylation of 5 beta-cholestane-3 alpha, 7 alpha, 12 alpha-triol, but the importance of this pathway in bile acid biosynthesis is unclear.