Selective inhibition of mitochondrial 27-hydroxylation of bile acid intermediates and 25-hydroxylation of vitamin D3 by cyslosporin A

It was demonstrated recently that cyclosporin A blocks bile acid synthesis in cultured rat and human hepatocytes by specific inhibition of chenodeoxycholic acid formation. The site of inhibition was found to be the 27-hydroxylation of cholesterol catalysed by the liver mitochondrial 27-hydroxylase. In this paper the mechanism by which cyclosporin A blocks mitochondrial 27-hydroxylation was further investigated. It is shown that cyclosporin A inhibited 27-hydroxylation of bile acid intermediates, depending on their polarity. In isolated rat liver mitochondria, 27-hydroxylation of cholesterol was dose-dependency blocked by the drug, giving half-maximal inhibition at 4μM,whereas 27-hydroxylation of 5β-cholestane-3α,7α,12α,triol was not affected. A similar observation was made using electrophoretically homogeneous cytochrome P-45027 isolated from rabbit liver mitochondria, excluding the possibility that cyclosporin A interfered with transport of substrates into the mitochondrion. Kinetic studies showed that inhibition of the 27-hydroxylation of cholesterol by cyclosporin A was of a noncompetitive type. The drug also inhibited the 25-hydroxylase activity towards vitamin D3, catalysed by the same enzyme preparation, to the same extent as 27-hydroxylation of cholesterol. These results suggest that cyclosporin A may interfere with binding of cholesterol, but not of 5β-cholestane-3α,7α,12αtriol, to the active site of the enzyme. These data provide an explanation for the selective inhibition of chenodeoxycholic acid synthesis.
Chemicals/CAS: chenodeoxycholic acid, 474-25-9; cholesterol, 57-88-5; colecalciferol, 1406-16-2, 67-97-0; cyclosporin A, 59865-13-3, 63798-73-2; cytochrome P450, 9035-51-2; oxygenase, 9037-29-0, 9046-59-7; Bile Acids and Salts; Cholecalciferol, 67-97-0; Cyclosporine, 59865-13-3; Cytochrome P-450 Enzyme System, 9035-51-2