Selective inhibition of mitochondrial 27-hydroxylation of bile acid intermediates and 25-hydroxylation of vitamin D3 by cyclosporin 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 [Princen, Meijer, Wolthers, Vonk and Kuipers (1991) Biochem J. 275, 501-505]. 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-dependently blocked by the drug, giving half-maximal inhibition at 4 microM, whereas 27-hydroxylation of 5 beta-cholestane-3 alpha, 7 alpha, 12 alpha-triol was not affected. A similar observation was made using electrophoretically homogeneous cytochrome P-450(27) 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 non-competitive 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 5 beta-cholestane-3 alpha, 7 alpha, 12 alpha-triol, to the active site of the enzyme. These data provide an explanation for the selective inhibition of chenodeoxycholic acid synthesis.
Dahlbäck-Sjöberg
H
H
Department of Pharmaceutical Biochemistry, University of Uppsala, Sweden.
Björkhem
I
I
Princen
H M
HM
D000818:Animals; D001647:Bile Acids and Salts / Q000737:chemistry / Q000378:metabolism*; D002478:Cells, Cultured; D002762:Cholecalciferol / Q000378:metabolism*; D016572:Cyclosporine / Q000494:pharmacology*; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D006900:Hydroxylation; D008099:Liver / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D008930:Mitochondria, Liver / Q000187:drug effects / Q000201:enzymology; D011817:Rabbits; D051381:Rats; D017208:Rats, Wistar
D001647:Bile Acids and Salts; D002762:Cholecalciferol; D016572:Cyclosporine; D003577:Cytochrome P-450 Enzyme System