Hepatic mitochondrial coumarin 7-hydroxylase: comparison with the microsomal enzyme

The specific activity of cytochrome P450-linked coumarin 7-hydroxylase (COH) of hepatic mitoplasts from DBA/2N mice is up to 55% as great as the microsomal activity. According to Western blot and immunodiffusion analysis and inhibition studies with anti-P450Coh and metyrapone, the mitoplastic P450Coh had the same molecular weight and immunochemical and catalytic properties as the corresponding microsomal enzyme. The inducibility of the two proteins by pyrazole and their genetic regulation, as studied with DBA/2N and AKR/J mice, appears to be similar. However, the mitochondrial electron transfer system was not able to support the COH activity of reconstituted microsomal P450Coh although the enzyme was fully active with the microsomal NADPH-cytochrome P450 reductase. This indicates some differences between the two proteins with respect to their interaction with the electron transfer system. This was confirmed by the ability of anti-adrenodoxin reductase antibody to effectively inhibit the mitoplastic COH but not the COH reconstituted with purified microsomal P450Coh and NADPH-P450 reductase. We have previously found that P450Coh does not react with anti-P450b or anti-P450c antibodies, which recognize respective isoforms in rat liver mitoplasts. While P450Coh from microsomes and mitoplasts possess a number of properties in common, the mitoplast P450Coh represents a new subspecies of mitochondrial P450. Some characteristics of mitoplast P450Coh may be the result of post-translational modifications necessary for processing and translocation into the mitochondria.