Suicidal Inactivation of Human Cytochrome P‐450 by Carbon Tetrachloride and Halothanein Vitro

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Abstract
A significant loss of human cytochrome P‐450 was observed during the anaerobic incubation of NADPH‐reduced human liver microsomes obtained from surgical samples, in presence of carbon tetrachloride or halothane. In order to prevent any interference in the classical spectrum of cytochrome P‐450 with CO, the method of Johannesen & DePierre (1978) was modified to obtain cytochrome P‐450 determination. The enzyme inactivation reaction showed pseudo‐first order kinetics and was accompanied by destruction of the haem tetrapyrrolic structure, as indicated by a significant loss of its porphyrin fluorescence. Values of about 200 and 700 were calculated for the partition ratio between metabolic turnover of the substrate and enzyme inactivation during reductive incubation of one of these microsomal preparations with limiting concentrations of CCl4and halothane, respectively. The results indicate that human liver cytochrome P‐450 can be inactivated reductivelyin vitroby CCl4and halothane reactive metabolites and suggest that a suicide type of mechanism, similar to that which was recently demonstrated to occur, for both substrates, with rat liver microsomes (Mannoet al.1988a & 1991), may also be involved in the inactivation of the human enzyme(s).