Ethanol oxidation by rat liver catalase (the ‘peroxidatic’ reaction) was studied quantitatively with respect to the rate of H2O2 generation, catalase haem concentration, ethanol concentration and the steady-state concentration of the catalase–H2O2 intermediate (Compound I). At a low ratio of H2O2-generation rate to catalase haem concentration, the rate of ethanol oxidation was independent of the catalase haem concentration. The magnitude of the inhibition of ethanol oxidation by cyanide was not paralleled by the formation of the catalase–cyanide complex and was altered greatly by varying either the ethanol concentration or the ratio of the rate of H2O2 generation to catalase haem concentration. The ethanol concentration producing a half-maximal activity was also dependent on the ratio of the H2O2-generation rate to catalase haem concentration. These phenomena are explained by changes in the proportion of the ‘catalatic’ and ‘peroxidatic’ reactions in the overall H2O2-decomposition reaction. There was a correlation between the proportion of the ‘peroxidatic’ reaction in the overall catalase reaction and the steady-state concentration of the catalase–H2O2 intermediate. Regardless of the concentration of ethanol and the rate of H2O2 generation, a half-saturation of the steady state of the catalase–H2O2 intermediate indicated that about 45% of the H2O2 was being utilized by the ethanol-oxidation reaction. The results reported show that the experimental results in the study on the ‘microsomal ethanol-oxidation system’ may be reinterpreted and the catalase ‘peroxidatic’ reaction provides a quantitative explanation for the activity hitherto attributed to the ‘microsomal ethanol-oxidation system’.

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