The temperature dependence of the components of the hepatic microsomal mixed-function oxidases

Abstract

The activation energies of ethylmorphine N-demethylase (16.8 kcal/mole) and aniline hydroxylase (17.6 kcal/mole) did not greatly differ. The endogenous NADPH oxidase (16.8 kcal/mole) and the oxidase in the presence of ethylmorphine (15.7 kcal/mole) and aniline (14.7 kcal/mole) are only slightly different. On the other hand, the activation energy for NADPH-cytochrome c reductase is only 8.53 kcal/mole, suggesting that the reduction of the flavin in this enzyme does not significantly contribute to the rate of the overall reaction. The addition of ethylmorphine reduces the NADPH-cytochrome P-450 reductase activation energy from 12.5 kcal/mole to 10.8 kcal/mole. This energy is slightly increased by the presence of aniline (14.7 kcal/mole). Since NADPH-cytochrome P-450 reductase in the presence of ethylmorphine is presumed to be the rate-limiting step in the N-demethylase, the large difference between the energy of activation of this reaction and the energy of activation of ethylmorphine N-demethylase would suggest the participation of two reactions of similar rate in the control of the overall rate of the oxygenase reaction.