Substrate specificity of the carbon monoxide-dependent cytochrome P-450 kinetics

The substrate-dependent kinetics of the carbon monoxide-inhibited cytochrome P-450 activity and its light reversibility is reinvestigated in microsomal preparations. In order to find out whether the substrate specificity is mediated by an isoenzyme-specific binding of carbon monoxide with different dissociation constants an experimental design has been chosen where it could be established that essentially the same isoenzyme component was involved in two different monooxygenase reactions, i.e., the O-dealkylation of 7-ethoxycoumarin and the 7-hydroxylation of coumarin. The dissociation constant kD(CO) of the ferrous cytochrome P-450 carbon monoxide complex is 6-fold higher in the presence of 7-ethoxycoumarin than in the presence of coumarin. But the light-induced relative changes of the Warburg partition coefficient for the 7-ethoxycoumarin deethylation and for coumarin 7-hydroxylation do not differ remarkably from each other. These relative changes are shown to represent the ratio of the photoinduced rate constant to the spontaneous rate constant of the dissociation for the ferrous cytochrome P-450 carbon monoxide complex. The differences in the dissociation constants are assigned to substrate specific effects on the carbon monoxide binding, indicating a substrate-specific change of the free binding enthalpy for carbon monoxide.