Calculation of competitive inhibition of substrate binding to cytochrome P-450 illustrated by the interaction of d,l-propranolol with d,l-hexobarbital.

Various forms of cytochrome P-450 exist. A substrate may bind to these forms completely or partially. In addition, during the interaction of two substrates, several cytochrome P-450 forms may also be involved, either partially or completely. Now a method is described which allows the estimation of the spectral dissociation constants for the binding of two compounds to cytochrome P-450 subforms common to both. The method is illustrated by the competitive interaction of d,l-hexobarbital and d,l-propranolol with cytochrome P-450 which show only a partial overlap for type I binding sites using rat hepatic microsomes. Different subforms of cytochrome P-450 may be characterized by means of crossover studies with several type I compounds using this method.