Cytochrome P-450 comprises a large super family of hemoproteins that collectively catalyze the metabolism of literally thousands of different compounds, including steroids, fatty acids, drugs and carcinogens. Although the general sequences of events that occur during the action of these enzymes are understood, and are common to all members of the family, the mechanism by which a substrate is converted to several metabolites is frequently obscure. Several mechanism may be envisioned. In each of the mechanisms, the substrate combines with the enzyme in different orientations to form complexes that then are activated to a set of (EOS) complexes. The fate of these (EOS) complexes determine the mechanisms. In the parallel pathway mechanism, the (EOS) complexes are rigid; they cannot be converted either directly or indirectly to complexes with different orientations; the orientations of the (ES) complexes thus determine which metabolite will be formed. In the nondissociative mechanisms, the complexes are not rigid; instead they undergo interconversion while the substrate remains in the active site of the enzyme. In the dissociative mechanisms, the (EOS) complexes dissociate to (EO) and (S) but recombine to form (EOS) complexes with either the same or different orientations. We have derived steady-state rate equations for each kinetic mechanism and used them to elucidate the mechanism by which P-450 2C11 converts dO-testosterone and d5- testosterone to A-ring and D-ring metabolites. Some of the results indicate that the metabolites are formed by the general dissociative mechanism, but other results indicate that the parallel pathway mechanism may be invoked under certain conditions. In addition, we have developed a way to determine the extent to which a primary metabolite-enzyme complex dissociates before it is converted to a secondary metabolite. Studies with P450 2C11 indicate that only about 85% of the androstenedione-P450 2C11 complex dissociates; about 15% remains intact and is converted to a 16a-hydroxyandrostenedione-P450 2C11 complex.
