Interactions between cytochromes P450 2B4 (CYP2B4) and 1A2 (CYP1A2) lead to alterations in toluene disposition and P450 uncoupling.

The goal of this study was to characterize the effects of CYP1A2·CYP2B4 complex formation on the rates and efficiency of toluene metabolism by comparing the results from simple reconstituted systems containing P450 reductase (CPR) and a single P450 to those using a mixed system containing CPR and both P450s. In the mixed system, the rates of formation of CYP2B4-specific benzyl alcohol and p-cresol were inhibited, whereas that of CYP1A2-specific o-cresol was increased, results consistent with the formation of a CYP1A2·CYP2B4 complex in which the CYP1A2 moiety has a higher affinity for CPR binding. Comparison of the rates of NADPH oxidation and production of hydrogen peroxide and excess water by the simple and mixed systems indicated that excess water formed at a much lower rate in the mixed system. The commensurate increase in the rate of CYP1A2-specific product formation suggested the P450·P450 interaction increased the rate of the putative rate-limiting step of CYP1A2 catalysis, abstraction of a hydrogen radical from the substrate. Cumene hydroperoxide-supported metabolism was measured to determine whether the effects of the P450·P450 interaction required the presence of CPR. Peroxidative metabolism was not affected by the interaction of the two P450s, even with CPR present. However, CPR did stimulate peroxidative metabolism by the simple system containing CYP1A2. These results suggest the major functional effects of the P450·P450 interaction are mediated by changes in the relative abilities of the P450s to receive electrons from CPR. Furthermore, CPR may play an effector role by causing a conformational change in CYP1A2 that makes its metabolism more efficient.