CYP2D6-CYP2C9 Protein-Protein Interactions and Isoform- Selective Effects on Substrate Binding and Catalysis

Cytochrome P450 (P450) protein-protein interactions have been observed with various in vitro systems. It is interesting to note that these interactions seem to be isoform-dependent, with some com-binations producing no effect and others producing increased or decreased catalytic activity. With some exceptions, most of the work to date has involved P450s from rabbit, rat, and other animal species, with few studies including human P450s. In the studies presented herein, the interactions of two key drug-metabolizing enzymes, CYP2C9 and CYP2D6, were analyzed in a purified, re-constituted enzyme system for changes in both substrate-binding affinity and rates of catalysis. In addition, an extensive study was conducted as to the “order of mixing ” for the reconstituted enzyme system and the impact on the observations. CYP2D6 coincubation inhibited CYP2C9-mediated (S)-flurbiprofen metabolism in a pro-tein concentration-dependent manner. Vmax values were reduce