Inhibition by SKF 525-A of 7-ethoxycoumarin O-deethylation in microsomal and the reconstituted monooxygenase systems from PCB-treated rat livers.

Addition of diethylaminoethyl 2,2-diphenylvalerate-HCl (SKF 525-A) to the incubation mixture containing liver microsomes or purified cytochrome P-450 (PCB P-450) from PCB (KC-500)-treated rats resulted in non-competitive inhibition of 7-ethoxycoumarin O-deethylation activity whereas the addition to the incubation mixture containing purified cytochrome P-448 (PCB P-448) showed a competitive inhibition. Fortification of PCB-induced microsomes with purified NADPH-cytochrome P-450 reductase enhanced the O-deethylation activity. With the reductase-fortified microsomes, SKF 525-A inhibited the O-deethylation in a competitive manner. Based on these results, we confirmed that SKF 525-A inhibits non-competitively and competitively, depending on the species of cytochrome P-450. Our results also support the view that in microsomes from PCB-treated rats, PCB P-450 rather than PCB P-448 is mainly involved in the O-deethylation reaction, presumably due to the presence of a limited amount of NADPH-cytochrome P-450 reductase in microsomes.