NADH-dependent O-deethylation of p-nitrophenetole with rabbit liver microsomes

Abstract

Liver microsomes are known to contain enzyme systems catalyzing monooxygenation of many foreign compounds including O-dealkylation which require NADPH and oxygen. In the present study, O-deethylation of p-nitrophenetole with rabbit liver microsomes proceeds well with NADH instead of NADPH as a cofactor. When both NADH and NADPH are used, deethylation increases markedly. The NADH-dependent system was shown to be different from the NADPH- and the NADH plus NADPH-dependent systems in several ways. For instance, in addition to the difference in optimum pH and oxygen dependency, the NADH-dependent system is not inhibited by CO, whereas the other two systems are inhibited markedly. Pretreatment of rabbits with CoCl 2 decreases both the content of cytochrome P-450 and the NADPH-linked deethylase activity but changes neither the cytochrome b 5 content nor the NADH-linked deethylase activity. After administration of 3-methylcholanthrene to the rabbit, the NADH-dependent deethylation activity is not enhanced, but the other two activities increase about twofold. KCN, at a concentration of 10 −4 m, on the other hand, does not affect any of the three activities. These results strongly suggest the possible involvement of a new type of NADH-dependent oxygenase, which is different from cytochrome P-450 and cyanide-sensitive factor, in the deethylation of p-nitrophenetole with rabbit liver microsomes.