Multiple drug metabolism: p-nitroanisole reversal of acetone enhanced aniline hydroxylation

Abstract

The unique effect of acetone on the p-hydroxylation of aniline was evaluated in microsomes prepared from control, phenobarbital- and 3-methylcholanthrene-pretreated animals. The existence of multiple forms of the hydroxylase was evaluated as an explanation of the acetone enhancement. Simultaneous metabolism of aniline in the presence of either p-nitroanisole (pNA) or ethylmorphine (EM) was evaluated to probe the participation of different mixed function oxidase systems. Aniline inhibited both N- and O-demethylation, while pNA and EM both inhibited p-hydroxylation of aniline. Acetone decreased the individual demethylation reactions, but enhanced aniline hydroxylation. In multiple drug reactions, acetone decreased N-demethylation and proportionately increased aniline p-hydroxylation. On the other hand, p-nitroanisole blocked the acetone enhancement of aniline metabolism. Kinetic evaluation of the acetone and p-nitroanisole effects on aniline metabolism indicated that each agent increased the apparent K m′ by 4- to 5-fold for aniline in the hydroxylation reaction, but only acetone increased the V max′. From the Eadie-Scatchard analysis of the rates of aniline hydroxylation, acetone appeared to produce a biphasic increase in the hydroxylation above 0.75 mM aniline, even in the presence of pNA. Thus, multiple forms of the aniline p-hydroxylase are indicated by their altered activities in the presence of other drugs, and acetone seemed to specifically alter a species having a higher K m′ for aniline.