Enzyme-mediated covalent binding of hydralazine to rat liver microsomes

Hydralazine was metabolized in vitro to a reactive metabolite(s) which bound to microsomal protein. The binding was dependent on mixed function oxidase activity, was proportional to time and microsomal protein concentration, and was not removed by acid washing. The apparent Km was 3.55 X 10(-5) M and the Vmax was 0.342 nmol/mg protein/min. Binding was inhibited by carbon monoxide and required oxygen and enzyme activity. Microsomes from animals pretreated with 3-methylcholanthrene showed a slight but significant increase in covalently bound hydralazine metabolite(s) whereas piperonyl butoxide treatment significantly reduced this binding. Glutathione, cysteine, and N-acetylcysteine all reduced the binding when added to incubations. Therefore, hydralazine is metabolized by the microsomal enzymes to a metabolite(s) capable of reacting covalently with cellular macromolecules.