Molecular Pharmacology, Vol 12, 911-920, Copyright © 1976 by the American Society for Pharmacology and Experimental Therapeutics

Oxidation of -Methyldopa and Other Catechols by Cytochrome P-450-Generated Superoxide Anion: Possible Mechanism of Methyldopa Hepatitis

¹ Laboratory of Chemical Pharmacology, National Heart and Lung Institute, National Institutes of Health, Bethesda, Maryland 20014

Renewed interest in the hepatic injury produced by -methyldopa has been stimulated by recent reports that the antihypertensive drug may initiate chronic active liver disease, occasionally with a fatal outcome. To determine whether the toxicity might be due to a reactive metabolite, [³H]-methyldopa was incubated with rat liver microsomes in the presence of an NADPH-generating system. A large amount of covalent binding occurred, but only in the presence of NADPH and oxygen (V_max = 0.5 nmol/mg/min; K_m = 50 µM). The binding was inhibited by a CO-O₂ atmosphere (9:1), indicating the involvement of cytochrome P-450. However, -methyldopa did not show P-450 binding spectra (type I, II, or III), and its covalent binding was inhibited by superoxide dismutase, ascorbic acid (1 mM), ethylenediamine (20 mM), and glutathione (1 mM). This indicated that -methyldopa was being oxidized by cytochrome P-450-generated superoxide anion to a reactive semiquinone and/or quinone. The covalent binding was inhibited by analogues such as l-dopa, dopamine, epinephrine, norepinephrine, and catechol, but not by 3-O-methyldopa, indicating a requirement for the unsubstituted catechol nucleus. Additional studies demonstrated that the rat microsomal system could be replaced by human hepatic microsomes or by a xanthine oxidase system and the binding was again inhibited by superoxide dismutase. Metabolic activation by superoxide anion may play a role in the hepatotoxicity of this and other catechols, including hydroxylated estrogens.