Reactivity and possible significance of hydroxylamine and nitroso metabolites of procainamide

We have demonstrated previously that procainamide is metabolized to a hydroxylamine. The reactivities of this hydroxylamine and of the closely related nitroso derivative toward biological molecules were investigated with the objective of exploring possible mechanisms of procainamide-induced lupus. The hydroxylamine of procainamide was found to bind covalently to microsomal protein to a much greater degree than did procainamide and, in contrast to procainamide, it did not require metabolic activation. However, the hydroxylamine is readily converted nonenzymatically to the nitroso derivative, and reducing agents such as ascorbate and NADPH, which reduce the nitroso derivative to the hydroxylamine, blocked covalent binding. This suggests that the nitroso derivative is the reactive species for covalent binding. Furthermore, glutathione had been shown previously to block covalent binding of procainamide metabolites, and the nitroso derivative, but not the hydroxylamine, reacted rapidly with glutathione forming a sulfinamide derivative. The covalent binding of the nitroso derivative to microsomal protein appears to involve sulfydryl groups, because it, like the glutathione adduct, was readily cleaved by mild acid. In contrast, the nature of the covalent binding to albumin and histone protein appears different from that to microsomal protein in that most of the binding was stable to mild acid. The reactivity toward DNA was much less than that to protein. The observation that both the reactivity of nitrosoprocainamide and the specificity of antinuclear antibodies in procainamide-induced lupus are to histone protein rather than the DNA supports the hypothesis that this reactive metabolite plays a role in the etiology of procainamide-induced lupus.