Peroxidase substrates stimulate the oxidation of hydralazine to metabolites which cause single-strand breaks in DNA.

Hydrazines are believed to be oxidized by peroxidases to reactive intermediates responsible for a variety of adverse side effects including cancer and drug-induced lupus. However, hydrazines are regarded as a poor peroxidase substrates because inactivation of the peroxidase occurs during oxidation of these compounds. We have investigated the hypothesis that efficient peroxidase substrates, termed mediators, may stimulate peroxidase-catalyzed oxidation of hydrazines to intermediates capable of causing DNA damage. Oxidation of hydralazine by horseradish peroxidase was stimulated, enzyme inactivation was significantly decreased, and DNA strand breakage was enhanced by the addition of chlorpromazine. Similar results were obtained using other peroxidases, mediators, and hydrazine derivatives. DNA damage required the addition of a minimum of 3 equiv of hydrogen peroxide, suggesting the involvement of a three-electron oxidation product of hydralazine in DNA damage. Efficient substrates may therefore play a critical role in peroxidase-dependent oxidative metabolism and subsequent damage to biological macromolecules by certain chemicals.