Captopril does not scavenge superoxide: captopril prevents O2-. production by chelating copper.

The purpose of this study was to use a direct method, that of electron spin resonance spectroscopy, to evaluate the ability of captopril, an angiotensin-converting enzyme inhibitor, to prevent the superoxide-mediated formation of phenyl radicals. Results indicate that, under certain conditions, captopril is a potent inhibitor of the generation of phenyl radicals, produced by the autoxidation of phenylhydrazine. The inhibitory effect of captopril, however, was better understood as a direct interaction of the drug with the metals that catalyze the autoxidation process rather than as a reaction of captopril with the free radicals generated. This last conclusion was supported by the finding that captopril was not able to inhibit the superoxide anion-mediated reduction of nitroblue tetrazolium.