Apocynin Is Not an Inhibitor of Vascular NADPH Oxidases but an Antioxidant

A large body of literature suggest that vascular reduced nicotinamide-adenine dinucleotide phosphate (NADPH) oxidases are important sources of reactive oxygen species. Many studies, however, relied on data obtained with the inhibitor apocynin (4'-hydroxy-3'methoxyacetophenone). Because the mode of action of apocynin, however, is elusive, we determined its mechanism of inhibition on vascular NADPH oxidases. In HEK293 cells overexpressing NADPH oxidase isoforms (Nox1, Nox2, or Nox4), apocynin failed to inhibit superoxide anion generation detected by lucigenin chemiluminescence. In contrast, apocynin interfered with the detection of reactive oxygen species in assay systems selective for hydrogen peroxide or…