Enzymatically derived oligophenols from apocynin can be effective inhibitors of human vascular NADPH oxidase (Nox). An isolated trimer hydroxylated quinone (IIIHyQ) has been shown to inhibit endothelial NADPH oxidase with an IC50 ˜ 30 nM. In vitro studies demonstrated that IIIHyQ is capable of disrupting the interaction between p47phox and p22phox, thereby blocking the activation of the Nox2 isoform. Herein, we report the role of key cysteine residues in p47phox as targets for the IIIHyQ. Incubation of p47phox with IIIHyQ results in a decrease of ˜ 80% of the protein free cysteine residues; similar results were observed using 1,2- and 1,4-naphthoquinones, whereas apocynin was unreactive. Mutants of p47phox, in which each Cys was individually replaced by Ala (at residues 111, 196, and 378) or Gly (at residue 98), were generated to evaluate their individual importance in IIIHyQ-mediated inhibition of p47phox interaction with p22phox. Specific Michael addition on Cys196, within the N–SH3 domain, by the IIIHyQ is critical for disrupting the p47phox–p22phox interaction. When a C196A mutation was tested, the IIIHyQ was unable to disrupt the p47phox–p22phox interaction. However, the IIIHyQ was effective at disrupting this interaction with the other mutants, displaying IC50 values (4.9, 21.0, and 2.3 μM for the C111A, C378A, and C98G mutants, respectively) comparable to that of wild-type p47phox.