Human myeloperoxidase activity is inhibited in vitro by quercetin. Comparison with three related compounds.

Quercetin is an effective inhibitor of human myeloperoxidase (MPO) activity, both with purified enzyme (IC50 = 3.5 microM) and in a system using stimulated human neutrophils. Quercetin is significantly more potent than three other related compounds (rutin, rutin sulfate and troxerutin) and than methimazole, a previously-known myeloperoxidase inhibitor. The inhibitory activity of quercetin is of the competitive type. Moreover, quercetin is directly able to scavenge hypochlorous acid (HOCl), a chlorinated species generated by the MPO/H2O2/Cl- system.