Eosinophil peroxidase-dependent hydroxyl radical generation by human eosinophils

Eosinophil production of superoxide (O₂/^-·) and hydrogen peroxide (H₂O₂) is important in host defense. The present study assessed the potential of eosinophils to generate another potent cytotoxic species, the hydroxyl radical (·OH). ·OH formation by phorbol myristate acetate (PMA)- stimulated eosinophils was demonstrated using an α-(4-pyridyl-1-oxide)-N- tert-butyl nitrone/ethanol spin trapping system. Additionally, ·OH was spin trapped following the addition of purified eosinophil peroxidase (EPO) to a cell-free O₂/^-·/H₂O₂ generating systems. Effects of superoxide dismutase, catalase, azide, aminotriazole, chloride-depleted buffer, and extensive metal chelation were consistent with ·OH formation via the reaction of O₂/^-· and EPO-generated hypohalous acid. Under chloride- depleted conditions, physiologic concentrations of Br^- increased ·OH formation by both PMA-stimulated eosinophils and the cell-free EPO system. Physiologic concentrations of SCN^-, however, did not increase ·OH formation, and in the presence of both Br^- and SCN^-, ·OH formation was similar to SCN^- only. Eosinophils appear to form ·OH via an EPO-dependent mechanism, the magnitude of which varies with the availability of various EPO substrates. Given the highly reactive nature of this radical and the ability of EPO to adhere to cell membranes, even small amounts of ·OH formed at such sites could contribute to eosinophil-mediated cytotoxicity.