PP29 - Regulation of Myeloperoxidase NO Oxidase Activity by Pharmacological Agents: Novel Biochemical Basis for NO Preservation by Phenolics and Nitroxides (Tempol)

Abstract

Introduction The leukocyte-derived heme enzyme myeloperoxidase (MPO) plays a key role in impairing NO bioavailability during inflammatory conditions. MPO can consume NO as a direct enzyme substrate as well as by generating NO-consuming free radicals. While our recent studies reveal that endogenous agents (thiocyanate, ascorbate) inhibit this NO oxidase activity, their protective action is incomplete (FRBM, 2014;72:91-103). Aims We sought to identify pharmacological agents that can afford additional protection against MPO NO oxidase activity in complex physiological fluids. We also sought to identify drugs that stimulate this activity, as these may have unforeseen side-effects. Methods Diverse pharmacological agents with established redox activities as MPO substrates and/or radical scavengers were screened for their effects on MPO NO oxidase activity in human plasma and physiological model systems containing endogenous MPO substrates and antioxidants. Results Unexpectedly, the hydrazide MPO inhibitors ABAH and isoniazid, and the peroxidase-activated NO donor hydroxyurea all greatly stimulated MPO NO oxidase activity under physiological conditions. Melatonin marginally increased NO consumption. The phenolic drugs acetaminophen and resveratrol, which are excellent peroxidase substrates, initially stimulated NO consumption but ultimately limited the extent of NO losses. Tempol and related nitroxides were uniquely able to inhibit the rate of MPO-catalyzed NO consumption in ascorbate-replete fluids. Discussion Kinetic analyses revealed the mechanistic basis of drug activities and identified criteria for developing drugs with improved activity. Overall, our data reveal that widely-used pharmacological agents strongly influence MPO NO oxidase activity in complex physiological fluids and identify novel mechanisms by which phenolic drugs and nitroxides may preserve NO bioavailability during inflammatory conditions.