Investigations into the spin trapping of nitric oxide and superoxide: models to explore free radical generation by nitric oxide synthase

Abstract

Nitric oxide synthase catalyzes the oxidative metabolism of L-arginine to L-citrulline and NO˙. During cycling, this enzyme, besides generating NO˙, also secretes O₂˙⁻. These free radicals react at diffusion controlled rates to produce ONOO⁻, which has been shown to decompose to give, among a variety of products, small amounts of HO˙. Thus, during oxidation of L-arginine by nitric oxide synthase, NO˙ and O₂˙⁻ will be secreted, and as a result, produce H₂O₂ from the dismutation of O₂˙⁻, and ONOO⁻ from the reaction of NO˙ and O₂˙⁻. Hydroxyl radical can then be formed either by the reaction of H₂O₂ with transition metal ions or decomposition of ONOO⁻. Using different spin traps unique to each of these free radicals, EPR spectroscopy has been used to identify NO˙, O₂˙⁻ and HO˙.