Oxidation of biological electron donors and antioxidants by a reactive lactoperoxidase metabolite from nitrite (NO2-): An EPR and spin trapping study

We report that a lactoperoxidase (LPO) metabolite derived from nitrite (NO2 -) catalyses one-electron oxidation of biological electron donors and antio xidants such as NADH, NADPH, cysteine, glutathione, ascorbate, and Trolox C . The radical products of the reaction have been detected and identified us ing either direct EPR or EPR combined with spin trapping. While LPO/H2O2 al one generated only minute amounts of radicals from these compounds, the yie ld of radicals increased sharply when nitrite was also present. In aerated buffer (pH 7) the nitrite-dependent oxidation of NAD(P)H by LPO/H2O2 produc ed superoxide radical, O-2(.-), which was detected as a DMPO/(O2H)-O-. addu ct. We propose that in the LPO/H2O2/NO2-/biological electron donor systems the nitrite functions as a catalyst because of its preferential oxidation b y LPO to a strongly oxidizing metabolite, most likely a nitrogen dioxide ra dical (NO2)-N-., which then reacts with the biological substrates more effi ciently than does LPO/H2O2 alone. Because both nitrite and peroxidase enzym es are ubiquitous our observations point at a possible mechanism through wh ich nitrite might exert its biological and cytotoxic action in vivo, and id entify some of the physiological targets which might be affected by the per oxidase/H2O2/nitrite systems. Published by Elsevier Science Inc.