Initiation of a superoxide-dependent chain oxidation of lactate dehydrogenase-bound NADH by oxidants of low and high reactivity

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In cells, NADH and NADPH are mainly bound to dehydrogenases such as lactate dehydrogenase (LDH). In cell-free systems, the binary LDH–NADH complex has been demonstrated to produce reactive oxygen species via a chain oxidation of NADH initiated and propagated by superoxide. We studied here whether this chain radical reaction can be initiated by oxidants other than LDH largely increased the oxidation of NADH (but not of NADPH) by O2, H2O2 and during the intermediacy of HNO2. LDH also increased the oxidation of NADH by peroxynitrite. The increases in NADH oxidation were completely prevented by superoxide dismutase (SOD). In contrast, the nitrogen dioxide-dependent oxidation of NADH and NADPH was decreased by LDH in a SOD-independent manner. These experimental data strongly indicate that oxidation of LDH-bound NADH can be induced from reaction of either weak oxidants with LDH-bound NADH or of strong oxidants with free NADH thus yielding which is highly effective to propagate the chain. Our results underline the importance of SOD in terminating superoxide-dependent chain reactions in cells under oxidative stress.