PROOXIDANT AND ANTIOXIDANT ACTIVITIES OF THE MITOCHONDRIAL RESPIRATORY-CHAIN - FACTORS INFLUENCING NAD(P)H-INDUCED LIPID-PEROXIDATION

This paper is a study of factors influencing the rate of lipid peroxid ation in beef heart submitochondrial particles induced by NAD(P)H via the NADH-ubiquinone oxidoreductase (Complex I) of the respiratory chai n. In accordance with earlier observations, both NADH and NADPH initia ted lipid peroxidation in the presence of ADP-Fe3+. The rate of the re action, measured as oxygen consumption and formation of thiobarbituric acid reactive substances, was biphasic as a function of NADH concentr ation, reaching a maximum at low NADH concentrations and then declinin g. In contrast, the NADPH-initiated lipid peroxidation showed a monoph asic concentration profile of hyperbolic character. Rotenone did not e liminate the biphasicity of the NADH-induced reaction, indicating that this was not due to an antioxidant effect of reduced ubiquinone at hi gh NADH concentrations. This conclusion was further supported by the d emonstration that extraction of ubiquinone from the particles did not relieve the inhibition of lipid peroxidation by high NADH concentratio ns. However rhein, another inhibitor of Complex I, eliminated the biph asicity, and even caused a substantial stimulation of the NADH-induced lipid peroxidation in the particles upon extraction of ubiquinone by pentane. No similar effect occurred in the case of NADPH-induced lipid peroxidation. Furthermore, rhein facilitated both NADH- and NADPH-ind uced lipid peroxidation even in the absence of added ADP-Fe3+, in a fa shion similar to that earlier reported with succinate in the presence of theonyltrifluoroacetone. Based on these findings and measurements o f the redox states of ubiquinone and cytochromes in the presence of KC N and NADH or NADPH, it is concluded that Complex I may distinguish be tween electron input from NADH and NADPH by differences in the site(s) of substrate binding and in the pathways and rates of NADH and NADPH oxidation.