Chromium ions inactivate electron transport and enhance superoxide generation in vivo in pea (Pisum sativum L. cv. Azad) root mitochondria

The effect in vivo of hexavalent chromium (Cr6+) on the respiratory electron transport activity and production of superoxide (O-2(-)) radicals, was studied in submitochondrial particles (SMPs) prepared from mitochondria isolated from roots of 15-day-old pea (Pisum sativum L. cv. Azad) plants exposed to environmentally relevant (20 muM) and acute (200 muM) concentrations of chromium for 7 d. A concentration-dependent inactivation of electron transport activity from both NADH to O-2 (NADH oxidase) and succinate to O-2 (succinate oxidase) was observed. The electron transport activity was more sensitive to Cr6+ with NADH as the substrate than with succinate as the substrate. Although NADH dehydrogenase and succinate dehydrogenase were less affected, NADH: cytochrome c oxidoreductase and succinate: cytochrome c oxidoreductase activities were prominently affected by Cr6+. Cytochrome oxidase was the most susceptible complex of mitochondrial membranes to Cr6+, exhibiting maximal inactivation of activity both at 20 and 200 muM chromium concentrations. Cr6+ increased the generation of O-2(-) radicals. This effect was more evident at 200 than at 20 pm. A significant increase in lipid peroxidation of mitochondrial membranes at 200 muM Cr6+ was the physiological impact of the metal-induced enhanced generation of O-2(-) radicals. An increase in superoxide dismutase (SOD) activity at 20 muM Cr6+ towards enhanced production of O-2(-) radicals appeared to be a defence response in pea root mitochondria that, however, could not be sustained at 200 muM Cr6+. The results obtained concerning inactivation of mitochondrial electron transport and subsequent enhancement in the generation of O-2(-) radicals suggest that root mitochondria are an important target of Cr6+-induced oxidative stress in pea.