Origin of cadmium-induced reactive oxygen species production: mitochondrial electron transfer versus plasma membrane NADPH oxidase

Cadmium (Cd2+) is an environmental pollutant that causes increased reactive oxygen species (ROS) production. To determine the site of ROS production, the effect of Cd2+ on ROS production was studied in isolated soybean (Glycine max) plasma membranes, potato (Solanum tuberosum) tuber mitochondria and roots of intact seedlings of soybean or cucumber (Cucumis sativus). The effects of Cd2+ on the kinetics of superoxide (O-2(center dot-)), hydrogen peroxide (H2O2) and hydroxyl radical ((OH)-O-center dot) generation were followed using absorption, fluorescence and spin-trapping electron paramagnetic resonance spectroscopy. In isolated plasma membranes, Cd2+ inhibited O-2(center dot-) production. This inhibition was reversed by calcium (Ca2+) and magnesium (Mg2+). In isolated mitochondria, Cd2+ increased O-2(center dot-) and H2O2 production. In intact roots, Cd2+ stimulated H2O2 production whereas it inhibited O-2(center dot-) and (OH)-O-center dot production in a Ca2+-reversible manner. Cd2+ can be used to distinguish between ROS originating from mitochondria and from the plasma membrane. This is achieved by measuring different ROS individually. The immediate (<= 1 h) consequence of exposure to Cd2+ in vivo is stimulation of ROS production in the mitochondrial electron transfer chain and inhibition of NADPH oxidase activity in the plasma membrane.