Cu2+ but not Cd2+ ions at low concentrations increase mitochondrial inner membrane ion permeability by inducing reactive oxygen species production and lipid peroxidation

Abstract

The integrity of the mitochondrial inner membrane is essential for efficient ATP production in mitochondria. In this study, we analyze the effect of heavy metal ions Cd2+ and Cu2+ at low concentrations on the mitochondrial inner membrane permeability and test the hypothesis that the mechanism of action of Cd2+ and Cu2+ ions is different due to the ability of the latter to undergo redox-cycling and consequently to stimulate the production of reactive oxygen specines (ROS) and to induce lipid peroxidation. We show that 5 nmol Cu2+ per mg of protein but not 5 nmol Cd2+ per mg of protein increase membrane ion permeability in the whole range of the physiologically relevant membrane potential (Δψ) values, especially at high Δψ. Moreover, Cu2+ ions at this concentration enhance the formation of H2O2 in the mitochondrial matrix by 43% and induce lipid peroxidation as indicated by an increase in the amount of thiobarbituric acid reactive substances (TBARS) by 26%. Meanwhile, Cd2+ ions at the same concentration have no significant effect on H2O2 formation or lipid peroxidation. Only a higher amount of Cd2+ions significantly stimulates the formation of H2O2. However, this is not accompanied by an increase in lipid peroxidation, which suggests that, in contrast to Cu2+-induced ROS production, the mechanism of Cd2+-induced ROS production most likely does not involve formation of the hydroxyl radical. In conclusion, we show that Cu2+ but not Cd2+ ions at a low concentration stimulate ROS production and induce accumulation of TBARS, suggesting that, indeed, Cu2+ions increase membrane ion permeability by stimulating lipid peroxidation. Keywords: Cu2+ ions, Cd2+ ions, mitochondria, inner membrane ion permeability, reactive oxygen species, lipid peroxidation