Interactions of binary mixtures of metals on rainbow trout (Oncorhynchus mykiss) heart mitochondrial H2O2 homeodynamics

Abstract

For continuous pumping of blood, the heart needs a constant supply of energy (ATP) that is primarily met via oxidative phosphorylation in the mitochondria of cardiomyocytes. However, sustained high rates of electron transport for energy conversion redox reactions predisposes the heart to the production of reactive oxygen species (ROS) and oxidative stress. Mitochondrial ROS are fundamental drivers of responses to environmental stressors including metals but knowledge of how combinations of metals alter mitochondrial ROS homeodynamics remains sparse. We explored the effects and interactions of binary mixtures of copper (Cu), cadmium (Cd), and zinc (Zn), metals that are common contaminants of aquatic systems, on ROS (hydrogen peroxide, H₂O₂) homeodynamics in rainbow trout (Oncorhynchus mykiss) heart mitochondria. Isolated mitochondria were energized with glutamate-malate or succinate and exposed to a range of concentrations of the metals singly and in equimolar binary concentrations. Speciation analysis revealed that Cu was highly complexed by glutamate or Tris resulting in Cu²⁺ concentrations in the picomolar to nanomolar range. The concentration of Cd²⁺ was 7.2–7.5 % of the total while Zn²⁺ was 15 % and 21 % of the total during glutamate-malate and succinate oxidation, respectively. The concentration-effect relationships for Cu and Cd on mitochondrial H₂O₂ emission depended on the substrate while those for Zn were similar during glutamate-malate and succinate oxidation. Cu + Zn and Cu + Cd mixtures exhibited antagonistic interactions wherein Cu reduced the effects of both Cd and Zn, suggesting that Cu can mitigate oxidative distress caused by Cd or Zn. Binary combinations of the metals acted additively to reduce the rate constant and increase the half-life of H₂O₂ consumption while concomitantly suppressing thioredoxin reductase and stimulating glutathione peroxidase activities. Collectively, our study indicates that binary mixtures of Cu, Zn, and Cd act additively or antagonistically to modulate H₂O₂ homeodynamics in heart mitochondria.