THE MECHANISM OF THE STIMULATION OF STATE-4 RESPIRATION BY CADMIUM IN POTATO-TUBER (SOLANUM-TUBEROSUM) MITOCHONDRIA

The effect of different concentrations of cadmium on non-phosphorylating respiration in purified potato tuber mitochondria (Solanum tuberosum) was investigated in vitro: respiration rate, membrane potential, the pH gradient, the protonmotive force and the mitochondrial volume were determined after the cadmium effects had reached a steady state. No effects of cadmium on the volume and insignificant changes in the pH gradient could be detected in the concentration range used; the membrane potential and the protonmotive force were decreased and non-phosphorylating respiration was increased. The pH gradient was small compared to the membrane potential, which dominated the protonmotive force regardless of the cadmium concentrations present. Since respiration in potato tuber mitochondria in the absence of ADP (state 4) is oligomycin insensitive, this rate represents the proton leak rate and the phosphorylation system does not operate. The increase in state 4 respiration rate with increasing concentrations of cadmium was due to an increase in the passive permeability of the mitochondrial inner membrane to protons (''proton leak'') only, and not to a cadmium induced change in the number of protons pumped out of the matrix by the respiratory chain per oxygen consumed (''redox slip'').