EFFECTS OF CADMIUM ON THE CONTROL AND INTERNAL REGULATION OF OXIDATIVE-PHOSPHORYLATION IN POTATO-TUBER MITOCHONDRIA

The effect of cadmium on the distribution of control over oxidative ph osphorylation in potato tuber mitochondria was quantified by measuring control coefficients using top-down metabolic control analysis. Oxida tive phosphorylation was divided into three subsystems, namely substra te oxidation, the phosphorylation reactions and the proton leak. The c ontrol exerted by each of these subsystems over the system fluxes, the value of the protonmotive force and the effective P/O ratio was quant ified in the presence of different concentrations of free cadmium (up to 21 mu M). Cadmium is known to stimulate the proton leak and inhibit the substrate oxidation reactions, but it had little effect on the di stribution of control over the system variables except to shift the pa ttern to lower rates. Control exerted by particular subsystems appeare d to change or to stay the same as cadmium was varied, depending on wh ether the control coefficients were presented as a function of respira tion rate or protonmotive force. The regulatory strength of protonmoti ve force on the system variables was also calculated, as partial inter nal response coefficients. These coefficients changed with ATP turnove r rate and with cadmium concentration, showing how the internal regula tion of oxidative phosphorylation shifts under different conditions. T he values of control coefficients and partial internal response coeffi cients show where control lies and how intermediates regulate the syst em variables under different conditions of ATP demand and external eff ector (i.e. cadmium) concentration. However, they are not useful for i dentifying the sites of action of external effecters, for which elasti city and regulation analysis must be used.