THE EFFICIENCY OF OXIDATIVE-PHOSPHORYLATION IN POTATO-TUBER MITOCHONDRIA IS DIFFERENT FOR SUCCINATE AND EXTERNAL NADH

We have determined the efficiency of oxidative phosphorylation in potato-tuber mitochondria using two respiratory substrates: succinate or NADH. Since plant mitochondria contain a NADH dehydrogenase that faces the intermembrane space but which does not translocate protons, the value for the P/0 is theoretically the same for succinate and NADH despite the difference in redox potential. We found that in practice the P/0 is slightly lower for NADH. Furthermore, the overall efficiency of oxidative phosphorylation is lower with NADH than with succinate. The value for the coupling coefficient is approx. the same for both substrates, namely 0.960 for succinate and 0.956 for NADH. The values for the phenomenological stoichiometry were 1.90 and 1.83 for succinate and NADH, respectively. With both substrates, the low value for the phenomenological stoichiometry indicates that in potato-tuber mitochondria a slip occurs in the proton-pumps of the electron-transport chain. Only part of the free energy available from substrate oxidation is used, effectively: this amounts to 92 kJ/mol (60%) and 94 kJ/mol (45%) when succinate or NADH are used as substrate, respectively.