Effect of protonmotive force on the relative proton stoichiometries of the mitochondrial proton pumps.

The rate of phosphorylation of ADP by isolated mitochondria respiring on succinate was set by addition of ATP, ADP or ADP plus malonate. We measured the rates of phosphorylation and respiration and the protonmotive force under each of these conditions. We measured the oxygen consumption required to drive the proton leak at the protonmotive force reached under each condition and subtracted it from the respiration rate during phosphorylation to determine the oxygen consumption driving phosphorylation. By dividing the rate of phosphorylation by the rate of respiration driving phosphorylation we calculated the mechanistic P/O ratio (number of molecules of ADP phosphorylated per oxygen atom reduced). This ratio was the same at high, intermediate and low values of protonmotive force, indicating that the relative stoichiometries of the mitochondrial protonmotive-force-producing and protonmotive-force-consuming pumps (i.e. H+/O:H+/ATP) are independent of the protonmotive force. This greatly weakens the case for a decrease in stoichiometry, or 'slip', in the mitochondrial proton pumps at high protonmotive force.