Evaluation of the relationship between the intra- and extramitochondrial [ATP]/[ADP] ratios using phosphoenolpyruvate...

The ratio of free ATP to free ADP in the mitochondrial matrix [( ATPf]/[ADPf]) has been measured in suspensions of isolated mitochondria under conditions of active phosphorylation of extramitochondrial ADP. These measurements utilized phosphoenolpyruvate carboxykinase which is present in the matrix of mitochondria from the livers of guinea pigs, chickens, and pigeons. Mitochondria isolated from these sources also contain nucleoside diphosphate kinase, malate dehydrogenase, and glutamate dehydrogenase or 3-OH-butyrate dehydrogenase. Together these enzymes catalyze the synthesis of phosphoenolpyruvate and CO2 from oxaloacetate with oxidative phosphorylation as an energy source. These reactions have been shown to be fully reversible in suspensions of mitochondria isolated from the above sources. With oxidative phosphorylation as the source of ATP, phosphoenolpyruvate was synthesized from malate and conversely addition of phosphoenolpyruvate, ADP, and CO2 led to synthesis of malate and ATP. The forward and reverse reactions were allowed to continue until the rate of change of metabolite concentrations was minimal and then the latter were measured. The intramitochondrial [Mg-ATPf]/[MgADPf] was calculated from the equilibrium constants for the reactions and the measured steady state concentrations of the metabolites in both the intra- and extramitochondrial spaces. The value of the intramitochondrial [MgATPf]/[MgADPf] was found to exceed the extramitochondrial value (adjusted to the same free Mg2+ concentration) by a factor (+/- S.E.) of 0.83 +/- 0.22 (n = 17) for the forward reaction and 1.81 +/- 0.54 (n = 11) for the reverse reaction. It is concluded that the adenine nucleotide translocase catalyzes electroneutral exchange of ATP for ADP and that this reaction does not contribute significantly to the energetics of mitochondrial oxidative phosphorylation.