Relationship of the intra- and extramitochondrial adenine nucleotide ratios during synthesis of phosphoenolpyruvate using extramitochondrial ATP.

Mitochondria prepared from the livers of guinea pig, chicken, and pigeon all actively synthesize phosphoenolpyruvate from oxalacetate and GTP, utilizing phosphoenolpyruvate carboxykinase. It was previously shown (Wilson, D. F., Erecińska, M., and Schramm, V. L. (1983). J. Biol. Chem. 258, 10464-10473) that phosphoenolpyruvate carboxykinase is freely reversible and that, in conjunction with nucleoside diphosphate kinase and malate dehydrogenase, which are also present in the mitochondria, it can be used to measure the intramitochondrial [ATPfree]/[ADPfree]. In this study, synthesis of phosphoenolpyruvate by guinea pig liver mitochondria was studied under conditions for which the only source of GTP was extramitochondrial ATP via adenine nucleotide translocase and nucleoside diphosphate kinase (the mitochondria were treated with rotenone, oligomycin, uncoupler, and fluorocitrate). When the extramitochondrial [ATP]/[ADP] was greater than the intramitochondrial [ATPfree]/[ADPfree] calculated from the phosphoenolpyruvate carboxykinase reaction, there was net synthesis of phosphoenolpyruvate, but when it was less, there was net disappearance of phosphoenolpyruvate. Thus, the intramitochondrial [ATPfree]/[ADPfree] was equal to the extramitochondrial value at the point of reversal of the phosphoenolpyruvate carboxykinase reaction. This equality of the intra- and extramitochondrial adenine nucleotide ratios occurred with a measured mitochondrial membrane potential of approximately -36 mV, whereas in the previous experiments, equality was observed for conditions in which the measured membrane potential was -111 to -125 mV. Thus, adenine nucleotide translocation was not dependent on the transmembrane electrical potential and must, therefore, have occurred by electroneutral exchange.