Mitochondrial respiratory control. Evidence against the regulation of respiration by extramitochondrial phosphorylation potentials or by [ATP]/[ADP] ratios.

Abstract

To explore how mitochondria can respire at high physiological, extramitochondrial phosphorylation potentials, two series of experiments were conducted. In the first, intact rat liver mitochondria were incubated in oxygraph medium containing 5 mM succinate (+rotenone), 1.0 mM ATP, 20 mM glucose, pH 7.2, at 37 degrees C. Yeast hexokinase (0.02 to 1.0 IU) was added to establish steady state rates of respiration. Samples were removed, assayed for ATP, ADP, and Pi content, and ratios were calculated. As previously reported, low rates of respiration were observed at high phosphorylation potential ([ATP]/[ADP] x [Pi]) or [ATP]/[ADP] ratio values, and the rates of respiration increased as these values declined. In a second series of experiments, only sufficient hexokinase was added to potentially stimulate respiration to 90% of the ADP State 3 rate. At constant hexokinase, 0.35 IU, ATP (5 microM to 10.0 mM) was titrated into the medium to establish steady state rates of oxygen consumption. Under these conditions, low rates of respiration correlated with low [ATP]/[ADP] ratios and extramitochondrial phosphorylation potentials, while maximum rates of respiration were observed at high values of these ratios, the opposite of the previous experimental case. Therefore, it may be concluded that these extramitochondrial parameters per se exert little or no regulatory influence on the rates of respiration, and thus matrix ATP synthesis. In both cases, the concentrations of ADP correlated with respiratory rates. Double reciprocal plots were used to estimate the apparent KmADP for respiratory stimulation. The values are 56 microM for constant [ATP] and 15 microM at constant hexokinase. The value calculated from direct ADP pulses was 25 microM. Together, these results suggest that the most plausible explanation of respiratory control is the availability of ADP and the kinetics of its transport by the adenine nucleotide translocase, a hypothesis first proposed by Chance and Williams more than 25 years ago (Chance, B., and Williams, G. R. (1955) J. Biol. Chem. 217, 385-393).