RESUMEN
The respiration of rat liver mitochondria was stimulated by three different ways of energy drain: (a) partial uncoupling (equivalent to direct collapse of the proton-motive force), (b) intramitochondrial utilization of ATP for citrulline synthesis, and (c) extramitochondrial utilization of ATP for glucose phosphorylation. At identical rates of respiration, the intramitochondrial ATP : ADP ratios were the same in all three systems. Furthermore, the proton-motive force was the same in partially uncoupled mitochondria and in the presence of hexokinase plus glucose up to a respiration rate amounting to about 60% of that of the fully active state. However, external ATP : ADP ratios were considerably different in various systems at comparable rates of oxygen uptake, being the lowest under conditions when ATP was being utilized externally. On this basis, it is concluded that the respiratory rate is controlled directly by the proton-motive force and the mitochondrial ATP-synthesizing system operates under near-equilibrium conditions with respect to the membrane energy state parameters. However, a disequilibrium exists at the step of the transport of ATP from mitochondria to the external (cytoplasmic) compartment.