Relationship between the energy cost of ATP transport and ATP synthesis in mitochondria

Liver mitochondria form rats kept on a high-protein diet exhibit an increased rate of respiration upon addition of ornithine in the presence of HCO2 and NH+4. This is the manifestation of intramitochondrial utilization of ATP for the synthesis of citrulline. State 3 respiration of these mitochondria could be adjusted to the same rate as that produced by ornithine by either using limiting amounts of hexokinase or titration with atractyloside. Under such conditions, in both systems the proton-motive force, the intramitochondrial ATP/ADP ratio and the redox state of the respiratory chain were the same. In contrast to this, the ATP/O ratio (equal to 2 X citrulline/O ratio) in mitochondria synthesizing citrulline was higher than the glucose 6-phosphate/O ratio in the system where ATP was trapped extramitochondrially. The ratio of these two ratios was close to 1.5 with both glutamate and succinate as respiratory substrates. From these results it can be concluded that the translocation of ATP against ADP and phosphate utilizes an amount of the chemiosmotic proton gradient equal to half of that needed for the synthesis of ATP in the inner compartment.