Effect of the extramitochondrial adenine nucleotide pool size on oxidative phosphorylation in isolated rat liver mitochondria

The effect of the concentration of extramitochondrial adenine nucleotides on oxidative phosphorylation was studied in isolated rat liver mitochondria. Mitochondria were incubated with succinate and hexokinase or creatine kinase at constant or varying extramitochondrial adenine nucleotide concentrations ranging over 0.3-5 mM. As parameters of oxidative phosphorylation, rate of respiration, membrane potential as well as intra-and extra-mitochondrial adenine nucleotide concentrations were determined. Below a threshold concentration of extramitochondrial adenine nucleotides of 2 mM, the free Gibb's energy for the adenine nucleotide transport increased but the extramitochondrial ATP/ADP ratio decreased at intermediate rates of respiration with decreasing extramitochondrial adenine nucleotide concentrations. In this range the rate of respiration was dependent on the extramitochondrial ADP concentration. No effect of the extramitochondrial adenine nucleotide concentration on the relationships between the rate of respiration and the membrane potential, the intramitochondrial adenine nucleotide pool and the intramitochondrial ATP/ADP ratio was found. This suggests that the decline of extramitochondrial ATP due to adenine nucleotide degradation and the limitation of adenine nucleotide transport may be of importance in the postischemic phase as nucleotide resynthesis and reorganization of physiological ion distribution are ATP consuming processes.