The subcellular distribution of
ATP,
ADP,
creatine phosphate and
creatine was studied in normoxic control,
isoprenaline-stimulated and
potassium-arrested guinea-pig hearts as well as during
ischemia and after reperfusion. The mitochondrial
creatine phosphate/
creatine ratio was closely correlated to the oxidative activity of the hearts. This was interpreted as an indication of a close coupling of
mitochondrial creatine kinase to oxidative phosphorylation. To further investigate the functional coupling of
mitochondrial creatine kinase to oxidative phosphorylation, rat or guinea-pig heart mitochondria were isolated and the mass action ratio of
creatine kinase determined at active or inhibited oxidative phosphorylation or in the presence of high
phosphate, conditions which are known to change the functional state of the mitochondrial
enzyme. At active oxidative phosphorylation the mass action ratio was one-third of the equilibrium value whereas at inhibited oxidative phosphorylation (N2,
oligomycin,
carboxyatractyloside) or in the presence of high
phosphate, the mass action ratio reached equilibrium values. These findings show that oxidative phosphorylation is essential for the regulation of the functional state of
mitochondrial creatine kinase. The functional coupling of the
mitochondrial creatine kinase and oxidative phosphorylation indicated from the correlation of mitochondrial
creatine phosphate/
creatine ratios with the oxidative activity of the heart in situ as well as from the deviation of the mass action ratio of the mitochondrial
enzyme from
creatine kinase equilibrium at active oxidative phosphorylation in isolated mitochondria is in accordance with the proposed operation of a
creatine shuttle in heart tissue.