Changes in the contents of adenine nucleotides and intermediates of glycolysis and citric acid cycle in flight muscle of the locust upon flight and their relationship to the control of the cycle

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Abstract
The contents of some intermediates of glycolysis, the citric acid cycle and adenine nucleotides were measured in the freeze-clamped locust [Schistocerca gregaria] flight muscle at rest and after 10 s and 3 min flight. The contents of G-6-P, pyruvate, alanine and especially fructose bisphosphate and triose phosphates increased markedly upon flight. The acetyl-CoA content decreased after 3 min flight. The acetylcarnitine content markedly decreased after 10 s flight, but returned towards the resting value after 3 min flight. The content of citrate markedly decreased after both 10 s and 3 min flight, whereas that of isocitrate changed very little after 10 s and increased by 50% after 3 min. The content of oxaloacetate was very low in insect flight muscle and was measured by a sensitive radiochemical assay. The content of oxaloacetate increased about 2-fold after 3 min flight. A similar change was observed in the content of malate. The content of ATP decreased about 15%, whereas those of ADP and AMP increased about 2-fold after 3 min flight. Calculations based on O2 uptake of the intact insect indicate that the rate of the citric acid cycle must be increased > 100-fold during flight. Consequently, if citrate synthase catalyzes a non-equilibrium reaction, the activity of the enzyme must increase > 100-fold during flight. Changes in the concentrations of possible regulators of citrate synthase, oxaloacetate, acetyl-CoA and citrate (which is an allosteric inhibitor), are not sufficient to account for this change in activity. There may be much larger changes in the free concentration of oxaloacetate than indicated by the changes in the total content of this metabolite or other unknown factors might play an additional role in the regulation of citrate synthase activity. The increased content of oxaloacetate could be produced via pyruvate carboxylase, which may be stimulated during the early stages of flight by the increased concentration of pyruvate. The decreases in the concentrations of citrate and .alpha.-oxoglutarate indicate that isocitrate dehydrogenase and oxoglutarate dehydrogenase may be stimulated by factors other than their pathway substrates during the early stages of flight. Calculated mitochondrial and cytosolic NAD+/NADH ratios are both increased upon flight. The change in the mitochondrial ratio indicates the importance of the intramitochondrial ATP/ADP concentration ratio in the regulation of the rate of electron transfer in this muscle.