When deprived of oxygen, the desert locust (Schistocerca gregaria) enters a state of apparent dormancy, from which it can recover when oxygen is again made available. Calorimetric measurements show that with an atmosphere of 100% nitrogen, energy flow in locusts reaches a new steady-state rate of heat dissipation of only 6 – 7% of aerobic values. Metabolite analysis shows large decreases in concentrations of arginine phosphate and adenosine triphosphate during anoxia, while adenosine monophosphate, lactate, α-glycerophosphate, and Pi accumulate to significantly elevated levels during anoxia. Time-course studies on whole animals and flight muscle paradoxically show declining metabolic rates when changes in metabolite (especially adenylate) concentration would normally be expected to activate metabolism.
The physiological importance of urea biosynthesis differs widely among ureogenic animals. Many terrestrial animals synthesize urea as a mechanism for detoxifying NH3 and (or) [Formula: see text], while elasmobranchs and some species of amphibians with osmoconforming strategies produce urea and retain it within the body as an osmolyte. The ornithine–urea cycle is highly constrained in terms of enzyme composition and tissue localization. Other features of the pathway vary adaptively among organisms that produce urea for different functions. Adaptive features of the pathway include the occurrence of different isozymes, subcellular compartmentation of enzymes, and the regulation of urea synthesis. This review identifies the constrained and variable features of the urea cycle and attempts to place these features within an adaptive and evolutionary framework.
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