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Patterns of anaerobic metabolism in molluscan muscle Fields, Jeremy Harold Austin

Abstract

Anaerobic metabolism in cephalopod muscle and in bivalve adductor muscle depends on the coupling of carbohydrate and amino acid metabolism. In cephalopod muscle this is acheived by octopine dehydrogenase (E.C.1.5.1.11) , whereas in the oyster adductor muscle it is acheived by transaminases and malate dehydrogenase (E.C.1.1.1.37). Therefore studies of the catalytic properties (a) of octopine dehydrogenase from muscle of a group of cephalopods, and (b) of cytoplasmic aspartate aminotransferase (E.C.2.6.1.1) and malate dehydrogenase from adductor muscle of the oyster, Crassostrea gigas, were undertaken. Higher activities of octopine dehydrogenase were found in the mantle of Octopus ornatus than in the mantle of Symplectoteuthis oualaniensis, but the catalytic properties of both enzymes were similar. The affinity for pyruvate was low (Km approx. 1.7 mM), but increased with increasing concentrations of arginine; the affinity for arginine similarly increased with increasing concentrations of pyruvate. Octopine dehydrogenase from the spadix muscle of the chambered nautilus, Nautilus pompilius, had a higher affinity for pyruvate (Km approx. 0.3 mM), and this was also increased by increasing arginine concentrations. It is suggested that octopine dehydrogenase maintains redox balance in a manner analogous to lactate dehydrogenase (E.C.I.1.1.27), and closely couples glycolysis with arginine phosphate metabolism, such that an anaerobic reserve is provided for high intensity "burst" work. The octopus mantle relies on this mechanism more so than does the mantle of the oceanic squid, S. oualaniensis, and the Nautilus spadix muscle appears to use this anaerobic process for most of its energetic requirements. In contrast to cephalopod muscle, oyster adductor muscle maintains redox balance through coupling aspartate and alanine metabolism with carbohydrate fermentation. Adductor aspartate aminotransferase had a higher affinity for aspartate than for glutamate, and a higher affinity for 2-ketoglutarate than for oxaloacetate, suggesting that it would function more readily in the direction of aspartate utilization. Adductor malate dehydrogenase had a higher affinity for oxaloacetate than did aspartate aminotransferase, hence the major fate of oxaloacetate produced would be conversion to malate, and this would direct the flow of aspartate carbon towards succinate. Since adductor alanine aminotransferase (E.C.2.6.1.2) is kinetically adapted for alanine formation, these enzymes couple glycolysis with aspartate mobilisation, such that alanine is formed from glucose and succinate from aspartate. In addition,it was found that pyruvate had another possible fate during anoxia in the adductor, that is conversion to an as yet unidentified compound that is produced by a dehydrogenase requiring NADH, alanine and pyruvate as substrates. This enzyme has an extremely low affinity for alanine, and is potently inhibited by succinate at low pH; hence during anoxia production of this compound would be limited, and the pathway leading to succinate production favoured.

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