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Anaerobic metabolism in the oyster heart Collicutt, Janet Margaret

Abstract

Previous studies indicate that the intertidal oyster, Crassostrea gigas, is a facultative anaerobe. The end-products of anaerobiosis in the oyster and in other intertidal bivalves are succinate and alanine. The production of these compounds is energetically advantageous over lactate production in that ATP-producing steps, other than those in glycolysis, appear to be coupled to succinate production. In this study, anoxia adaptations of the ventricle, a tissue with a high aerobic capacity, were examined. Two sources of energy are available to the anoxic ventricle : glycogen stores and a large free amino acid pool. ¹⁴C-labelled glucose, aspartate, and glutamate were tested as substrates of anaerobic metabolism in the isolated ventricle. Glucose and aspartate are readily metabolized by the anoxic ventricle. The major end-product of glucose metabolism is alanine while aspartate is metabolized mostly to succinate. Glutamate is a poor substrate of anaerobic metabolism. Correspondingly, the intracellular pool of aspartate decreases in anoxia while the glutamate pool is not depleted; It is proposed that the simultaneous mobilization of one mole of glucose and one mole of aspartate to two moles of alanine and one mole of succinate respectively, can maintain the system in redox balance, account for the production of 2-3 times more alanine than succinate in the anoxic ventricle, and provide the ventricle with an ATP-yielding step at the fumarate reductase reaction. Amino groups are transfered from aspartate to alanine through the combined functioning of aspartate and alanine aminotransferases. However, the total number of amino groups needed for alanine synthesis cannot be accounted for by anoxic changes in the pools of aspartate, other amino acids, or the adenylates. A minimal accumulation of succinate-¹⁴C from glutamate-¹⁴C as the precursor shows that forward functioning of the Krebs cycle from α-ketoglutarate is of minor importance in anoxia and the potential for generating GTP at the succinyl thiokinase reaction cannot be utilized. A third anaerobic end-product is produced from both glucose-¹⁴C and aspartate-¹⁴C. The compound appears to be metabolically closely linked to alanine or pyruvate but it could not be identified. Aerobic ATP concentrations are not maintained in anoxia. The concentration of ATP decreases while ADP and AMP concentrations increase. These changes would activate glycolytic enzymes in anoxia and reactivate the Krebs cycle immediately upon return to an aerobic environment.

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