Role of amino acids in cell volume control in the ribbed mussel: alanine and proline metabolism

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Greenwalt, Dale
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The euryhaline mollusc Modiolus demissus uses amino acids as osmotic solutes in cell volume regulation. During the initial stages of cell volume regulation, during hyperosmotic shock, isolated heart and gill tissues of M. demissus accumulate alanine, proline, (beta)-alanine and glycine. The biosynthetic pathways of alanine and proline synthesis were investigated through the use of specific metabolic inhibitors and ('14)C-labelled precursors, intermediates and end products;Inhibition of aminotransferase reactions during hyperosmotic shock resulted in only a small inhibition of the increase in the total intracellular free amino acid pool while inhibiting the accumulation of alanine and proline. Amino acids were identified as precursors of alanine and proline during shock. A number of ('14)C-labelled amino acids were converted to ('14)C-alanine during hyperosmotic shock and the conversion of ('14)C-glutamate, ornithine, arginine and proline to ('14)C-alanine was inhibited by aminotransferase inhibitors. Inhibition of aminotransferase reactions during hyperosmotic shock did not result in large accumulations of glutamate. Therefore, fixation of free NH(,3) by alanine dehydrogenase or glutamate dehydrogenase does not appear to be required for the synthesis of alanine during shock. ('14)C-alanine was synthesized from ('14)C-glucose and this conversion was partially inhibited by iodoacetic acid, an inhibitor of glycolysis. Alanine appears to be synthesized from both glucose and amino acid-derived tricarboxyllic acid cycle intermediates during hyperosmotic shock. The catabolism of alanine was inhibited during hyperosmotic shock. Radoactivity from ('14)C-alanine did not accumulate in alanopine ((alpha),(alpha)-iminodipropionic acid), proline, octopine, serine, glycine, taurine or (beta)- alanine in tissues incubated at either high or low salinity;Inhibitors of aminotransferase reactions resulted in the accumulation of ornithine in both heart and gill tissue during hyperosmotic shock. ('14)C-glucose and glutamate were not converted to proline, taurine or (beta)-alanine. Radioactive ('14)C-arginine and ornithine accumulated in proline during hyperosmotic shock. This conversion was inhibited by aminotransferase inhibitors. Therefore, the preferred route of proline biosynthesis from ornithine takes place via the transaminase route rather than the L-amino acid oxidase pathway or by synthesis from glutamate. During hyperosmotic shock, proline is synthesized primarily from arginine released from protein and/or the phosphoarginine stores and from the proline released during protein turnover. Proline appeared to be metabolized at higher rates during hyperosmotic shock.

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Thu Jan 01 00:00:00 UTC 1981