Effects of calcium, S-adenosylmethionine, S-(2-aminoethyl)-L-cysteine, methionine, valine and salt concentration on rice aspartate kinase isoenzymes

The activities of two aspartate kinase (EC 2.7.2.4) isoenzymes that have be en partially purified from developing rice seeds, were studied in the prese nce of calcium, calmodulin inhibitors, S-adenosylmethionine, S-(2-aminoetyl )-L-cysteine, methionine, valine and increased salt concentrations. None of the compounds tested was able to produce any significant alteration in thr eonine-sensitive aspartate kinase activity. On the other hand, the activity of the lysine-sensitive aspartate kinase was slightly increased by calcium . The increase in activity was not observed when EGTA was added in combinat ion with calcium. S-adenosylmethionine alone inhibited the activity by 12% and intensified the inhibition caused by lysine. S-(2-aminoethyl)-L-cystein e also inhibited the activity of aspartate kinase, but not to the same exte nt of lysine. Methionine and valine stimulated slight increases in activity , whereas KCl up to 500 mM did not cause any change in aspartate kinase act ivity. These results with rice aspartate kinase indicate that lysine-sensit ive aspartate kinase is also synergistically inhibited by S-adenosylmethion ine, as observed for other plants species. Although some increase in aspart ate kinase activity was observed in the presence of calcium, the magnitude of the alterations was not sufficient to indicate a regulatory role of calc ium on aspartate kinase. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.