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

Abstract

The activities of two aspartate kinase (EC 2.7.2.4) isoenzymes that have been partially purified from developing rice seeds, were studied in the presence 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 threonine-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 combination with calcium. S-adenosylmethionine alone inhibited the activity by 12% and intensified the inhibition caused by lysine. S-(2-aminoethyl)- l-cysteine also inhibited the activity of aspartate kinase, but not to the same extent of lysine. Methionine and valine stimulated slight increases in activity, whereas KCl up to 500 mM did not cause any change in aspartate kinase activity. These results with rice aspartate kinase indicate that lysine-sensitive aspartate kinase is also synergistically inhibited by S-adenosylmethionine, as observed for other plants species. Although some increase in aspartate kinase activity was observed in the presence of calcium, the magnitude of the alterations was not sufficient to indicate a regulatory role of calcium on aspartate kinase.