Aspartokinase of Lemna paucicostata Hegelm. 6746

A sensitive and specific method was developed for assay of aspartokinase (EC 2.7.2.4) in crude extracts of Lemna paucicostata. Lysine inhibited approximately 93%, and threonine approximately 6%; together, these amino acids inhibited 99%. Inhibition by lysine was synergistically increased by S-adenosylmethionine, which by itself had no effect on activity. Essentially complete inhibition of threonine-resistant activity was obtained with lysine, and of lysine-resistant activity with threonine. Inhibition by lysine and threonine was additive, with no indication of concerted inhibition. Aspartate concentration had no effect on the relative proportions of lysine- and threonine-sensitive activities. Aspartokinase activity was in large excess of that reported by other workers, the maximum capacity (V(max)) far exceeding the in vivo requirements. Estimations of rates of aspartokinase in vivo suggest that the step catalyzed by this enzyme may not be the overall ;rate-limiting' one for entry of 4-carbon units into the aspartate family of amino acids, and that feedback inhibition of this enzyme by lysine and threonine may not be a major factor in regulating flux through this step.