Amino Acid levels in carrot cell suspension culture: no correlation with aspartate kinase isoenzyme levels

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.

The regulation of methionine biosynthesis and metabolism in plants and bacteria

The amino acids biosynthetically derived from asparate including methionine are all essential in the diet of monogastric animals. Most of this requirement is met by plant foods. The methionine biosynthetic pathways in plants and bacteria are outlined and compared. Regulation in bacterial systems is by a combination of repression and feedback inhibition whereas in plants repression is unimportant. Several enzymes in the branched pathway to methionine in plants are regulated by feedback inhibition; others are yet to be investigated. In plants may amino acid biosynthetic enzymes are localized in plastids and this is also likely for methionine biosynthesis. Methionine occupies an important position in cellular metabolism where the processes of one-carbon transfer via S-adenosylmethionine, protein synthesis, protein initiation and ethylene synthesis are interlocked. Attempts to increase the levels of free methionine have been made by selecting for plant mutants resistant to lysine plus threonine. One dominant mutation causes elevation of free amino acid levels in vegetative tissues but also has undesirable side-effects. The potential of such approaches is discussed.