Amino Acid Metabolism of Lemna minor L. 1: I. Responses to Methionine Sulfoximine

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When Lemna minor L. is supplied with the potent inhibitor of glutamine synthetase, methionine sulfoximine, rapid changes in free amino acid levels occur. Glutamine, glutamate, asparagine, aspartate, alanine, and serine levels decline concomitantly with ammonia accumulation. However, not all free amino acid pools deplete in response to this inhibitor. Several free amino acids including proline, valine, leucine, isoleucine, threonine, lysine, phenylalanine, tyrosine, histidine, and methionine exhibit severalfold accumulations within 24 hours of methionine sulfoximine treatment. To investigate whether these latter amino acid accumulations result from de novo synthesis via a methionine sulfoximine insensitive pathway of ammonia assimilation (e.g. glutamate dehydrogenase) or from protein turnover, fronds of Lemna minor were prelabeled with [15N]H4+ prior to supplying the inhibitor. Analyses of the 15N abundance of free amino acids suggest that protein turnover is the major source of these methionine sulfoximine induced amino acid accumulations. Thus, the pools of valine, leucine, isoleucine, proline, and threonine accumulated in response to the inhibitor in the presence of [15N]H4+, are 14N enriched and are not apparently derived from 15N-labeled precursors. To account for the selective accumulation of amino acids, such as valine, leucine, isoleucine, proline, and threonine, it is necessary to envisage that these free amino acids are relatively poorly catabolized in vivo. The amino acids which deplete in response to methionine sulfoximine (i.e. glutamate, glutamine, alanine, aspartate, asparagine, and serine) are all presumably rapidly catabolized to ammonia, either in the photorespiratory pathway or by alternative routes.

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