A Comparison of the Characteristics of Glutamine Synthetase and Glutamate Dehydrogenase From Lemna Minor L.

SUMMARY The high affinity of Lemna minor glutamine synthetase for ammonium (K app m 1.2 – 1.5 × 10 −5 M) and its strong inhibition by ADP and 5‐AMP contrasts with the low affinity (K app m 3 × 10 −2 M) for ammonium and lack of inhibition by ATP shown by glutamate dehydrogenase. Glutamine synthetase was inhibited by alanine, aspartate, glycine and serine while glutamate dehydrogenase was not affected by any amino acid, other than the product of the reductive amination reaction, glutamate. The properties of glutamine synthetase and glutamate dehydrogenase are consistent with the view that the glutamine pathway is the major route for ammonia assimilation when there is a restricted availability of ammonia. The characteristics of glutamate dehydrogenase, when contrasted with those of glutamine synthetase, and considered in relation to the concurrent regulation of the two enzymes, support the view that the glutamate pathway may contribute towards ammonia assimilation under conditions of high ammonia availability or low energy charge. Comparison of the properties of glutamate dehydrogenase purified from ammonium‐ and nitrate‐grown plants suggests that only a single form of the enzyme is present.