Lupin glutamate synthase

Glutamate synthase activity has been detected in the plant fraction of lupin nodules (Robertson et al., 1975b Radyukinaer al., 1977) and the level of this enzyme also increases during nodule development. Glutamate synthase has been purified from the plant fraction of lupin nodules (Boland and Benny,... 

In contrast the enzyme purified from lupin root nodule cytosol is reported to consist of a single polypeptide chain of molecular weight 235,000 (Boland and Benny, 1977). No information is available at present regarding the possible subunit structure of other eukaryote glutamate synthases although the ferredoxin-dependent enzyme from V. faba has a molecular weight of 150,000, as determined by gel filtration (Wallsgrove et al., 1977) and the pyridine nucleotide-dependent enzyme of Saccharomyces cerevisiae has a sedimentation coefficient of 14.6 S (Roon et al., 1974). 

Neither lupin nodule cytosol or V. faba glutamate synthases are reported to utilize ammonia (see Boland and Benny, 1977 Wallsgrovee/ al., 1977) and neither of these enzymes appear to have been examined for glutaminase activity. 

The catalytic activity of glutamate synthases from a variety of sources has been found to be unstable (Miller and Stadtman, 1972 Boland and Benny, 1977 Wallsgrove et al., 1977). The E. coli enzyme was found to be more stable in the presence of glutamine or reducing agents (Miller and Stadtman, 1972). Very high concentrations of mercaptoethanol or dithiothreitol were found to be necessary during purification of the lupin nodule enzyme and routine assays of this enzyme are carried out in the presence of 1% mercaptoethanol (Boland and Benny, 1977). The lupin enzyme is reported to be more stable (20% loss per week) in phosphate compared with Tris buffers. 

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