Glutamate synthase in rice root extracts and the relationship among electron donors, nitrogen donors and its activity

The presence of glutamate synthase (GOGAT) in rice root extracts and the relationship among electron donors, nitrogen donors and its activity were studied using 15N-amido-labelled glutamine, asparagine, 14C-2-oxoglutarate and inhibitors The high molecular fraction of rice root extracts prepared by Sephadex G-50 column showed ferredoxin-dependent GOGAT activity, but pyridine nucleotide dependent activity could not be detected in it. Asparagine did not act as a nitrogen donor for rice root GOGAT. Methyl viologen could be a substitute for ferredoxin, but GOGAT activity with it was about 1/4 of that with ferredoxin. Accordingly, rice root GOGAT was considered to be the same type as that observed in leaves of many higher plants, but different from that discovered in pea roots and cultured carrot tissues. The extracts showed high GDH activity, which was reduced by the addition of glutamine. The GDH did not act with glutamine and asparaginc in the presence of aminooxy-acetate and did not show GOGAT-like activity.