METABOLISM OF GLUTAMATE AND ASPARTATE IN BACTEROIDS ISOLATED FROM SOYBEAN ROOT-NODULES

Bacteriods were isolated anaerobically from root nodules of soybean (Glycine max L. Merr.) and supplied with C-14- or N-15-labelled glutamate or aspartate under microaerobic conditions where bacteroids retained a high activity of N2 fixation. Glutamate and aspartate significantly stimulated N2 fixation (C2H2 reduction) and respiration (CO2 evolution) of isolated bacteroids. They were both utilized as substrates for bacteroid respiration and their nitrogen was rapidly released from the bacteroids as NH3. Amino-oxyacetate (AOA) strongly inhibited glutamate utilization, but aspartate utilization was only slightly affected by AOA. Substantial activity of aspartate ammonia-lyase (aspartase) was detected in the cell-free extract of bacteroids. These results suggest that the major pathway of glutamate utilization in bacteroids is transamination to form aspartate followed by direct deamination of aspartate by aspartase. It is also suggested that the 4-aminobutyrate pathway (GABA-shunt) is partly responsible for the catabolism of glutamate by soybean nodule bacteroids. The results are discussed in terms of the possible relationships between C4-dicarboxylates and glutamate in carbon and nitrogen metabolism in soybean nodule bacteroids.