SUMMARY: Bacteroids were isolated anaerobically from root nodules of soybean ( L. Merr.) and supplied with C- or N-labelled glutamate or aspartate under microaerobic conditions where bacteroids retained a high activity of N fixation. Glutamate and aspartate significantly stimulated N fixation (CH reduction) and respiration (CO evolution) of isolated bacteroids. They were both utilized as substrates for bacteroid respiration and their nitrogen was rapidly released from the bacteroids as NH 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 C-dicarboxylates and glutamate in carbon and nitrogen metabolism in soybean nodule bacteroids.


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