Evidence for the Glutamine Synthetase/Glutamate Synthase Pathway during the Photorespiratory Nitrogen Cycle in Spinach Leaves

Spinach leaf (Spinacia oleracea L.) discs infiltrated with [(15)N]glycine were incubated at 25 degrees C in the light and in darkness for 0, 30, 60 and 90 minutes. The kinetics of (15)N-incorporation into glutamine, glutamate, asparagine, aspartate, and serine from [(15)N]glycine was determined. At the beginning of the experiment, just after infiltration (0 min incubation) serine, and the amido-N of glutamine and asparagine were the only compounds significantly labeled in both light- and dark-treated leaf discs. Incorporation of (15)N-label into the other amino acids was observed at longer incubation time. The per cent (15)N-enrichment in all amino acids was found to increase with incubation. However, serine and the amido-N of glutamine remained the most highly labeled products in all treatments. The above pattern of (15)N-labeling suggests that glutamine synthetase was involved in the initial refixation of (15)NH(3) derived from [(15)N]glycine oxidation in spinach leaf discs.The (15)N-enrichment of the amino-N of glutamine was found to increase rapidly from 0 to 19% during incubation in the light. There was a comparatively smaller increase (4-9%) in the (15)N-label of the amino-N of glutamine in tissue incubated in darkness. Furthermore the total flux of (15)N-label into each of the amino acids examined was found to be greater in tissue incubated in the light than those in the dark. The above evidence indicates the involvement of the glutamine synthetase/glutamate synthase pathway in the recycling of photorespiratory NH(3) during glycine oxidation in spinach leaves.