Asparagine synthetase from root nodules of alfalfa

Asparagine synthetase, the enzyme which catalyzes the formation of asparagine from aspartate and glutamine (the preferable N donor), was partially purified to 300-fold from root nodules of alfalfa (Medicago sauva L.). The enzyme has Km values for aspartate, glutamine, and ammonium of 1.25, 0.16, and 2.70 mM, respectively. The ratio of glutamine- to ammonium-dependent activity is 2.5 and the pH optimum is between 7.8 and 8.2. Its subcellular location is the cytoplasm. The activity of asparagine synthetase increased in parallel with that of glutamine synthetase and the amounts of organic N in the nodules during their development. Maximum levels were formed at about 3 weeks following innoculation with Rhizobium meliloti. These values decreased rapidly after removal of the shoots: asparagine synthetase reached its lowest value (10% of original) while glutamine synthetase retained 45% of its activity at day 7. Treatment with an argon atmosphere (80% Ar + 20% O2, v/v) to prevent N2 fixation by nodules, and the resulting inhibition of NH4+ production, caused a decrease in asparagine synthetase activity to 10% of control, while this treatment only slightly affected glutamine synthetase (80% of control activity remained).Key words: alfalfa nodules, asparagine synthetase.