Metabolism of nitrogen fixed by root nodules of alfalfa (Medicago sativa L.): I. Utilization of [14C, 15N]glutamate and [14C, 15N]glutamine in the synthesis of γ-aminobutyrate

Abstract

The fate of nitrogen and carbon from [14C, 15N]glutamate and glutamine, two primary assimilation products of ammonia from fixed nitrogen, was studied by vacuum infiltration of the compounds into alfalfa nodule slices. The amide group of glutamine is an important precursor in the synthesis of asparagine, a major transport compound in alfalfa; this reaction, catalyzed by asparagine synthetase, also produces glutamate. Glutamate is also synthesized by the action of glutamate synthase. Transamination plays an important role in the redistribution of the nitrogen groups to yield a range of amino acids. The rapid transfer of 15N from glutamate to aspartate provides the substrate for asparagine synthesis. Some glutamate was used in glutamine synthesis, indicating the operation of glutamine synthetase. Glutamate is also metabolized by decarboxylation to γ-aminobutyric acid (Gaba), a nonprotein amino acid abundant in alfalfa nodules; Gaba is further metabolized by transamination. Considerable amounts of carbon from both glutamine and glutamate enter the pool of organic acids and are utilized in the synthesis of amino acids. There is relatively little metabolism of glutamate by isolated bacteroids.