Amino Acid metabolism of pea leaves: labeling studies on utilization of amides

Short term (2-hour) incorporation of nitrogen from nitrate, glutamine, or asparagine was studied by supplying them as unlabeled ((14)N) tracers to growing pea (Pisum sativum L.) leaves, which were previously labeled with (15)N, and then following the elimination of (15)N from various amino components of the tissue. Most components had active and inactive pools. Ammonia produced from nitrate was assimilated through the amide group of glutamine. When glutamine was supplied, its nitrogen was rapidly transferred to glutamic acid, asparagine, and other products, and there was some transfer to ammonia. Nitrogen from asparagine was widely distributed into ammonia and amino compounds. There was a rapid direct transfer to glutamine, which did not appear to involve free ammonia. Alanine nitrogen could be derived directly from asparagine, probably by transamination. Homoserine was synthesized in substantial amounts from all three nitrogen sources. Homoserine appears to derive nitrogen more readily from asparagine than from free aspartic acid. A large proportion of the pool of gamma-aminobutyric acid turned over, and was replenished with nitrogen from all three supplied sources.