New evidence for the metabolic conversion of aspartate-4-14C to glutamine-l-14C during the initial period of ammonia assimilation in the roots of barley plants

Metabolic conversion of aspartate was investigated in rootsof barley plants fed a 30-min pulse of aspartate-4-¹⁴C, followedby transfer to an ammonia or nitrate medium for 1 hr. Glutamate was the predominant labeled amino acid after the pulse;whereas, glutamine was after ammonia assimilation. Each labelappeared almost entirely at the C-1 position. The organic acidfraction was also labeled with¹⁴C. Negligible labeling of asparagineshowed that it was not a primary product of ammonia assimilation. The data demonstrated that aspartate-4-¹⁴C was transformed intoglutamate-1-¹⁴C by the tricarboxylic acid cycle mechanism, withglutamate being converted to glutamine-1-¹⁴C during ammoniaassimilation. The physiological significance of this metabolicconversion in plant metabolism is discussed. This conversionplays an important role in preventing a drop in the oxalacetatelevel when -ketoglutarate has been drained off by rapid synthesisof glutamate and glutamine during the initial period of ammoniaassimilation. This paper also presents, for die first time, evidence for theexistence of a new pathway, named the "aspartate-glutamine pathway",in higher plants. (Received August 5, 1972; )