Carbon Metabolism of C14-Labeled Amino Acids in Wheat Leaves. II. Serine & its Role in Glycine Metabolism

Abstract

When glycolate-2-C14 was fed to wheat leaves, it was converted rapidly to glycine and serine. Serine was formed rapidly from glycine-2-C14 and possessed 39% of the total radioactivity in the amino acid fraction after 10 minutes, whereas glycine retained only 25%; after 25 minutes, serine had 29% and glycine 12%. In contrast, little glycine was formed from serine-1.-C14, and it possessed only 1.3%, 0.5%, or 0.7% of the total radioactivity of the amino acids after 10, 25, or 45 minutes, respectively. Serine-3-C14, which was not expected to yield labeled glycine by direct conversion, gave glycine with the same amount of C14. Furthermore, the intramolecular distribution of C14 in glycine produced from serine-1-C14 showed a major portion of the isotope residing in the [alpha]-C of glycine rather than in the expected position in the carboxyl C. Apparently glycine is synthesized by 1 metabolic pathway and oxidized by another. Analysis of the organic acid fraction formed by wheat leaves from glycine-2-C14, and serine-3-c14 showed that glyceric acid was labeled quickly and possessed the highest radioactivity after 10 minutes of exposure. Much C14 was transferred to phosphoserine from serine-1-C14, serine-3-C14, giy-cine-2-C14, and glycolate-2-C14. These results, as well as the intramolecular distribution of C14 in glucose produced from serine-1-C14, agree with the previously proposed pathway of glyoxylate-serine metabolism and clearly demonstrate the important role of serine in glycine metabolism in wheat leaves.