Glycolate, Glycine, Serine, and Glycerate Formation during Photosynthesis by Tobacco Leaves

Formation of 3-phosphoglycerate, glycolate, glycerate, and serine by tobacco leaves was analyzed between 4 and 60 sec of 14CO2 fixation during photosynthesis. With decreasing time, the percentage of the total 14C incorporated into 3-phosphoglycerate increased; the percentage into the other compounds, including glycolate, decreased to nearly zero. At 4 or 11 sec 3-phosphoglycerate and glycerate were predominantly carboxyl labeled; glycolate was uniformly labeled. At 4 sec the specific activity of the carboxyl group of 3-phosphoglycerate was about 100-fold greater than that of either carbon atom of glycolate. Carbon atoms 2 and 3 of 3-phosphoglycerate were uniformly labeled at 4 or 11 sec and had a 10-fold higher specific activity than the carbon atoms of glycolate. 3-Phosphoglycerate became uniformly labeled between 30 and 60 sec, and the specific activities of carbon atoms of 3-phosphoglycerate and glycolate became similar. Addition of α-hydroxy-2-pyridinemethanesulfonate did not alter the initial rates of labeling or specific activity of 3-phosphoglycerate or glycolate, but glycolate-14C accumulated while the percentage of 14C decreased in products formed from glycolate, such as glycine, serine, glycerate, and sucrose. Glycine, like glycolate, was uniformly labeled even at the shortest times. After 4 sec serine was equally labeled in carbon atoms 1 and 2. Carbon atom 3 was slightly less active. In 11 sec serine was uniformly labeled. Results were consistent with serine formation from glycolate and glycine and a small unlabeled C1 pool. After 4 sec in the presence of the glycolate oxidase inhibitor, the small amount of labeled serine was slightly carboxyl labeled, as if part of the serine came from 3-phosphoglycerate. In 60 sec, tobacco leaves incorporated 12% of the total 14C in glycerate. The percentage distribution of 14C in glycerate carbon atoms was similar to that in 3-phosphoglycerate. The specific activity of glycerate carbon atoms was one-one hundredth that of 3-phosphoglycerate at 4 and 11 sec and one-tenth that of 3-phosphoglycerate at 60 sec.