Effects of Glyoxylate, Glycine, and Serine on the Assimilation of 14CO2 in Mesophyll Cells of Chenopodium album

Abstract

Mesophyll cells from Chenopodium album were isolated within 1 h in two media containing macerase, hemicellulase, sorbitol, albumine, polyvinylpyrrolidone, potassium dextran sulfate, a mixture of inorganic salts, and MES buffer, pH 5.8. Cells with insufficient activity could be recognized by the absence of phototaxis of their chloroplasts; these cells predominated in preparations from older leaves. The effect of various glycollate metabolites (glycollate, P-glycollate, P-glycolaldehyde, glyoxylate, P-glyoxylate, glycine, and serine) on the assimilation of 14CO2 from H14CO3- at pH 7.3 was investigated. Glyoxylate, glycine, and serine exhibited most pronounced effects depending on the concentration of H14CO3- the total fixation of 14C (control: 34.4 μmoles of CO2/mg of chlorophyll × h) was reduced by 0.01 M solutions of glyoxylate (28.7 μmoles), glycine (23.9 μmoles), and serine (12.9 μmoles). Glyoxylate diminishes the incorporation into amino acids but not into sugars and starch, whereas labelling of sucrose and starch is reduced by glycine and serine. After 1 h of photosynthesis at 5 mM H14CO3- the amino acid fraction above all consists of glutamate and aspartate (67.8 and 13.6%, resp.) followed by alanine (11.0%). The very low radioactivity in serine and glycine (4.0 and 1.2%, resp.) indicates a suppressed glycollate pathway. In 1 mM H14CO3- net photosynthesis is reduced and only little influenced by external glyoxylate, glycine, or serine (control: 8.1 μmoles of CO2/mg of chlorophyll × h; glyoxylate: 7.0; glycine: 8.4; serine: 10.6). Label in glycine (control: 14% of the total amino acids) is reduced in the presence of 0.01 M serine, glycine, or glyoxylate (5.3, 6.3, and 6.7%, resp.). It is suggested that the glycollate pathway might exert a regulatory effect on the net rate of photosynthesis. An increase of net photosynthesis in the presence of glyoxylate could never be observed.