Role of Glycine and Glyoxylate Decarboxylation in Photorespiratory CO2 Release

Abstract

Mechanically isolated soybean leaf cells metabolized added glycolate by two mechanisms, the direct oxidation of glyoxylate and the decarboxylation of glycine. The rate of glyoxylate oxidation was dependent on the cellular glyoxylate concentration and was linear between 0.58 and 2.66 micromoles glyoxylate per milligram chlorophyll. The rate extrapolated to zero at a concentration of zero. The concentration and, therefore, the rate of oxidation of glyoxylate could be decreased by adding glutamate or serine to the cells. These substrates were amino donors for the transamination of glyoxylate to glycine. In the presence of these amino acids more CO₂ was released from added glycolate via the glycine decarboxylation reaction and less by the direct oxidation of glyoxylate. Leaves from soybean plants of various ages grown under different nitrogen regimes had glyoxylate concentrations of about 80 to 100 nanomoles per milligram chlorophyll. Using the isolated cells as a model to determine the relationships between the glyoxylate concentration and rate of its decarboxylation indicated that about 2.5% of the photorespiratory CO₂ would arise from this reaction. This percentage would not be expected to vary greatly with growth conditions.