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Abstract
The mechanism of induction of glycolate excretion by aminooxyacetate (AOA) during photosynthesis in Euglena gracilis Z has been studied enzymologically and metabolically. AOA inhibited glutamate: glyoxylate aminotransferase competitively with glutamate with Ki’s of 0.7 and 9.5 μm. Glycolate dehydrogenase and NADPH: glyoxylate reductase were also competitively inhibited by AOA with glycolate and glyoxylate, respectively. The Ki℉s were 1.2 and 0.4 mm, respectively. These inhibitions of the glycolate pathway enzymes could not be the immediate cause of the induction of the excretion. Intracellular concentrations of glycolate and glyoxylate were 10 and 3 nmol/mg chlorophyll in photosynthesizing Euglena. Addition of AOA to the cells caused increases of both metabolites to 13 and 7 nmol/mg chlorophyll, respectively. Glycolate dehydrogenase was strongly inhibited by glyoxylate competitively with glycolate. The Ki was 10 μm, less than one-fifteenth of the Km of glycolate dehydrogenase for glycolate. These results suggest that inhibition of glutamate: glyoxylate aminotransferase increases the glyoxylate concentration which inhibits glycolate oxidation and such a disturbance of an intracellular equilibrium of glycolate and glyoxylate levels makes a glycolate-excreting transporter operate.