No abstract

Aminooxyacetate induced excretion of glycolate from airgrown cells of Euglena gracilis in both air and 1% CO2 atmospheres. The rate of the excretion reached 70% of the photosynthetic rate in the air on a carbon basis, and was 10% in 1% CO2. The compulsory loss of photosynthetically fixed carbon as glycolate at the high rate in air in the presence of aminooxyacetate caused a decrease of the rate of synthesis of paramylon, the reserve polysaccharide. Analyses of the steady levels of photosynthetic intermediates showed that a decrease of the 3-phosphoglycerate level was the cause of the slow rate of paramylon synthesis under these conditions. The photosynthetic CO2 fixation enzyme, Rubisco,2 inevitably fixes O2 in the presence of 02. The resulting phosphoglycolate is hydrolyzed in the chloroplast to glycolate, the substrate of photorespiration. The rate of glycolate synthesis reaches 25 to 50% ofthe rate ofCO2 fixation in air atmosphere in the organisms, where Rubisco is the primary C02-fixing enzyme in photosynthesis (2,5,7,20).Glycolate is metabolized via the glycolate pathway in C3 higher plants (8 function of the glycolate pathway in these experiments. However, since any metabolite accumulated in excess in the presence of the inhibitors or in the mutants can affect growth and photosynthesis through influencing nonspecified steps (3), it is difficult for these studies to reach firm conclusions on the metabolic and physiologic functions ofthe metabolism. Elimination of the secondary effects of accumulated metabolites must be essential to obtain evidence for the hypotheses.Euglena and algal cells excrete glycolate, unlike C3 plants, when its metabolism is inhibited. We have found that AOA induces the excretion of glycolate with a small accumulation of glycolate and glyoxylate in the cells of E. gracilis (13,14). Therefore, the metabolic study of the effects of AOA on photosynthesis in Euglena may lead to an understanding of the metabolic relationship between the glycolate pathway and the PCR cycle. This paper describes our results.