The photooxidation of glyoxylate by envelope-free spinach chloroplasts and its relation to photorespiration

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Abstract

Large quantities of CO2 are released within many photosynthesizing tissues in the light by the process of photorespiration. This CO2 arises largely from the carboxylcarbon atom of glycolate, which is synthesized in chloroplasts during photosynthesis. Glyoxylate is then produced by the glycolate oxidase reaction. The glyoxylate may be directly decarboxylated to CO2, but some investigators believe the glyoxylate must first be converted to glycine before CO2 is released during photorespiration. Spinach chloroplasts with their envelope membranes removed in dilute buffer solution have now been shown to carry out the oxidative decarboxylation of [1-14C]glyoxylate, in the presence of light and manganous ions in an atmosphere containing oxygen, to yield 1 mole each of 14CO2 and formate. Rates of enzymatic decarboxylation exceeding 50 μmoles of 14CO2 mg chlorophyll−1 hr−1 were obtained at pH 7.6; hydrogen peroxide is probably the oxidant in the reaction. Heated chloroplasts are inactive under the standard conditions and there is an almost absolute requirement for each of the components listed above. Conditions for some other nonenzymatic decarboxylations of glyoxylate have also been described. [1-14C]Glycine is decarboxylated by the enzymatic system at only 1% of the rate of [1-14C]glyoxylate. Maize chloroplast preparations are much less active than spinach chloroplasts. The high rates of CO2 produced by the spinach system directly from glyoxylate, as well as the need for light and oxygen, suggest that this reaction functions in photorespiration, and that CO2 arises during photorespiration without glycine as a mandatory intermediate.

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