The Glyceraldehyde 3-Phosphate and Glycerate 3-Phosphate Shuttle and Carbon Dioxide Assimilation in Intact Spinach Chloroplasts

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Abstract
The regulation of CO2 assimilation by intact spinach (Spinacia oleracea) chloroplasts by exogenous NADP-linked nonreversible d-glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.9) was investigated. This dehydrogenase mediated a glyceraldehyde 3-phosphate/glycerate 3-phosphate shuttle for the indirect transfer of NADPH from chloroplast to the external medium. The rate of NADPH formation in the medium reflected glyceraldehyde 3-phosphate efflux from the chloroplast. Increasing enzyme concentrations stimulated NADP reduction and, in turn, CO2 fixation. Pyrophosphate increased CO2 fixation by apparently inhibiting glyceraldehyde 3-phosphate efflux. Increasing the glycerate 3-phosphate concentration above 0.1 mm stimulated glyceraldehyde 3-phosphate efflux but inhibited CO2 fixation. Addition of up to 0.5 mm orthophosphate enhanced both glyceraldehyde 3-phosphate efflux and CO2 fixation while each was inhibited by higher orthophosphate concentrations. The mechanism by which the extent of glyceraldehyde 3-phosphate efflux regulated the rate of CO2 fixation in chloroplasts was discussed.

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