Uptake and reduction of glycerate by isolated chloroplasts

Abstract

Intact chloroplasts of spinach (Spinacia oleracea L.) evolve O2 in the light in a glycerate-dependent reaction at rates usually close to 10 μmolxmg(-1) chlorophyllxh(-1). Glycerate isfirst phosphorylated and the resulting phosphoglycerate reduced to the sugar level. Products of the reaction are the intermediates of the Calvin cycle and glycolate. The ratio of triosephosphates to phosphoglycerate is higher under low light or at a low pH than under high light or at a high pH. Chloroplasts contain activities of glycerate kinase which approximately correspond to observed glycerate reduction rates at light saturation. The main part of the glycerate kinase of leaf cells is localized in the chloroplasts, but considerable activity also resides outside these organelles. Glycerate can enter intact chloroplasts of spinach as the anion and the undissociated acid. It can thus mediate indirect proton transfer across the chloroplast envelope. In the presence of slowly permeating cations it is taken up mainly in an anion exchange reaction. Chloride and acetate anions permeate faster than the glycerate anion. The relation between glycerate reduction and photorespiration is discussed.