Rates and properties of endogenous cyclic photophosphorylation of isolated intact chloroplasts measured by CO 2 fixation in the presence of dihydroxyacetone phosphate

Abstract

1. Dihydroxyacetone phosphate in concentrations ⩾ 2.5 mM completely inhibits CO 2-dependent O 2 evolution in isolated intact spinach chloroplasts. This inhibition is reversed by the addition of equimolar concentrations of P i, but not by addition of 3-phosphoglycerate. In the absence of P i, 3-phosphoglycerate and dihydroxyacetone phosphate, only about 20% of the 14C-labelled intermediates are found in the supernatant, whereas in the presence of each of these substances the percentage of labelled intermediates in the supernatant is increased up to 70–95%. Based on these results the mechanism of the inhibition of O 2 evolution by dihydroxyacetone phosphate is discussed with respect to the function of the known phosphate translocator in the envelope of intact chloroplasts. 2. Although O 2 evolution is completely suppressed by dihydroxyacetone phosphate, CO 2 fixation takes place in air with rates of up to 65μ mol · mg −1 chlorophyll · h −1. As non-cyclic electron transport apparently does not occur under these conditions, these rates must be due to endogenous pseudocyclic and/or cyclic photophosphorylation. 3. Under anaerobic conditions, the rates of CO 2 fixation in presence of dihydroxyacetone phosphate are low (2.5–7 μmol · mg −1 chlorophyll · h −1), but they are strongly stimulated by addition of dichlorophenyl-dimethylurea (e.g. 2 · 10 −7 M) reaching values of up to 60 μmol · mg −1 chlorophyll · h −1. As under these conditions the ATP necessary for CO 2 fixation can be formed by an endogenous cyclic photophosphorylation, the capacity of this process seems to be relatively high, so it might contribute significantly to the energy supply of the chloroplast. As dichlorophenyl-dimethylurea stimulates CO 2 fixation in presence of dihydroxyacetone phosphate under anaerobic but not under aerobic conditions, it is concluded that only under anaerobic conditions an “overreduction” of the cyclic electron transport system takes place, which is removed by dichlorophenyl-dimethylurea in suitable concentrations. At concentrations above 5 · 10 −7 M dichlorophenyl-dimethylurea inhibits dihydroxyacetone phosphate-dependent CO 2 fixation under anaerobic as well as under aerobic conditions in a similar way as normal CO 2 fixation. Therefore, we assume that a properly poised redox state of the electron transport chain is necessary for an optimal occurrence of endogenous cyclic photophosphorylation. 4. The inhibition of dichlorophenyl-dimethylurea-stimulated CO 2 fixation in presence of dihydroxyacetone phosphate by dibromothymoquinone under anaerobic conditions indicates that plastoquinone is an indispensible component of the endogenous cyclic electron pathway.