Abstract
(14)CO(2) photoassimilation in the presence of MgATP, MgADP, and MgAMP was investigated using intact chloroplasts from Sedum praealtum, a Crassulacean acid metabolism plant, and two C(3) plants: spinach and peas. Inasmuch as free ATP, ADP, AMP, and uncomplexed Mg(2+) were present in the assays, their influence upon CO(2) assimilation was also examined. Free Mg(2+) was inhibitory with all chloroplasts, as were ADP and AMP in chloroplasts from Sedum and peas. With Sedum chloroplasts in the presence of ADP, the time course of assimilation was linear. However, with pea chloroplasts, ADP inhibition became progressively more severe, resulting in a curved time course. ATP stimulated assimilation only in pea chloroplasts. MgATP and MgADP stimulated assimilation in all chloroplasts. ADP inhibition of CO(2) assimilation was maximal at optimum orthophosphate concentrations in Sedum chloroplasts, while MgATP stimulation was maximal at optimum or below optimum concentrations of orthophosphate. MgATP stimulation in peas and Sedum and ADP inhibition in Sedum were not sensitive to the addition of glycerate 3-phosphate (PGA).PGA-supported O(2) evolution by pea chloroplasts was not inhibited immediately by ADP; the rate of O(2) evolution slowed as time passed, corresponding to the effect of ADP on CO(2) assimilation, and indicating that glycerate 3-phosphate kinase was a site of inhibition. Likewise, upon the addition of AMP, inhibition of PGA-dependent O(2) evolution became more severe with time. This did not mirror CO(2) assimilation, which was inhibited immediately by AMP. In Sedum chloroplasts, PGA-dependent O(2) evolution was not inhibited by ADP and AMP. In chloroplasts from peas and Sedum, the magnitude of MgADP and MgATP stimulation of PGA-dependent O(2) evolution was not much larger than that given by ATP, and it was much smaller than MgATP stimulation of CO(2) assimilation. Analysis of stromal metabolite levels by anion exchange chromatography indicated that ribulose 1,5-bisphosphate carboxylase was inhibited by ADP and stimulated by MgADP in Sedum chloroplasts.The appearance of label in the medium was measured when [U-(14)C] ADP-loaded Sedum chloroplasts were challenged with ATP, ADP, or AMP and their Mg(2+) complexes. The rate of back exchange was stimulated by the presence of Mg(2+). This suggests that ATP, ADP, and AMP penetrate the chloroplast slower than their Mg(2+) complexes. A portion of the CO(2) assimilation and O(2) evolution data could be explained by differential penetration rates, and other proposals were made to explain the remainder of the observations.