STARCH DEGRADATION IN CHLOROPLASTS ISOLATED FROM C-3 OR CAM (CRASSULACEAN ACID METABOLISM)-INDUCED MESEMBRYANTHEMUM-CRYSTALLINUM L

C-3 or crassulacean acid metabolism (CAM)-induced Mesembryanthemum cry stallinum plants perform nocturnal starch degradation which is linear with time. To analyse the composition of metabolites released by isola ted leaf chloroplasts during starch degradation we developed a protoco l for the purification of starch-containing plastids. Isolated chlorop lasts from C-3 or CAM-induced M. crystallinum plants are also able to degrade starch. With respect to the endogenous starch content of isola ted plastids the rate of starch degradation in these organelles is clo se to the observed rates of starch degradation in intact leaves. The c ombined presence of P-1, ATP, and oxaloacetate is identified to be the most positive effector combination to induce starch mobilization. The metabolic flux through the oxidative pentose-phosphate pathway in chl oroplasts isolated from CAM-induced M. crystallinum is less than 3.5% compared with other metabolic routes of starch degradation. Here we re port that starch-degrading chloroplasts isolated from CAM-induced M. c rystallinum plants use exogenously supplied oxaloacetate for the synth esis of malate. The main products of starch degradation exported into the incubation medium by these chloroplasts are glucose 6-phosphate, 3 -phosphoglyceric acid, dihydroxyacetone phosphate and glucose. The ide ntification of glucose 6-phosphate as an important metabolite released during starch degradation is in contrast to the observations made on all other types of plastids analysed so far, including chloroplasts is olated from M. crystallinum in the C-3 state. Therefore, we analysed t he transport properties of isolated chloroplasts from M. crystallinum. Surprisingly, both types of chloroplasts, isolated from either C-3 or CAM-induced plants, are able to transport glucose 6-phosphate in coun ter exchange with endogenous P-i, indicating the presence of a glucose 6-phosphate translocator as recently demonstrated to occur in other t ypes of plastids. The composition of metabolites released and the stim ulatory effect of oxaloacetate on the rate of starch degradation are d iscussed with respect to the acidification observed for CAM leaves dur ing the night.