II. Lokalisation von Enzymen des reduktiven und dem oxydativen Pentosephosphat-Zyklus in den Chloroplasten und Permeabilität der Chloroplasten-Membran gegenüber Metaboliten

Abstract

1. The interrelationship of metabolic activities in chloroplasts and cytoplasm of leaf cells of spinach, sugar beet and Elodea has been investigated. Different methods have been adopted to study the intracellular localization of enzymes and the flow of phosphorylated intermediates across the chloroplast membrane. The flow of substrates was investigated by determining the rates of the conversion of substrates added to aqueously isolated chloroplasts, prior to and after destruction of the outer chloroplast membrane. The observed differences yielded information as to whether a substrate could traverse the chloroplast membrane. Two methods mere used to investigate the localization of enzymes : a) The percentage distribution of photosynthetic and respiratory enzymes in chloroplasts and cytoplasm was calculated from data on enzyme activities in non-aqueous cell fractions. b) Low levels of enzymes in chloroplasts in the presence of high cytoplasmatic levels were detected by assaying enzyme activities in preparations of aqueously isolated chloroplasts prior to and after ultrasonic destruction of the outer chloroplast membrane. 2. If chloroplasts are isolated in aqueous sucrose buffer, their outer membranes act as an efficient barrier against the penetration of NADP, RuDP, GAP and, in some but not all experiments, of FMP and GMP. PGA, DHAP and, probably to a lesser extent, aspartate, ɑ-ketoglutarate, oxaloacetate and FDP can traverse this membrane. Chloroplast membranes are significantly altered when isolated in NaCI-buffer systems and do not correspond to the in vivo situation. 3. The conversion of Ri-5-P to RuDP occurs exclusively or nearly exclusively in the chloroplasts indicating that phosphoribulokinase and/or ribosephosphate isomerase are located only there. 4. The conversion of Ri-5-P to GAP and SuMP, which is catalyzed by the enzymes ribosephosphate isomerase, xylulosephosphate epimerase and transketolase, proceeds likewise only or at least predominantly in the chloroplasts and not, or only to a small extent, in the cytoplasm. 5. The major parts of glucose-6-phosphate dehydrogenase and of 6-phosphogluconate dehydrogenase reside in the cytoplasm. However, a small, but significant, level of these enzymes is to be found also in the chloroplasts. Hexokinase and transaldolase are also present there. Pyruvate kinase and phosphofructokinase appear to be absent from chloroplasts. 6. Since, with the presence of glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, hexokinase, transaldolase and enzymes of the Calvin cycle, the enzymic machinery of the oxidative pentose phosphate pathway is complete in the chloroplasts, the results suggest that chloroplasts are engaged in the oxidative decomposition of carbohydrates. 7. In the dark the oxidative pentose phosphate pathway requires the control of NADPH formation and the transfer of hydrogen across the chloroplast membrane. 8. The available data on the intracellular localization of enzymes and on the kinetics of the distribution of labelled intermediates show that the photosynthetic carbon cycle operates exclusively within the chloroplasts. There is nothing to suggest that enzymes of chloroplasts and cytoplasm cooperate in the cyclic regeneration of the carbon acceptor molecule. However, the existence of phosphorylated transport metabolites suggests that secondary reactions of photosynthesis such as sucrose and amino acid synthesis, which proceed, at least in part, outside the chloroplasts, are directly linked with chloroplastic reactions by activated (phosphorylated) intermediates.