Chloroplast Integrity and ATP-Dependent CO2 Fixation in Spinacia oleracea

Abstract
Washed whole chloroplasts of Spinacia oleracea isolated and assayed in a tris (hydroxymethyl aminomethane)-HCl buffered sucrose solution exhibited low dark CO2 fixing activity, whereas washed whole chloroplasts isolated in the same buffer but assayed in that buffer without sucrose exhibited much greater dark CO2 fixing activity. The lowered activity could be attributed to the impermeability of the chloroplast membrane to ribose-5-phosphate or ATP. The preservation of the integrity of the chloroplast membrane, as reflected by its impermeability in either or both of the above mentioned compounds, was measured by the fixation of 14CO2 into acid-stable products in the presence of ribose-5-phosphate and ATP by the whole chloroplast as compared with fixation by the chloroplast extract. An effect (i.e., apparent resistance to the passage of ribose-5-phosphate or ATP into the chloroplast) similar to, but less pronounced than, that produced by the presence of sucrose in the isolation medium was observed upon the addition of MnCl2 or CaCl2 to the buffered sucrose isolation medium. The addition of KC1 enhanced slightly the effect produced by addition of sucrose alone to the isolation medium. The presence of MgCl2 in the isolation medium, however, either caused the chloroplasts to become leaky or more fragile since more of the activity of the carboxylative phase enzymes appeared in the cytoplasm. When a mixture of all of the metal ions was added to the buffered sucrose suspending medium, the chloroplasts exhibited the same response observed with MgCl2 alone. The addition of EDTA or dithiothreitol appeared to alter the permeability of the chloroplast membrane nonspecifically when the assay was conducted in the absence of sucrose. Specific activities ([mu]moles CO2 fixed/mg chlorophyll x hr) as high as 329.6 have been observed for dark fixation by chloroplasts. The phosphoenolpyruvate carboxylase activity in the chloroplasts was only one-seventh that of ribulose diphosphate carboxylase. The phosphoenolpyruvate carboxylase activity in the cytoplasm was 5 times that of the chloroplasts.