Causes for the Disappearance of Photosynthetic CO2 Fixation with Isolated Spinach Chloroplasts

When isolated spinach chloroplasts are illuminated, photosynthesis and CO(2) fixation die off within 30 to 90 minutes. Even when air levels of CO(2) are used which maintain high and rate-saturating amounts of ribulose 1,5-bisphosphate inside the plastids, CO(2) fixation declines. The decline begins with a drop in activity of the ribulose 1,5-bishosphate carboxylase/oxygenase, specifically loss of the enzyme-activator CO(2)-Mg(2+) form. Next, the light reactions cause gradual leakage of the carboxylase and other stromal proteins to the suspending medium. The chloroplast outer envelope appears to reseal and protect the thylakoids since there is little change in the ferricyanide-dependent Hill reaction. In the dark, under otherwise identical conditions, leakage of carboxylase does not occur.