Regulation of adenylate levels in intact spinach chloroplasts

Adenylate levels and oxygen evolution in isolated intact spinach chloroplasts were measured in the presence of nitrite or oxaloacetate, which do not consume ATP during reduction, and of bicarbonate and glycerate the reduction of which requires ATP. The following results were obtained: 1. In chloroplasts incubated in the dark with nitrite, oxaloacetate or bicarbonate, the concentration of AMP was high and that of ATP low. The concentration of ADP was usually but not always lower than that of AMP. Light induced a rapid increase in the level of ATP which was accompained by a marked decrease in AMP and a slight decrease in ADP. The ratio of ATP/ADP rarely increased above 3 in the light, even in the absence of ATP consumption. Glycerate differed from other substrates in that it drastically decreased the chloroplast ATP level in the dark and kept its concentration low, even under illumination. The rate of glycerate-dependent oxygen evolution was high. 2. Mass action ratios (ATP) (AMP)/(ADP)(2) were close to 0.5 in the dark and in the presence of various substrates, except glycerate, indicating that adenylates were close to adenylate kinase equilibrium. When CO2 served as the substrate, (ATP) (AMP)/(ADP)(2) values were often higher than 0.5 in the light. In the presence of glycerate, the values were always much below 0.5. 3. The energy charge [2(ATP)+(ADP)]/2[(ATP) +(ADP)+(AMP)] of the chloroplast adenylate system was between 0.27 and 0.6 in the dark. It increased, but scarcely exceeded 0.8, in the light. Energy charge was usually higher with bicarbonate than with phosphoglycerate, and always low with glycerate, both in the dark and in the light.