Properties of ATPase in chloroplasts

1. 1. Mg²⁺-ATPase (light-triggered Mg²⁺-dependent ATPase) activity in chloroplasts was stimulated by atebrin, NH₄Cl and gramicidin when the uncouplers were added after light triggering. The stimulation was followed in time by inhibition when the reaction took place in the dark. 2. 2. This inhibition of Mg²⁺-ATPase activity was overcome when the reaction was carried out under continuous illumination. 3. 3. The energy transfer inhibitors of photophosphorylation, phlorizin and Dio-9, inhibited Mg²⁺-ATPase activity and the extent of inhibition increased with time. The inhibition by Dio-9 was partially reversed by light while that of phlorizin was not. 4. 4. Light-triggered ATP-P_i exchange activity in chloroplasts was inhibited by both atebrin and phlorizin. The extent of the inhibition increased with time. 5. 5. The activity of a soluble Ca²⁺-ATPase was inhibited by Dio-9, phlorizin, NH₄Cl and atebrin. The kinetics of activity was linear with time, except in the presence of phlorizin. 6. 6. These results are interpreted as indicating a requirement of a high-energy state for triggering and maintenance of Mg²⁺-ATPase and ATP-P_i exchange reactions. The relation of ATPase activity to the coupling mechanism in chloroplasts is discussed.