Light-Triggered and Light-Dependent Atpase Activities in Chloroplasts

This chapter discusses the activities of light-triggered and light-dependent adenosine triphosphatase (ATPase) in chloroplasts. Chloroplasts prepared from leaves of plants such as lettuce or spinach catalyzes the hydrolysis of adenosine triphosphate (ATP) to adenosine diphosphate (ADP) and P_i at very low rates. However, in the presence of sulfhydryl reagents, such as cysteine, lipoic acid, or dithiothreitol, light triggers an ATPase activity in chloroplasts. This light-triggered and magnesium-dependent ATPase activity is maintained in the dark after light-triggering. Light also induces a calcium-activated, light-dependent ATPase activity which ceases as soon as the light is turned off. Both the light-triggered and the light-dependent ATPase activities represent a reversal of some of the reactions involved in the latter stages of photophosphorylation. ATPase activity is assayed either by a colorimetric determination of the P_i released from ATP or by the more sensitive determination of [³²P] P_i released from [³²P] ATP.