Further Studies on Chloroplast Adenosine Triphosphatase Activation by Acid-Base Transition

Data are presented on some further characteristics of the chloroplast ATP hydrolysis induced by a combination of sulfhydryl compounds and a transition from acid to base. These studies include questions of substrate specificity and stoichiometry of the hydrolysis, Mg++ and dithiothreitol concentration requirements, the effect of uncouplers, and the need for a distinct pH difference between the two stages. The evidence is consistent with a close similarity between the acid-base-induced ATPase and the light-requiring counterpart, and in turn suggests that a proton gradient formed in the light may be the basis for the light-induced ATPase activation. It was found that the acid to base transition induced a 32Pi-ATP exchange reaction as well as ATP hydrolysis. This exchange is not activated by sulfhydryl compounds, and its rate is only a small fraction of that of ATP hydrolysis. The significance of this finding for a postulated sequence for ATP hydrolysis is discussed.