Adenosine Triphosphatase Activity in Chloroplasts.

Abstract

Two distinct systems with ATP-ase activity are observed in spinach chloroplasts. They are different in their pH optima and their response to MgCl2 and chlorpromazine. At pH 7.5 the ATP-ase activity is rather constant in different chloroplast preparations. It is strongly stimulated by MgCl2 and inhibited by chlorpromazine. The pyro-phosphatase activity at pH 7.5 is very low under the conditions of the ATP-ase activity measurements. Thus it can be excluded that the release of phosphate from ATP at pH 7.5 involves a liberation and a subsequent hydrolysis of inorganic pyrophosphate. Dinitrophenol does not stimulate the ATP-ase activity, either in intact or in broken chloroplasts. At pH 5.5 a variable ATP-ase activity is found. It is unaffected by MgCl2 and chlorpromazine and parallels the pyrophos-phatase activity at this pH. Results of kinetic experiments on the Mg++ stimulated ATP-ase activity of spinach chloroplasts are given. It is discussed whether the ATP-ase activity of chloroplasts has a functional relationship to the enzymatic mechanism by which phos-phorylation of ADP is coupled to electron transport during light-induced phosphorylation.