Acid/Base-Induced Exchange of Adenine Nucleotides on Chloroplast Coupling Factor (CF1)

Abstract

Abstract Exchange of CF1-bound adenine nucleotides was shown to take place when isolated broken chloroplasts were subjected to an acid/base treatment. This process was measured in forward (in­ corporation of 14C-labeled nucleotides) as well as back exchange reactions (release of bound labeled nucleotides). In the presence of unlabeled ADP and Pi a high precentage of labeled ATP was found to be released into the medium. Exchange is highly specific for exogenous ADP and ATP. Acid/base-induced adenylate exchange and acid/base-induced phosphorylation were measured in dependence of several external factors. The substrate constant for ADP in exchange was 1.6 μм, whereas an apparent Km of 5 μм was determined for ADP in the phosphorylation process. Acid/base-induced exchange of adenine nucleotides and acid/base-induced phosphorylation depend in a similar way on the pH values of the acid and base stages. Acid/base-induced ATP formation was sensitive to prolonged treatment in the acid stage whereas exchange was not. Mg2+ was strictly required in phosphorylation but less important in exchange. The phosphorylation ability decreased within seconds after acid/base transition; however, the ability of [14C]ADP incorporation persisted for minutes. The formation of ATP depended on an acid/base transition, whereas some adenylate exchange was also induced by an acid treatment alone. The results suggest that appropriate pH values at the inner and outer side of the thylakoid membrane rather than energy from a pH gradient is sufficient for adenylate exchange to take place on membrane-bound CF1.