The stimulation of post-illumination ATP synthesis by valinomycin

Abstract

A high-energy state or intermediate (XE) is formed in illuminated chloroplasts which can drive ATP formation in a dark reaction [l] . Since the characteristics of X, and of the light-induced H+ uptake [2] are similar, it has been proposed that XE is a gradient in H+ activity across chloroplast membranes. This view is substantiated by the finding that ATP formation may be driven in total darkness by an artificiallyproduced H+ gradient [3]. Nelson et al. [4] reported that the treatment of lettuce chloroplasts with digitonin inhibited H+ uptake and post-illumination ATP synthesis more severely than photophosphorylation. In our previous studies on phosphorylation in subchloroplast particles [ 5,6] , we concluded that H+ uptake in these particles was electrogenic and, furthermore, that a membrane potential might play a role in photophosphorylation. To test this idea further, the effects of valinomycin and K+ on post-illumination ATP synthesis and H+ uptake in subchloroplast particles were tested. These reagents were found previously [7] to enhance H+ uptake in R.rubrum chromatophores. In this paper, we report that valinomycin in the presence of K+ stimulated H+ uptake as well as the formation of X, in subchloroplast particles.