Determination of pH in chloroplasts. I. Distribution of ( 14 C) methylamine

Light-dependent redistribution of ions in suspensions of chloroplast thylakoid membranes

Ion movements associated with the pH rise that is observed upon illumination of thylakoid suspensions at low pH have been studied by a multiparameter technique. Light-dependent, dark-reversible fluxes of H(+), Cl(-), Na(+), K(+) and divalent cations were monitored, together with simultaneous changes in the optical density of the suspension. Extensive uptake of Cl(-) and efflux of Mg(2+) accompany the apparent inward movement of H(+) in the light. Only minor efflux of K(+) is seen and Na(+) appears immobile. The Cl(-) and Mg(2+) fluxes together compensate for most of the charge transferred as H(+), contributing respectively about 49% and 43% on an equivalent basis. The ratio of Cl(-) influx to Mg(2+) efflux is variable, but usually >1.0. The Mg(2+) flux can be supplanted by (1) K(+) flux, if the K(+)/Mg(2+) activity ratio in the suspension is high, and (2) Ca(2+) flux, if the thylakoids are equilibrated with suspending media containing Ca(2+). The affinity of the divalentcation-binding sites, or carrier mechanism, is greater for Ca(2+) than for Mg(2+). Schemes can be drawn up to account for the observed ion movements on the basis of either a chemical or a chemiosmotic mechanism for energy transduction in chloroplasts. In intact chloroplasts, light-dependent control of Mg(2+) distribution between thylakoid and stroma could serve to regulate enzyme activities in the carbon fixation pathway, and hence photosynthesis.

Proton-coupled accumulation of galactoside in Streptococcus lactis 7962

When cells of the anaerobe Streptococcus lactis 7962 are deprived of their normal fermentable energy source, active transport of galactosides is completely abolished although the membrane carriers are still capable of facilitating the equilibration of sugars across the cell membrane. In these nonmetabolizing cells it was possible to test the Mitchell hypothesis of obligatory coupling of proton movement with sugar transport. This hypothesis was supported by alkalinization of the medium observed when thiomethylgalactoside was added to a lightly buffered suspension of S. lactis cells. Conversely, addition of protons resulted in active transport of thiomethylgalactoside. Accumulation of thiomethylgalactoside to a concentration more than 20-times that in the external medium was induced by suddenly exposing cells to a medium at pH 6; no accumulation of thiomethylgalactoside was observed with cells exposed to pH 8.Active transport of thiomethylgalactoside occurred in the absence of energy metabolism when S. lactis cells were treated with valinomycin. This ionophore allowed intracellular K(+) to flow out, thus imposing a membrane potential (inside negative). This potential resulted in a proton uptake and an associated active transport of galactoside. The membrane potential was measured from the distribution ratio (inside/outside) of K(+) in the presence of valinomycin. The pH gradient was measured from the distribution ratio of [(14)C]methylamine. The protonmotive force, calculated from the membrane potential and the pH gradient, was found to be directly related to the accumulation of galactoside, in accordance with the chemiosmotic hypothesis.