The uncoupling of photophosphorylation by valinomycin and ammon-ium chloride.

1. Ammonium chloride is a poor uncoupler of photophosphorylation in subchloroplast particles. In the presence of valinomycin the inhibition of photophosphorylation by NH&l is greatly enhanced. 2. The enhancement of the ammonium chloride uncoupling of phosphorylation by valinomycin was less pronounced in chloroplasts than in subchloroplast particles. 3. The light-dependent uptake of ammonium ions by chloroplasts was abolished by valinomycin which also caused a rapid release of the ions accumulated during illumination. The uptake of ammonium ions in subchloroplast particles was only 5 to 10% of that in chloroplasts, whereas phosphorylation in subchloroplast particles was generally about 50% of that in chloroplasts. 4. The light-dependent uptake of hydrogen ions by chloroplasts, assayed in the presence or absence of ammonium chloride, was not affected by valinomycin. In subchloroplast particles, valinomycin had no effect on hydrogen ion uptake in the absence of ammonium chloride. In the presence of this uncoupler, valinomycin enhanced hydrogen ion uptake. 5. The uncoupling of ferricyanide-dependent phosphorylation in subchloroplast particles by ethylamine was not affected by valinomycin. In the presence of valinomycin, the inhibition of phosphorylation by ammonium chloride was partly relieved by ethylamine. 6. These results are interpreted in view of the chemiosmotic theory of oxidative phosphorylation. It is proposed that phosphorylation in chloroplasts is dependent upon a hydrogen ion gradient. In subchloroplast particles, however, a membrane potential may be required for adenosine triphosphate synthesis.