Effect of pyridine on the light-induced pH rise and postillumination ATP synthesis in chloroplasts

Pyridine reduces the rate of formation and the rate of decay of the reversible light-dependent pH change in isolated chloroplasts. Under the same conditions the amount of protons taken up by the chloroplast is markedly (up to 20-fold) increased. The postillumination formation of ATP (X_E) is also markedly increased. Concentrations of pyridine which stimulate appreciably the extent of the pH increase, inhibit only slightly the formation of ATP in the conventional one-step assay. Pyridine causes a large light-dependent absorbancy decrease. The effects of pyridine can be accounted for by assuming that it diffuses into the chloroplast as the uncharged moiety and binds protons inside the chloroplast thylakoids. This action increases the "buffer capacity" of the chloroplast and causes an increase in pH and X_E. The effects of pyridine differ significantly from those of other amines presumably because of the low pK_a of the former. The bearing of the present results on the chemiosmotic theory of phosphorylation is discussed.