[Studies on the Light-induced ATP-Pi Exchange Reaction with Spinach Chloroplasts]

Light induced the formation of an energy pool in chloroplast fragments which was emptied within 20-30 min in the dark, while an ATP-Pi exchange was going on.The exchange reaction was dependent on Mg(2+) and was inhibited by NH 4 (+) but not by ADP. The optimum pH in the light and in the dark stage of the reaction lay between pH 7.5 and 8.The high energy intermediate was formed only in the presence of DTT in the light stage; phosphate had no influence on the formation of the intermediate in that period, but increased its stability in the dark.The relation between both high energy intermediates, that of the light-induced ATP-Pi exchange reaction and that of the light-induced ATP synthesis, is discussed in its meaning for the hypothesis of chemiosmotic phosphorylation.