A Light‐Triggered Adenosine Triphosphate‐Phosphate Exchange Reaction in Chloroplasts

Chloroplasts were triggered by light in the presence of dithiol reagents to catalyze an ATP‐P_i exchange reaction in a following dark period. This light‐triggered ATP‐P_i exchange was related to photophosphorylation, and to light‐triggered ATPase by the similarity in requirements for magnesium ions, phenazine methosulfate and flavin mononucleotide, and the inhibition by uncouplers and electron transport inhibitors. The ATP‐P_i exchange reaction showed substrate specificity for ATP and was competitively inhibited by ADP. The light‐triggered condition decayed in the dark with a half‐life of several minutes. The rate of the decay was not affected by uncouplers or energy transfer inhibitors. The light‐triggering phase required the presence of a dithiol‐reagent, and was stimulated by magnesium and electron transfer catalysts. The dark phase did not require the presence of dithiol reagents. The energy transfer inhibitors DIO‐9 and phloridzin as well as the uncoupler atebrin and the inhibitor n‐butyl‐3,5‐diiodo‐4‐hydroxybenzoate inhibited only the dark phase. The uncoupler, NH₄CI, and the inhibitor 4,5,6,7‐tetrabromo‐2‐trifluoromethylbenzinamidazol inhibited both the light‐triggering phase and the dark phase. The overall pH optimum for ATP‐P_i exchange was about 8. However, the pH optimum of the light stage alone was about 9 and that of the dark stage — about 7.5. Chloroplast fragments which retain their capacity for photophosphorylation possessed light‐triggered ATP‐P_i exchange activity but had no light‐triggered ADP‐ATP exchange.