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

Abstract

Chloroplasts were triggered by light in the presence of dithiol reagents to catalyze an ATP-Pi exchange reaction in a following dark period. This light-triggered ATP-Pi 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-Pi 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-Pi 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-Pi exchange activity but had no light-triggered ADP-ATP exchange.