Light-driven ATP formation from 32Pi by chloroplast thylakoids without detectable labeling of ADP, as measured by rapid mixing and acid quench techniques.

The labeling of ATP and ADP in the first few milliseconds of exposure of chloroplast thylakoids to light, 32Pi, and ADP has been measured. At least 4 mol of ATP/mol of coupling factor 1 (CF1)-ATPase can be formed without detectable labeling of membrane-bound or free ADP. Such results are consistent with ADP and not AMP as the primary acceptor of Pi in photophosphorylation. Evidence is presented demonstrating that quenching with perchloric acid, as used in these and earlier experiments, is satisfactory for the measurement of the amount and nature of membrane-bound nucleotides.