MEDIUM ADP AND NOT ADP ALREADY TIGHTLY BOUND TO PHYLAKOID MEMBRANES FORMS INITIAL ATP IN CHLOROPLAST PHOSPHORYLATION

Measurements were reported on the source of the ADP and .gamma.-phosphoryl moieties of the initial ATP formed when chloroplast thylakoid membranes are energized by an acid-base transition. Millisecond mixing and quenching experiments demonstrate that most or all of the initial ATP, formed in amounts considerably less than the amount of CF1[chloroplast fragment 1]-ATPase present, arises from medium ADP and medium Pi. With no or low amounts of added medium ADP, the tightly-bound ADP present in thylakoid membranes is released to the medium on energization, then subsequently forms ATP. These results rule out the possible conversion of the ADP tightly-bound to CF1-ATPase to tightly-bound ATP as a step in the main pathway of chloroplast phosphorylation. Less complete experiments indicate a similar behavior of tightly-bound ADP of submitochondrial particles.