Pyridoxal 5-phosphate, Phenyl Phosphate and Acetyl Phosphate, as Inhibitors of Photophosphorylation Competitive with Phosphate1

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Abstract
Pyridoxal 5-phosphate, phenyl phosphate and acetyl phosphate, as well as rβ-naphthyl monophosphate, inhibited photophosphorylation of spinach chloroplasts competitively with Pi and noncompetitively with ADP. The apparent dissociation constant of the inhibitor-enzyme complex (Ki) values of pyridoxal 5-phosphate, phenyl phosphate and acetyl phosphate for the Pi site were 1.1, 3.8 and 2.4 mM, respectively. These organic phosphates inhibited Ca2+-ATPase of the isolated coupling factor 1 (CF1) (EC 3.6.1.3) noncompetitively with ATP. AMP, creatine phosphate, fructose 1,6-bisphosphate, glucose 6-phosphate, 3-phosphoglyceric acid, ribose 5-phosphate and PPi did not significantly inhibit photophosphorylation. Like rβ-naphthyl monophosphate, pyridoxal 5-phosphate and phenyl phosphate inhibited photophosphorylation and the coupled electron transport, but were almost without effect on the basal electron transport. On the other hand, acetyl phosphate considerably inhibited photophosphorylation, but had almost no effect on the coupled electron transport rate and the basal rate. The results suggest that these organic phosphates inhibit photophosphorylation by binding at the Pi site on the active center of CF1 and that their binding inhibits the ATPase activity of isolated CF1. These four organic phosphates which inhibited photophosphorylation competitively with Pi could not substitute for ADP or ATP in inhibiting ferricyanide photoreduction by decreasing H+-permeability through CF1 and in protecting the ATPase of isolated CF1 against cold-anion inactivation.