Cyclic and non-cyclic photophosphorylation in chloroplasts distinguished by use of labeled oxygen

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Abstract
Oxygen isotope exchanges during photophosphorylation by spinach chloroplasts have been followed with a mann-spectrometer. O2 exchange is equivalent to or greater than phosphate uptake when flavin mononucleotide or menadione is used as the catalyst if one assumes a P/O ratio of 1.0. With phenazine methosulfate or pyocyanine as the catalyst, phosphate uptake is from 5 to 15 times as great as the O2 exchange. It is concluded that flavin mononucleotide and menadione catalyze a non-cyclic phosphorylation in which molecular O2 is the final electron acceptor, and that phenazine methosulfate and pyocyanine catalyze a cyclic phosphorylation in which molecular O2 plays no part.