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

Since it was first reported that isolated chloroplasts utilize light energy in the formation of ATP5 (2) a number of catalysts of the reaction have been described (3). It was thought that these catalysts functioned as electron carriers in a cyclic process which does not involve molecular oxygen. The proposed process consisted of their reduction by a photochemiiically produced reductant and their subsequent reoxidation, in a series of phosphorylation linked reactions, by the associated photochemically produced oxidaant (1). Phosphorylation was attributed to the reoxidation of the reduced catalyst on the basis of an analogy to oxidative phosphorylation in mitochondria. The non-participation of molecular oxygen in the cycle was postulated partly on the basis of an analogy to photophosphorylation in the chromatophores of Rhodospirillum (9) and partly on the basis of two experimental observations. These observations were A: the fact that phosphorylation by chloroplasts was independent of exogenous oxygen, especially at high concentrations of catalysts such as FMN (21), and B: the fact that experiments with labeled oxygen revealed none of the exchange which would result from its continuous production and consumption (14). When it was discovered that phosphorylation also could be coupled to non-cyclic reactions such as the photochemical reduction of ferricyanide and TPN + by chloroplasts (3, 4) a re-evaluation of the earlier concept of cyclic phosphorylation became necessary. In the absence of any reoxidation process phosphorylation still occurred and consequently the site of phosphorylation (if only one) had to be assigned to the