DANIEL I. ARNON 505 



7. Catalysts of Photosynthetic Phosphorylation 



The most significant conclusion drawn from the recent investiga- 

 tions with isohited chloroplasts was that photosynthetic phosphoryhi- 

 tion rejircsents a major mechanism for converting light into useful 

 chemical energy independently of COo assimilation. We shall now 

 discuss what has been learned about the mechanism and the physio- 

 logical significance of this direct conversion of light into pyrophos- 

 jihate bond energy. 



The first factors which were found to stimulate cyclic photophos- 

 phorylation without themselves being consumed in the reaction were 

 magnesium ions and ascorbate (11, 15): the next to be recognized 

 were FMN and vitamin K compounds (170, 16). 



In searching for factors of photosynthetic phosphorylation by 

 isolated chloroplasts special attention was given to normal constituents 

 of chloroplasts and green leaves. Magnesium and ascorbate have long 

 been known to be present in chloroplasts (124) . FMN is widely 

 distributed in green leaves (134) . Ohta and Losada in our labora- 

 tory (unpub.) have found FMN to be a regular constituent of chloro- 

 plasts. Of unusual interest, however, was the antihemorrhagic fac- 

 tor, vitamin K, which occupied, since its discovery in plants, a unique 

 position among other vitamins in being specifically associated with 

 chloroplasts^ (40) . Moreover, Martins and others have recently as- 

 signed a role to vitamin K in oxidative phosphorylation (103, 104; cf. 

 10, review) . 



The working hypothesis first formulated to account for the role 

 of cofactors in photosynthetic phosphorylation visualized the light 

 reaction as resulting in the simultaneous generation, through a photo- 

 lysis of water, of a reducing agent (H) and an oxidizing agent (OH) 

 at opposite ends of an electron transport chain; vitamin K, FMN, 

 ascorbate, and cytochromes were tentatively proposed as members of 



^Bishop, who earlier presented evidence that vitamin K is an essential factor 

 for the photochemical activity of isolated chloroplasts (27), has reported in a 

 more recent publication (28) that spinach chloroplasts do not contain the naph- 

 thoquinones of the vitamin K type but contain instead the benzoquinone (X-2v)i) 

 ("plastoquinone"). which Crane (39) and Folkers and his associates {\:A) found in 

 green tissues and which Crane also found to be specifically concentrated in chloro- 

 plasts (39). Bishop (28) has reported that Q-255 activates the Hill reaction. The 

 role of 0-255 in photosynthetic phosphorylation is still unknown, but it should 

 be noted that from the standpoint of the mechanism of photosynthetic piiosphory- 

 lation (see next section), either a naphthoquinone of the vitamin K type or a 

 benzoquinone would appear suitable as an electron carrier in the process. A clari- 

 fication of the disagreement between the earlier reports of vitamin K distribution 

 in chloroplasts (40) and the recent reports on Q-255 will be awaited. 



