Indophenols as Electron Donors and Catalysts of Photophosphorylation in Chloroplast Reactions.

Photophosphorylation by the chloroplast is postulated to require electron flow through the electron transport chain between the weak reductant of Light Reaction II and the oxidant of Light Reaction I (terminology according to Duysens, Amesz, and Kamp (1)). This investigation was initiated to find reagents that could react chemically as electron donors with components of the electron transport chain, thus making it possible to localize experimentally the site of phosphorylation. One promising reagent was reduced 2,6-dichlorophenolindophenol, since the oxidation of this reagent had previously been shown to be coupled to triphosphopyridine nucleotide reduction by the chloroplast (2,3). More recently, Arnon (4) and Losada, Whatley, and Arnon (5) have reported that adenosine triphosphate formation was coupled to the reduction of TPN by a similar system in which oxygen evolution was inhibited by pchlorophenyl-1 ,1-dimethylurea and electrons supplied by reduced 2,6-dichlorophenolindophenol, and that the stoichiometry between adenosine triphosphate and TPNH was unity. However, earlier work by Krogmann (see Fig. 3 of Reference 6) indicated that the ATP formation in this system may have been due entirely to cyclic phosphorylation catalyzed by the 2,6-dichlorophenol-indophenol (7-11). Therefore an investigation of the role of indophenol dyes as electron donors for chloroplast reactions was undertaken. The results of these experiments indicate that ATP formation observed in a system with indophenols as electron donors can be accounted for entirely by cyclic photophosphorylation. These results do not exclude a phosphorylation coupled to TPN reduction and the oxidation of indophenols, but such coupling cannot be directly shown. Very recent reports by Wessels (12) and Avron (13) indicate that they have reached similar conclusions.