HILL REACTION AND ITS RELATIONSHIP TO PHOTOPHOSPHORYLATION 425 



obtained in a somewhat different way, and they demonstrate clearly the 

 need for CO2 in the photoreduction of trichlorophenol indophenol, a 

 different Hill reagent from the quinone and ferricyanide employed by 

 Warburg. The demonstrated need for COo in all Hill reactions examined 

 cannot easily be explained in terms of other, previously described CO.,- 

 fixing reactions, nor can it easily be explained away as an artifact or side 

 effect, 



I have already indicated the advantages gained from adding Warburg's 

 mechanism for the Hill reaction to the schemes we have been using to 

 represent the process of photophosphorylation. The diagram shown in 

 Scheme 8 represents our present working hypothesis regarding the manner 

 in which the Hill reaction operates in grana, and the locus of the ATP- 

 generating phosphorylation site in relation to the other reactions. The 

 diagram is subject to amplification and modification. We may speculate 

 that X might include cytochrome /. This would be in keeping with 

 Kamen's first postulated chemical step after photon absorption [57]. (See 

 also the paper by Kamen in the present volume.) One might, in fact, 

 borrow Kamen's first step and insert this initial step for the oxidation- 

 reduction between cytochrome and chlorophyll directly into Scheme 8, 

 equating R with reduced chlorophyll, and OX with oxidized cytochrome. 

 Two such reactions would be required, however, for the two electron 

 change shown. Scheme 8 has been drawn to show a second photon- 

 requiring step at the stage where the peroxide of carbonate splits out 

 oxygen. Warburg has not stated explicitly [31] which of the reactions in 

 Scheme 7 require photons, but it seems reasonable to conclude that the 

 second step in Scheme 7 would require light. As Scheme 8 is drawn, the 

 dismutation of OR and X to R and OX would also require light. Two 

 different photon-requiring reactions would be in line with the phenomena 

 of the so-called "second" Emerson effect [57-60], which is possibly 

 related to the activation by blue light described by Warburg [61]. The 

 latter phenomenon appears to be a catalytic effect, however, whereas the 

 Emerson effect does not. In this connection one can speculate about the 

 possibility that the COo "sub-cycle" in Scheme 8 might to some extent 

 operate independently of the generation of R and OX. This does not 

 appear to be impossible. 



The relationship of phosphate to the COo "sub-cycle" requires some 

 special comment. Warburg has stated that in addition to CO.,, the Hill 

 reaction also requires a cataKtic amount of phosphate [31]. This is 

 apparently the basis on which he brings orthophosphate into the system to 

 participate in the formation of the precursor of O2. In our experiments on 

 the CO2 stimulation of the Hill reaction we found that added orthophos- 

 phate had little effect on the photoreduction reaction rate, but none of our 

 grana preparations was completely free of traces of orthophosphate, so 



