HILL REACTION AND ITS RELATIONSHIP TO PHOTOPHOSPHORYLATION 427 



the chloroplasts contain a large amount of the chemically related lipid Q255 

 or plastoquinone [19, 20]. This is bound in the grana structure, and Bishop 

 has shown that it is essential for the Hill reaction [19]. We have postulated 

 an associated role in the generation of high-energy phosphate. But it does 

 not play a role equivalent to the cofactor which must be added to elicit the 

 photophosphorylation. Examination of the soluble constituents of leaves 

 has shown, however, that the leaf does contain a substance, or substances, 

 which can function as excellent cofactors for photophosphorylation. Mv 

 associates, Drs. David Krogmann and Mary Stiller, are currently engaged 

 in a study of this "naturally occurring" cofactor. They find that though 

 this material is present almost exclusively in the supernatant when chloro- 

 plasts are centrifuged out of an aqueous medium, there is an appreciable 

 amount of it retained by chloroplasts isolated in non-aqueous medium 

 [62, 63]. Such chloroplasts do not carry out photophosphorylation. The 

 cofactor can be extracted from them and added back to an equivalent 

 amount of chlorophyll in the form of active chloroplasts. Calculated on 

 this basis, there is sufficient cofactor in the chloroplasts isolated in non- 

 aqueous medium to elicit photophosphorylation at at least one-quarter of 

 the maximum rate achieved with FMN or menadione. With larger amounts 

 of natural cofactor, the maximum photophosphorylation rate is as high as 

 the maximum achieved with FMN or menadione. The photophosphoryla- 

 tion with the natural cofactor is oxygen-dependent. In some respects its 

 behaviour suggests that it is an orthohydroquinone derivative. Such 

 substances are widelv distributed in leaves. Among them are the flavonoids 

 quercetin and catechin, and related compounds, and the ortho-dihy- 

 droxycinnamic acid derivative, cafi^eic acid, with the related depside, 

 chlorogenic acid. All of these substances, when tested, proved to be good 

 cofactors for photophosphorylation. It is probably in these groups of 

 compounds that we will find a substance or substances which might serve 

 in the leaf to elicit ATP formation. Before we understand completely how 

 the energy of the photons is transmitted chemically to the energy con- 

 suming steps in metabolism, we may have to learn a good deal more about 

 the nature of the interaction of the "natural cofactor or cofactors" with 

 the oxidation-reduction components of the grana. In this surmise, as in 

 others, we are following the lead of Warburg, who hinted many years ago 

 at a functional role in photosynthesis for a naturally occurring ortho- 

 hydroquinone [64]. 



References 



1. Hill, R., Nature, Lond. 139, 881 (1937). 



2. Hill, R., Proc. roy. Soc. B 127, 192 (1939). 



3. Arnon, D. I., Allen, M. B., and Whatley, F. R., Nature, Loud. 174, 394 (1954). 



4. Frenkel, A. W.,^. Amer. chem. Soc. 76, 5568 (1954). 



