DIFFERENT OXIDANTS 1569 



Gerretsen (1950^) observed that crude chloroplast preparations from Avena, which 

 slowly absorbed oxygen in the dark (c/. below section 5(c)), increased this absorption 2-3 

 times in light (instead of replacing it by oxygen liberation, as noted bv Hill and others 

 with washed chloroplasts). Addition of glucose did not increase this photautoxidation ; 

 it was, however, stimulated by asparagine. Addition of manganese salts — which Gerret- 

 sen (1950') had found to increase strongly the effect of light on the redox potential of the 

 same material — often produces a marked temporary increase in the rate of oxygen uptake 

 in light. 



(d) Quinones 



An important new group of oxidants capable of serving in the Hill re- 

 action was found by Warburg and Liittgens. They noted (1944) that 

 chloroplasts isolated from spinach or sugar beet leaves can reduce p- 

 benzoquinone — probably to hydroquinone — with the liberation of an 

 equivalent amount of oxygen. A description of these experiments was 

 given in a paper published in Russian (Warburg and Liittgens, 1946) and 

 later reprinted in W^arburg's book Heavy Metals as Prosthetic Groups 

 (1946; English edition, 1949). Warburg and Liittgens discovered the 

 quinone reaction while studying the "respiration" of juices obtained by mac- 

 eration of spinach leaves. The oxygen consumption of these juices de- 

 clined with time; in an attempt to maintain it, various oxidation sub- 

 strates were added, and it was noted that with hydroquinone (or pyro- 

 catechol) the respiration was sustained in the dark, but markedly decreased 

 in light. This was attributed to photochemical reversal of the oxygen- 

 consuming process — i. e., to a (chloroplast-sensitized) photochemical oxida- 

 tion of water by quinone: 



Chl, light 

 (35.32) 2 C6H4O2 + 2H2O , 2 CeHeOz + O2 -52 kcal./mole 



dark 



Similar observations were made with some naphthaquinones, e. g., naph- 

 thaquinone sulfonic acid. No change in oxygen consumption was noted in 

 light, neither in the liquid fraction from the maceration of spinach leaves 

 (with or without pyrocatechol or hydroquinone) nor in the chloroplast- 

 containing fraction without added polyphenol; but in the chloroplast 

 fraction to which pyrocatechol was added, illumination caused 50% in- 

 hibition of the oxygen uptake (fig. 35.18); and if both pyrocatechol and 

 hydroquinone were added, a complete inhibition of respiration or even a 

 liberation of oxygen was observed in light. 



After it was first noted that chloride may be needed as a "co-enzyme" 

 for the quinone reaction (c/. above, sect, (c)), 0.05% KCl was added by 

 Warburg and Liittgens to all washed preparations. (If the centrifuged 

 suspensions are not washed, natural chloride content is high enough to ac- 

 tivate the photochemical reaction.) 



