284 



LIBERATION OF OXYGEN CHAP. 11 



that of hydrogen peroxide is undesirable, even though, in the absence of 

 peroxidase, hydrogen peroxide is a rather inert compound (probably 

 because of the high energy of the radical, H3O2, which is the first product 

 of its stepwise reduction). 



If hydrogen peroxide were formed as an intermediate in photosynthesis, 

 this would add 46 kcal or 40% to the energy requirement of the over-all 

 process (c/. Chapter 3, page 48). Furthermore, it would mean the 

 production in the cells of an oxidant with an extremely negative potential 

 (— 1.35 volt) on a scale ten or twenty times larger than that at which 

 it could ever be produced by respiration. These two considerations 

 argue a priori against the assumption (repeatedly made in the literature) 

 that hydrogen peroxide is an intermediate of photosynthesis. We shall 

 show presently that experimental evidence also speaks against the 

 "hydrogen peroxide hypothesis." 



It is well known that all green plants contain catalase. Warburg 

 thought that, if the dismutation of hydrogen peroxide were the rate- 

 limiting enzymatic reaction in photosynthesis, the capacity of plants for 

 photosynthesis (in strong light and in the presence of abundant carbon 

 dioxide) would be equal to their capacity for catalatic decomposition of 

 externally supplied hydrogen peroxide. In support of this view, Warburg 

 and Uyesugi (1924) quoted the observation that the catalytic activity of 

 Chlorella is affected by cyanide and urethane in approximately the same 

 way as the efiiciency of photosynthesis. The same is true according 

 to Kohn (1935), of hydrogen sulfide and iodoacetic acid. Yabusoe (1924), 

 working in Warburg's laboratory, found that the rate of the hydrogen 

 peroxide decomposition by Chlorella shows the same peculiar linear 

 increase with temperature which Warburg had previously attributed to 

 photosynthesis (c/. Vol. II, Chapter 31). 



The similarity in the response of photosynthesis and catalase activity 

 to different poisons was emphasized anew by Shibata and Yakushiji 

 (1933), Yakushiji (1933) and Nakamura (1938). The first-named 

 authors were led to investigate the sensitivity of photosynthesis to 

 hydroxylamine by the fact that this compound was known to be an 

 inhibitor of catalase; having found that it is also a specific poison for 

 photosynthesis, they became convinced that the rate-limiting dark reac- 

 tion in photosynthesis is the catalase-promoted dismutation of hydrogen 

 peroxide. Nakamura (1938) has given a comparison (Table 11.11) of the 

 effects of different poisons on photosynthesis and on the catalase activity 

 of Scenedesmus nanus. The table shows, however, that the effects of 

 poisons on catalase activity and photosynthesis are only approximately 

 the same. According to Warburg and Uyesugi, urethanes are 50% 

 more effective in inhibiting the catalase action of Chlorella than in 

 reducing its photosynthesis. Emerson and Green (1937) found that 



