286 • LIBERATION OF OXYGEN CHAP. 11 



effects in the two processes in table 11.11 must be considered fortuitous. 

 More significant are the results obtained with hydroxylamine, since this 

 poison affects the oxygen-liberating reaction (c/. page 313). However, 

 since this reaction is not rate limiting in nonpoisoned cells, no quantitative 

 parallel between the effects of hydroxylamine on photosynthesis and 

 hydrogen peroxide decomposition can be expected, even if the two effects 

 were due to one and the same reaction. On the other hand, a mere 

 qualitative similarity, as revealed by table 11.11, does not prove that the 

 affected enzymes are identical. The hydroxylamine-sensitive enzyme 

 in photosynthesis may be, for example, a "catalase" specifically adapted 

 to the dismutation of an organic peroxide, or even an oxidase (since 

 many oxidases contain hemin and are therefore capable of complex 

 formation with hydroxylamine). 



Thus, of all the arguments given above for or against the intermediary 

 formation of hydrogen peroxide in photosynthesis, there remains only 

 Gaffron's observation of the continued photosynthesis of certain Scene- 

 desmus strains in which the catalase was completely inhibited by cyanide. 

 This experiment provides a direct experimental support for the conception 

 — held plausible on general grounds on page 284 — that the catalatic 

 decomposition of hydrogen peroxide does not form a part of the chemical 

 mechanism of photosynthesis. Gaffron (1944) found indications that 

 hydrogen peroxide also does not occur as an intermediate in the oxy- 

 hydrogen reaction in adapted algae. 



If catalase does not take part in photosynthesis, what is the purpose 

 of its presence in all green plants? Gaffron suggested that its function 

 may be protection of the photosynthetic apparatus from injury which 

 can be caused by hydrogen peroxide (formed, for instance, by the 

 autoxidation of a yellow respiration enzyme). In confirmation of this 

 view, he reported that cyanide-treated Scenedesmus cells, which are 

 ordinarily able to continue photosynthesis indefinitely, ceased to evolve 

 oxygen upon the addition of a trace of hydrogen peroxide. This ob- 

 servation was confirmed by Weller and Franck (1941), who suggested 

 that the part of the photosynthetic apparatus which is destroyed by 

 hydrogen peroxide is the carboxylating enzyme, Ea (cf. page 318). 



Knoll, Matthews and Crist (1938) had claimed that the evolution of oxygen by 

 Chlorella can be enhanced by the addition of catalase; but this result — which, if correct, 

 would strongly support the hydrogen peroxide hypothesis — has never been elaborated 

 upon or confirmed. 



3. The Organic Peroxide Hypothesis 



Several hypotheses in which organic peroxides, rather than hydrogen 

 peroxide, were assumed as intermediates have been discussed in the 

 literature on photosynthesis. The earliest of them — the hypothesis of 



