HYDROXYLAMINE 311 



II, chapter 24. The effect of cyanide on photosynthesis in flashing light will be discussed 

 in more detail in chapter 34 (Vol. II) ; the influence of cyanide on photoxidation (Myers 

 and Burr 1940) will be mentioned on page 536, and the effect of this poison on respiration, 

 discussed earlier in the present chapter, will be considered again in chapter 20. 



3. Inhibition by Hydroxylamine 



Hydroxylamine (NH2OH) is an even stronger poison for photo- 

 synthesis than is cyanide, as first shown by Shibata and Yakushiji (1933), 

 who found that 2.2 X lO"* m./l. NHaOH-HCl suppresses all photo- 

 synthesis in Chlorella ellipsoidea (in strong light and in the presence of 

 abundant carbon dioxide), leaving respiration unaffected. Similar results 

 were obtained by Nakamura (1938), who found a 95% inhibition of pho- 

 tosynthesis by 1 X 10-4 m./l. NH2OH (c/. Table 11.11). The claim of 

 Shibata and Yakushiji that the effect of hydroxylamine proves the role 

 played in photosynthesis by catalase (cf. Chapter 11, page 284) cannot 

 be accepted as valid. In the first place, even if the poisoning were due 

 to catalase, the effect could be an indirect one (as was once suggested by 

 Gaffron for cyanide) : the presence of hydroxylamine could allow hydrogen 

 peroxide, produced by respiration, and normally destroyed by catalase, 

 to accumulate until it destroys one of the photosynthetic enzymes. 

 In the second place, it is not true that hydroxylamine inhibits only catalase 

 and no other enzymes. The work of Gaffron, which was discussed in 

 chapter 11, and which showed that, in some algae, catalase can be com- 

 pletely inhibited without a decline of photosynthesis, proves convincingly 

 that the effect of hydroxylamine on photosynthesis is caused by the 

 poisoning of another enzyme, and not of catalase. This hydroxylamine- 

 sensitive enzyme may bear a certain similarity to catalase in that its 

 function, too, is to assist in the liberation of oxygen. This is indicated by 

 experiments on the effect of hydroxylamine on the metabolism of hydro- 

 gen-adapted algae (e. g., Scenedesmus Dl). The photoreduction of car- 

 bon dioxide by these algae is much less sensitive to hydroxylamine than 

 is the normal photosynthesis of the same species. While the latter is 

 inhibited completely by 5 X 10"'* m./l. NH2OH, the reaction with 

 hydrogen is reduced by less than 50%, even in a 3 X 10"^ molar solution. 

 The reduction of carbon dioxide by bacteria (with hydrogen or hydrogen 

 sulfide as reductants) is also comparatively indifferent to hydroxylamine. 

 According to chapters 6 and 7, these processes share with ordinary 

 photosynthesis a common (or similar) primary photochemical process 

 which leads to the formation of a "primary oxidation product," {OH} 

 or Z. They differ, however, in the fate of this primary oxidation product 

 — which is decomposed with the liberation of oxygen in normal photo- 

 synthesis, but is reduced by hydrogen, hydrogen sulfide, or other re- 

 ductants, in the photoreduction of bacteria and adapted algae. The 



