318 CATALYST POISONS AND NARCOTICS CHAP. 12 



tration of iron salts. He claimed some success: for example, immersing 

 Fontinalis into a 5 X 10~'% ferrous ammonium citrate solution (after this 

 water moss was poisoned by fuming nitric acid) produced an increase in 

 photosynthesis from 35 to 51% of the normal rate. Poisoning by 

 phenylurethane (which, according to Noack, also promotes photoxida- 

 tion) was not relieved by iron salts. 



In another paper (1930), Noack attempted to support his theory by 

 experiments of a different kind: he tried to show that sulfur dioxide and 

 the nitrogen oxides actually affect the state of the iron in the chloroplasts. 

 These experiments (which will be described in Chapter 14) showed that 

 the percentage of water-soluble iron in the leaves is increased from 6 to 

 12% by the action of sulfur dioxide. Potassium cyanide caused an 

 increase to 10%, and potassium thiocyanide (according to Noack this 

 compound, too, is a specific poison for photosynthesis) to 12.6%. 



Noack saw in these experiments the confirmation of his theory that 

 simple iron compounds, rather than hemoprotein complexes, serve as 

 catalysts in photosynthesis. In this extreme form, Noack's hypothesis 

 certainly is incorrect, since true enzymes undoubtedly play an important 

 part in photosynthesis (the phenomena of reversible poisoning — e. g., 

 by cyanide or dinitrophenol — clearly indicate the participation of such 

 enzymes). Another and still open question is whether, in addition to 

 true enzymes, simple organic or inorganic iron salts also play a catalytic 

 part in photosynthesis. 



(d) Hydrogen Peroxide and Sodium Azide 



The effect of hydrogen peroxide was described in chapter 11. It can 

 be observed only when catalase is inhibited, as by cyanide (which can 

 be achieved in some strains of Scenedesmus without a simultaneous 

 inhibition of photosynthesis). It was stated on page 286 that the 

 peroxide-sensitive part of the photosynthetic apparatus is probably the 

 oxygen-liberating enzyme. The inhibition may be due either to oxida- 

 tions brought about by the high oxidation potential of the peroxide (of. 

 page 283) or, as suggested by Gaffron, to complex formation with the 

 affected enzyme (in competition with the normal substrate, but without 

 catalytic decomposition, in the same way as this was observed in the 

 inhibition of catalase by ethyl peroxide). 



Sodium azide also inhibits photosynthesis, even in algae which are 

 insensitive to cyanide (Gaffron 1944). 



5. lodoacetyl and Other Organic Poisons 



Kohn (1935) found that the iodoacetyl radical is a specific poison for 

 photosynthesis in Chlorella pyrenoidosa. He used iodoacetic acid, 

 (ICH2COOH), or its amine, (ICH2COONH2). The action was slow 



