310 CATALYST POISONS AND NARCOTICS CHAP. 12 



the "finishing" enzymes, Ec and Eo, so that its inactivation reduces the 

 rate of the over-all reaction before other cyanide effects become apparent. 

 We shall see in volume II, chapter 33, that inactivation of the enzyme, 

 Eo, may be responsible for most of the induction phenomena; therefore 

 cyanide effects observed during the induction period are likely to be due 

 to the poisoning of this enzyme. 



2. Effect of Cyanide on Hydrogen- Adapted Algae 



The hydrogen-adapted algae, whose metabolism was described in 

 chapter 6, provide an illustration of the fact that several partial processes 

 can be affected by the same inhibitor. According to Gaffron (1944^), the 

 two cyanide-sensitive processes are adaptation (the process by which the 

 hydrogenase system becomes "activated" during anaerobic incubation) 

 and carboxylation (which was held to be mainly or exclusively responsible 

 for the cyanide sensitivity under ordinary conditions). 



Cyanide is a highly specific poison for the "adaptation reaction." A 

 concentration of 1 X 10~* m./l. HCN, which only slightly reduces the 

 rate of normal photosynthesis or respiration in Scenedesmus Dl (cf. 

 Table 12. V) completely prevents its adaptation to hydrogen. The same 

 amount, added after adaptation, has no immediate effect on the rate of 

 hydrogen consumption. Larger concentrations affect it in the same way 

 as normal photosynthesis, i. e., probably through the inactivation of the 

 carboxylating enzyme, ^a- Cyanide accelerates de-adaptation, probably 

 by the following indirect mechanism: The adapted state is maintained 

 by continuous "re-adaptation" of oxidized hydrogenase molecules; in the 

 presence of cyanide, this re-adaptation is blocked, and the enzymatic 

 system gradually slides back into the "oxidized" state. 



The oxyhydrogen reaction, too, is affected by cyanide, but much less 

 strongly than is the " chemosynthetic'' reduction of carbon dioxide which 

 may be coupled with it. This, too, agrees with the hypothesis that 

 cyanide affects primarily the enzyme, Ex, which catalyzes the fixation 

 of carbon dioxide preliminary to its reduction (by photosynthesis or by 

 chemosynthesis) . 



According to Rieke and Gaffron (1943), the effect of cyanide on the 

 photoreduction of adapted algae in flashing light is the same as in ordinary 

 photosynthesis, a result which also is in agreement with Franck's hy- 

 pothesis. The identity of cyanide effects in photosynthesis (with 

 evolution of oxygen) and photoreduction (which proceeds without this 

 evolution) provides the best proof that the oxygen-liberating enzyme is 

 not responsible for the cyanide inhibition. 



The effect of cyanide on the fluorescence of chlorophyll in vivo was studied by 

 Kautsky and Hirsch (1937), Wassink, Vermeulen, Reman, and Katz (1938), Wassink 

 and Katz (1939), and Franck, French, and Puck (1941). It will be dealt with in volume 



