308 CATALYST POISONS AND NARCOTICS CHAP. 12 



with and without cyanide clearly favors the interpretation of Weller 

 and Franck (further confirmed by experiments of Rieke and Gaffron 

 1943) according to which a full yield can be obtained whenever the dark 

 intervals are long enough to allow the photosynthetic mechanism to be 

 "recharged" with a new quantity of carbon dioxide. This recharging 

 involves a reaction which was not rate limiting in the absence of the 

 poison, but became limiting when inhibited by cyanide. The suggestion 

 that this reaction is the primary fixation of carbon dioxide is supported 

 by the experiments of Ruben, Kamen, and Hassid (1940) on the effect 

 of cyanide on the reversible fixation of radioactive carbon dioxide in the 

 dark (c/. page 203) and the observation of Aufdemgarten (1939) that the 

 "pickup" of carbon dioxide by leaves in the dark after intense photo- 

 synthesis also is slowed down by 10~' m./l. of cyanide, from the usual 

 20-30 seconds to two or three minutes (c/. page 207). Thus, the as- 

 sumption that the cyanide-sensitive component of the photosynthetic 

 apparatus is the carboxylating enzyme Ea (Franck's "catalyst A") is 

 well supported by circumstancial evidence. 



The conclusion we derive from this discussion is that differences in 

 the cyanide sensitivity of photosynthesis in different species can be 

 attributed to variations in the content of an enzyme which is not rate 

 limiting under ordinary conditions, but becomes limiting when a large 

 fraction of it is inhibited by cyanide. In the case of respiration, the 

 explanation may be similar; but certain observations (see below) make 

 it probable that this process can proceed through different enzymatic 

 channels, some less sensitive to cyanide than others; and this also may 

 explain — at least in part — variations in the over-all sensitivity of 

 respiration to cyanide in different species. 



We now return to the alleged existence of cyanide-resistant residual 

 photosynthesis which, according to Warburg and van der Paauw, is just 

 sufficient to compensate for respiration. It is known (c/., for example, 

 the review of Commoner 1940) that many cells possess beside the main, 

 cyanide-sensitive respiration, a cyanide-insensitive respiration of com- 

 paratively minor importance. In Chlorella, the additional respiration, 

 caused by glucose feeding, proves to be much more sensitive to cyanide 

 than normal respiration (Genevois 1927, Emerson 1937). This points 

 to the existence of two alternative enzymatic channels of respiration. 

 Gaffron suggested that a similar situation may exist also in photo- 

 synthesis. From this point of view, the coincidence between the extent 

 of cyanide-resistant photosynthesis and that of respiration must be 

 fortuitous. However, another hypothesis is possible, which would make 

 this coincidence significant. This hypothesis suggests that a certain 

 fraction of photosynthesis is insensitive to cyanide because it uses a 

 different substrate — namely, unfinished products of respiration (rather than 



