CYANIDE INHIBITION OF PHOTOSYNTHESIS 309 



free carbon dioxide) — and not because it proceeds through a different 

 enzymatic channel. It was stated above that the cyanide-sensitive 

 stage in photosynthesis is the formation of the complex, {CO2}, probably 

 by carboxylation of an organic acceptor. On the other hand, it is known 

 (page 222) that carbon dioxide evolution in respiration occurs by 

 decarboxylation of organic acids. // these acids could be utilized in 

 photosynthesis before they are decarboxylated, the cyanide-sensitive stage 

 could be avoided. 



The problem of the cyanide-insensitive "residual" photosynthesis 

 certainly requires more experimental study; but if the results of Warburg 

 and van der Paauw were to be confirmed, this would throw new light 

 on the internal relationship between respiration and photosynthesis, by 

 showing that respiration products may become available for photo- 

 synthesis before they assume the form of free carbon dioxide. 



This interpretation of the cyanide-resistant residual photosynthesis 

 leads to a simple explanation of the discrepancy between the results 

 obtained with Chlorella and Stichococcus on the one hand, and Hormidium 

 and Scenedesmus on the other. Since the respiration of the last two 

 algae is more sensitive to cyanide than is their photosynthesis, no cyanide- 

 resistant photosynthesis can be expected in them — and none was found. 



Another disagreement between Warburg and van der Paauw remains to be clarified, 

 namely, the assertion of the latter that cyanide poisoning is almost equally efficient in 

 weak as in strong light. The shape of poisoning curves, given by van der Paauw for 

 Stichococcus in weak light, is peculiar: They show an inhibition of 50% at 5 X 10~'» 

 m./l. HCN, followed by a renewed increase in rate at the higher concentrations of the 

 inhibitor until, at 5 X lO"' m./l. HCN, the rate surpasses its normal value. No stimu- 

 lation of photosynthesis by high cyanide concentrations has ever been observed by 

 other investigators; and the reliability of van der Paauw's curve seems doubtful. 



Since cyanide is poisonous to many (although not all) catalysts 

 containing a heavy metal, and several such catalysts are probably 

 involved in photosynthesis, there is no reason to assume that the car- 

 boxylating enzyme, Ea, is the only cyanide-sensitive component of the 

 photosynthetic apparatus. The "catalase" or "deoxidase" which 

 catalyzes the evolution of oxygen in photosynthesis should, too, be 

 subject to cyanide poisoning. (It was mentioned in chapter 11 that the 

 sensitivity of photosynthesis to cyanide was the first argument in favor 

 of the interpretation of the "Blackman reaction" as the decomposition 

 of a peroxide by the action of catalase; but we have also seen, on page 

 286, that this argument was reversed by Gaffron when he found an 

 organism in which a complete suppression of catalatic activity by cyanide 

 could be achieved without inhibiting photosynthesis.) Apparently, the 

 effect of cyanide on the oxygen-liberating enzyme (or enzymes) remains 

 hidden, because the "preparatory" enzyme, Ea, is less abundant than 



