EFFECT OF CATALYST POISONS 



1909 



is enhanced (not only relatively, but even absolutely) by hydroxylamine 

 poisoning, suggesting, according to Calvin et at., a reversal, by this poison, 

 of the photochemical inhibition of respiration {cf. section 3 below). 



lodoacetate and lodoacetamide. Earlier experiments by Kohn, showing 

 especially high sensitivity of photosynthesis to these two compounds, were 



20 40 



30 



45 



X 



-\ 



o~«° 



Irs 



1 



X I 



90 



105 - 



120 - 



dork • 



60 



— T — 



— >■ mm 

 80 100 



"T 



120 



—I — 



no DNP 



m/20000 DNP 



light 



> l < 



dork 



Fig. 37D.8C. Inhibition of photosj^nthesis in Chlorella by 2,4-dinitrophenol 

 (after Holzer 1954). Conditions as in fig. 37D.8A, 20 mg. algae (dry weight). 



described on pp. 318-319. In chapter 36 (page 168G), reference was also 

 made to the observations (by Stepka and Calvin) of the effect of iodoacet- 

 amide on C(14)-fL\ation in light. These experiments, together with the 

 observations (of Arnon, and of Holzer) of the effect of the same poison on 

 the respiration of algae, w^ere discussed in chapter 36 in relation to a plausible 

 mechanism of photosynthesis, according to which the formation of PGA 

 is follow^ed by its reduction to triose, under the combined influence of photo- 

 chemically reduced TPN (or DPN), and of ATP, produced from ADP and 

 inorganic phosphate with the help of chemical energy derived from partial 

 reoxidation of the reduced pyridine nucleotides. This reaction is catalyzed 

 by triose dehydrogenase, specifically inhibited by the iodoacetyl radical. 



Fraser (1954) found, as expected, that quinone reduction by Chlorella 

 is much less sensitive to iodoacetate than photos3aithesis. 



Holzer (1954) described new experiments on the inhibition of photosyn- 

 thesis and respiration of Chlorella by iodoacetate (fig. 37C.8A). He sum- 



