EFFECT OF POISONS AND pU ON CO2 FIXATION 1687 



creased.) In glycolysis, iodoacetamide is known to inhibit the reaction by 

 which triose phosphate is oxidized to phosphopyruvate ; it thus could be ex- 

 pected to block the synthesis of sugars if it occurred by the reversal of this 

 reaction. The actually observed effect of iodoacetamide on the total 

 C*02 fixation can be attributed to this source. However, the lack of effect 

 on sucrose synthesis has then to be explained by the ad hoc assumption of 

 another partial reaction beyond the triose stage which, too, is affected by 

 iodoacetamide in such a way that the flow of intermediates through some 

 channel by-passing sucrose is slowed down, and ten times more than the 

 usual proportion of intermediates are converted into sucrose. 



The evidence concerning the effect of iodoacetamide on the respiration 

 of green cells also was contradictory until lately, when Arnon (1952, 1953) 

 and Holzer (cf. Holzer 1954) proved definitely that an iodoacetamide-sensi- 

 tive respiration path does exist in green plants. This was taken as proof 

 that their respiration, like that of animal tissues or yeast, proceeds, at least 

 in large part, via the triose-pyruvate step, with triose dehydrogenase as 

 catalyst, a pyridine nucleotide as hydrogen acceptor, and ADP as "energy 

 acceptor." It seems, however, that in green cells this reaction is not DPN- 

 specific, as usual, but can use either DPN or TPN. 



Because of the probable spatial separation of the sites of the main 

 catabolic and anabolic processes, the proof of the presence of a triose de- 

 hydrogenase in green cells is not a very strong argument for the participa- 

 tion of this enzyme in photosynthesis. More convincing are the observa- 

 tions on the inhibition of photosynthesis itself by iodoacetate (chapter 12, 

 section 5, and chapter 37D, section 2). In conjunction with the data of 

 Stepka et al. on the effect of iodoacetamide on the uptake of radiocarbon, 

 these observations can be considered as lending support to the — anyhow 

 plausible— assumption that the next step in photosynthesis after the for- 

 mation of PGA is its reduction to triose by an iodoacetamide-sensitive 

 hydrogenase. To a certain extent, these observations also make it plausi- 

 ble that the hydrogen donor in this reduction is a pyridine nucleotide 

 (DPNHa or TPNH2) and that consequently a high energy phosphate, such 

 as ATP, must be supplied to make the reduction possible; however, these 

 conclusions are by no means certain. 



A number of previous observations (cf. Vol. I, pages 301-311) led to 

 the conclusion that cyanide is a specific poison for the carboxylation reac- 

 tion in photosynthesis. Calvin and co-workers illuminated Scenedesmus 

 for 30 min. in C02-free air to build up the CO2 acceptor, then added 3 X 

 10 -* M KCN, and one minute later exposed the cells to C*02. The 

 tracer fixation was inhibited by 95%, but the compounds whose tagging 

 was least inhibited were alanine, malic acid and phosphoglyceric acid— 

 supposedly the immediate (or near-immediate) products of carboxylation ! 



