EFFECTS ON PHOTOSYNTHESIS 165 



directly was suggested; this is indicated by the dashed arrow in Scheme 3. 

 It is believed, at least in certain plants, that this is the normal pathway 

 taken in the synthesis of sucrose or polysaccharide, and that this pathway 

 is the most sensitive to iodoacetate, inhibition occurring between 0.001 and 

 0.01 mM. The various possible sites of inhibition, as given by Kandler et al. 

 (1961), are indicated in Scheme 3 by the plgo values. At low concentrations 

 only the keto acid ^- fructose- 1,6-diP reaction is blocked, but at higher 

 concentrations the inhibition may be more complex and involve at least 

 four sites. 



It has been thought that inhibition of CO2 incorporation might be due 

 to an inhibition of the formation of ribulose-l,5-diP, with which CO2 con- 

 denses, particularly by a block of the ribulose-5-P kinase. However, although 

 this enzyme is inhibited by iodoacetate, it is not as sensitive as total photo- 

 synthesis, and one must attribute a minor role to this site of inhibition. Of 

 course, formation of ribulose-l,5-diP may be decreased if any step in the 

 carbon cycle is inhibited, and this probably occurs if the site is the keto 

 acid— > fructose- 1,6-diP reaction. Furthermore, there is no accumulation of 

 ribose-5-P in iodoacetamide-poisoned plants (Gibbs and Calo, 1960 a). The 

 ribulose-l,5-diP carboxylase is unaffected by 10 mM iodoacetamide (Gibbs 

 and Calo, 1959 a) so this cannot be the site of inhibition. One is left with 

 only the keto acid -^ fructose-l,6-diP reaction as the sensitive step, but 

 no direct studies of this have been reported. 



The question of 3-P-glycerate levels during inhibition of photosynthesis 

 has been much discussed, and some have assumed that this substance should 

 accumulate, particularly if 3-PGDH is the site of inhibition. Change in con- 

 centration of intermediates in such complex systems is, however, often dif- 

 ficult to predict (see page 1-514). Here we have a cycle with pathways issu- 

 ing from some of the cycle intermediates, even neglecting for the moment 

 the postulated shunt across the carbon cycle. There is no necessity at all 

 for 3-P-glycerate to accumulate during iodoacetate inhibition; it may or it 

 may not. First, if its utilization along the EM pathway is decreased, more 

 will presumably proceed along other pathways, e.g., the formation of ser- 

 ine-P or pyruvate (and other amino acids derived from the tricarboxylate 

 cycle). This has actually, been observed and is undoubtedly the explana- 

 tion for the rise in C^^ incorporation into amino acids and malate. Second, 

 since CO2 incorporation is inhibited, less 3-P-glycerate would be formed 

 (assuming only the Calvin cycle); accumulation of a cycle intermediate is 

 usually very moderate when a cycle step is inhibited. If one now introduces 

 the keto acid ^ fructose- 1,6-diP shunt, a further consideration rises. If 

 this is the normal pathway for the keto acid, a block of this reaction will 

 increase the hydrolysis of the keto acid to 3-P-glycerate, and in this way 

 the level of 3-P-glycerate might tend to rise, particularly at low iodoacetate 

 concentrations. According to this picture, iodoacetate at low concentrations 



