158 1. lODOACETATE AND lODOACETAMIDE 



glycerate. Since formation of hexoses could occur through reduction of the 

 3-P-glycerate [with photochemically formed NAD(P)H and ATP] by a re- 

 versal of the EM pathway, it was evident that iodoacetate might inhibit 

 photosynthesis by blocking 3-PGDH, i.e., by the same mechanism as the 

 inhibition of glycolysis. If this were the case, one might expect some accu- 

 mulation of 3-P-glycerate in the presence of iodoacetate. However, Calvin 

 et al. (1951) reported that when 0.15 mM iodoacetamide has inhibited the 

 uptake of CO2 by 90% in Chlorella, the synthesis of sucrose is not decreased 

 and no 3-P-glycerate accumulates. This led them to postulate that iodo- 

 acetamide is acting on an enzyme other than 3-PGDH or the formation of 

 sucrose does not involve reversal of the EM pathway. However, Arnon 

 (1952) reexamined the data obtained by Stepka (1951) and claimed that 

 if the incubation of the cells with the inhibitor is longer, inhibition of su- 

 crose formation and accumulation of 3-P-glycerate occur, the data obtained 

 above having been on preparations not sufficiently inhibited. Nevertheless, 

 it is still difficult to understand how sucrose synthesis can be unaffected 

 while CO2 utilization is inhibited 90%, which means that the formation of 

 the sucrose from the CO, which is taken up is increased about 10 fold. One 

 would normally interpret these data as indicating iodoacetamide to block 

 various pathways for CO.. incorporation, allowing it to be channeled into 

 sucrose. In order to understand the effects of these inhibitors we must turn 

 to more recent investigations of the changes in the patterns of labeling. 



Localization of the Site of Action of Iodoacetate and Iodoacetamide on 

 Photosynthesis 



The effects of iodoacetate or iodoacetamide on the patterns of incorpora- 

 tion of C^^Oa during periods of illumination in several plants are shown in 

 Tables 1-26 to 1-29, and on the incorporation of inorganic P=^- in Table 1-30. 

 The relatively high concentrations of iodoacetate used in the study of sugar 



