166 1. lODO ACETATE AND lODOACETAMIDE 



forces photosynthesis into the Calvin carbon cycle. This could also contribute 

 to the greater formation of malate and amino acids. 



The fixation of C^^Oa by broken spinach chloroplasts in the presence of 

 either ribose-5-P or ribulose-l,5-diP and the effects of iodoacetamide on 

 this were studied by Trebst et al. (1960). It was stated that the carboxylative 

 phase is strongly inhibited but not the reductive phase. The accompanying 

 tabulation of the results shows that the fixation with ribose-5-P as sub- 



lodoacetamide ° " Inhibition of C»0, fixation 



strate is inhibited somewhat more than wnth ribulose-l,5-diP, which indi- 

 cates some inhibition, not of the carboxylative phase but of the formation 

 of ribulose-l,5-diP, an inhibition which could be on either pentose-P iso- 

 merase or ribulose-5-P kinase, probably the latter. On the other hand, the 

 carboxylation of ribuIose-l,5-diP is only weakly inhibited. Indeed, neither 

 of these inhibitions is very potent and could not explain the depression of 

 photosynthesis at concentrations below 0.1 mM. 



One must be careful in making generalizations about the mechanism of 

 iodoacetate inhibition, inasmuch as all plants do not photosynthesize in 

 exactly the same way. Mortimer (1960) found sugar beet and soybean leaves 

 to have somewhat different photosynthetic systems, the former depending 

 partially on 3-PGDH and the latter not at all; and Kandler et al. (1961) 

 reported quite different behavior in two species of Chlorella. Since only a 

 few species of plants have been studied thoroughly, it is very probable that 

 different patterns of response to iodoacetate will be found, and that the 

 above conclusions will not be generally valid. 



Nonphotosynthetic (Dark) COg Fixation 



Studies of photosynthesis have generally not included measurement of 

 CO2 fixed in the dark. Since the rate of dark fixation is probably much 

 lower than in the light, it is likely that it does not significantly complicate 

 the photosynthetic incorporation. However, the nonphotosynthetic fixation 

 of CO, may be quite sensitive to iodoacetate. Wood and Werkman (1940) 

 found that CO2 fixation in Pwpionihacterium Tpentosaceum (whereby suc- 

 cinate is formed from glycerol and COg) is inhibited 69% by 0.078 mM 

 iodoacetate. This is presumably not due to a direct inhibition on the car- 



