SPECIFICITY OF INHIBITION 167 



boxylation, for the new type of CO2 fixation reaction found in propionic 

 acid bacteria: 



Phosphoenolpyruvate + CO2 + P^ ^ oxalacetate + PP 



is not depressed by iodoacetamide up to 4 mM (Siu and Wood, 1962). The 

 fixation of C^^Oo by yeast endogenously is inhibited 35% by 0.046 mM iodo- 

 acetate, and that induced by ghicose is completely blocked at this concen- 

 tration (Stoppani et al., 1958 a). The action may not be on COo fixation 

 itself but on the formation of phosphoenolpyruvate, or other substances, 

 for condensation of the CO2. The usual carboxylation of phosphoenolpyru- 

 vate in plants is not affected by even 10 mM iodoacetamide (Tchen and 

 Vennesland, 1955). 



SPECIFICITY OF INHIBITION OF 3-PGDH AND EM PATHWAY 



There is no doubt that a large number of enzymes and metabolic path- 

 ways are appreciably inhibited by iodoacetate. The fundamental problem 

 in a particular instance is whether a concentration specifically blocking the 

 EM pathway can be found. The answer will depend on several factors — 

 the relative susceptibilities of the enzymes involved, the permeability of 

 the cells to iodoacetate, the pattern of metabolic activity under the experi- 

 mental conditions, the duration of contact with the inhibitor — and, in 

 addition, on what cellular process is being measured. It appears to be pos- 

 sible to produce a specific inhibition of 3-PGDH within the EM pathway 

 itself by using concentrations in the range 0.05-0.2 mM, although 3-PGDH 

 may not be completely blocked in all cases. It is also likely that the EM 

 pathway can be appreciably inhibited w^ithout interfering significantly with 

 the oxidation of pyruvate or the operation of the cycle, but this is difficult 

 and care must be taken in choosing the experimental conditions. In much 

 work attempting to correlate cellular activities with the EM pathway it is 

 quite certain that the cycle was inhibited to varying degrees. 



It is fairly difficult to test for the specificity of action on 3-PGDH 

 by determining the accumulation of intermediates since the situation is 

 frequently quite complex, as previously discussed, but it is occasionally 

 possible to differentiate the effects on the EM pathway and the cycle by 

 showing a response to added pyruvate. We shall have occasion to note the 

 results of such addition in studies of cell function, but unfortunately such 

 tests have seldom been done. Certain aspects of this problem have been 

 treated (page 1-877). One very crucial problem here is the permeability 

 of the cells under consideration to pyruvate, since the effect of pyruvate 

 may be limited by its rate of penetration. Another point to be taken into 

 account is the fact that pyruvate cannot completely restore normal condi- 



