168 1. lODOACETATE AND lODOACETAMIDE 



tions in iodoacetate-poisoned cells, because the ATP generated in the EM 

 pathway remains depressed; if compartmentalization of ATP occurs, that 

 produced in the cycle may not be able to replace that from the EM pathway. 

 Numerous examples of failure of pyruvate to counteract effects of iodoace- 

 tate have been reported; e.g., the depression of the Na+ pump and the 

 respiration in the marine alga Hormosira banksii by 0.1 mM iodoacetate is 

 not prevented by pyruvate (Bergquist, 1958). In this case a pH of 4.8 was 

 used to facilitate penetration (there was no effect at pH 7.5) and hence one 

 must consider the possibility of an acidification of the cell due to penetra- 

 tion of iodoacetic acid. Pyruvate will not entirely prevent the iodoacetate 

 depression of cardiac tissue; indeed, it will not completely counteract the 

 effects of 2-deoxyglucose, which is certainly specific for the EM pathway. 

 In these cases one can adduce compartmentalization or inadequate penetra- 

 tion, but there is no evidence either way. 



The specificity question becomes especially acute in long-term experi- 

 ments where many metabolic pathways are involved, as in the inhibition 

 of synthesis and growth. Many of the enzymes necessary for the over-all 

 function, maintenance, and growth of cells have not yet been examined 

 with respect to inhibition by iodoacetate. The time relations may be critical. 

 Selective attack on 3-PGDH or the EM pathway is often possible because 

 3-PGDH generally reacts more rapidly than other enzymes with iodoace- 

 tate, so that over a period of 30-60 min some specificity may be achieved, 

 but the longer the contact with iodoacetate, the greater the likelihood of 

 other loci of action. Many of the results obtained with iodoacetate are 

 meaningless and have led to erroneous interpretations because the factor 

 of time has been neglected. 



PENETRATION INTO CELLS AND THE EFFECTS OF PH 



Iodoacetic acid is a weak acid ipK^ = 3.12) and in the physiological pH 

 range would exist primarily in the ionized form of iodoacetate. Cells with 

 permeability barriers to anions should thus be relatively resistant to this 

 inhibitor, the effects increasing as the pH is reduced. The pH dependence 

 of iodoacetate inhibition was first noted in yeast fermentation by Neuberg 

 and Kobel (1931), who stated, "Das jodessigsaure Salz bei Verschiebung des 

 pH nach all'alischen Seite etwas in seiner Wirkung behindert ist,''"^ and the 

 quantitative aspects of fermentative and respiratory inhibition were studied 

 by Lundsgaard (1932). Many workers since then have reported essentially 

 complete inhibitions at pH 4.0-4.5 whereas little or no inhibition occurs if 

 the pH is raised above 7. Such is seen in yeast fermentation (Briicke, 1933; 



* "The action of the iodoacetic acid salt is reduced by a displacement of the pH 

 in the alkahne direction." 



