18 1. lODOACETATE AND lODOACETAMIDE 



was used as a basis for a cancer test, the iodoacetate index being defined in 

 terms of the iodoacetate required and the quantity of protein in the serum 

 (Huggins et al., 1949), but this test is not specific (it is positive in acute 

 inflammation, for example) and has recently been found to be unreliable. 



INHIBITION OF ENZYMES 



Iodoacetate and related substances have been shown to inhibit many en- 

 zymes but in very few instances has an analysis of the inhibition been made. 

 It has generally been assumed that inhibition implies the reaction of iodo- 

 acetate with SH groups, but seldom has a quantitative determination of the 

 SH groups actually reacted been made. On the other hand, many enzymes 

 which are usually classified as SH enzymes because of inhibition by other SH 

 reagents, do not react readily with iodoacetate and are not inhibited. We 

 shaU consider in this section the inhibition of a few important enzymes 

 and some investigations where an approach to the mechanism has been 

 made. Tables summarizing enzyme inhibitions will be presented in the sec- 

 tions on the different phases of metabolism to facilitate comparison of sen- 

 sitivities. By no means all of the studies on enzyme inhibition with iodo- 

 acetate will be included, but an attempt will be made to select data on 

 enzymes important in the major pathways of metabolism, and to present 

 only data which seem to be accurately obtained. It cannot be overempha- 

 sized that the values given in these tables are generally not comparable with 

 one another due to the different conditions under which the experiments 

 were run. Such factors as pH, temperature, period of incubation with the in- 

 hibitor, purity of the enzyme, and the presence of cofactors and coenzymes 

 vary greatly and, indeed, are quite often not even stated. The inhibitions 

 are often not quantitative in that they may represent some intermediate 

 inhibition on the way to a maximal inhibition — time curves of inhibition 

 are seldom run — and it is likely that secondary inactivation is commonly 

 superimposed on the effects of primary alkylation. They are to serve more 

 as references for those interested in the behavior of particular enzymes, and 

 at most are indicative of the presence or absence of marked inhibition. 

 Furthermore, the concentrations used are often higher than should be used 

 for blocking glycolysis, and this precludes the possibility of comparing en- 

 zyme susceptibilities at more practical concentrations, 



3-Phosphoglyceraldehyde Dehydrogenase (3-PGDH)* 



The early investigations of Embden and Meyerhof between 1933 and 1935 

 had shown that the site of the inhibition of glycolysis by iodoacetate is 



* The abbreviation 3-PGDH for 3-phosphoglyceraldehyde dehydrogenase will be 

 used tlu-oughout this chapter for convenience. 



