10 1. lODOACETATE AND lODOACETAMIDE 



The kinetics of the reactions between halogen acids and thioglycolate 

 -OOCCH2SH + XCH2COO- -» -OOCCH2— S— CH2COO- + CI- + H+ 



were thoroughly studied by Hellstrom (1931, 1933, 1934) in Stockholm. 

 The relative reaction rates are given in the accompanying tabulation. The 



Relative reaction rate of: 



rate increases with ionic strength — A; is proportional to s*^-^' — and with 

 the pH, indicating that the reaction may involve an R — S" ion. Some oxi- 

 dation by iodoacetate to the disulfide, -OOCCH2 — SS — CHgCOO-, also oc- 

 curs, whereas with iodoacetamide this reaction is very slow. Iodoacetate is 

 also able to react with SH groups complexed with metals; the rates of reac- 

 tion with the mercuric and zinc complexes of thioglycolate are actually 

 greater than with the free SH groups. 



The importance of SH groups in enzyme inhibition by iodoacetate was 

 brought to the fore by studies on glyoxalase under the mistaken impression 

 that this is an enzyme involved in the glycolytic pathway, methylglyoxal 

 being considered as the precursor of lactate. Dudley (1931) showed that 

 iodoacetate strongly inhibits glyoxalase and that this occurs in vivo follow- 

 ing injection of iodoacetate. Lohmann (1931) simultaneously demonstrated 

 that higher concentrations of iodoacetate are required to inhibit glyoxalase 

 than to block glycolysis, and felt that this was not the site of glycolytic 

 inhibition, but showed in the following year (Lohmann, 1932) that gluta- 

 thione is a cofactor for this enzyme. This led Dickens (1933 a,b) to study 

 the mechanism of inhibition of glyoxalase and he immediately found that 

 the inhibition can be reversed by glutathione; the purified enzyme, with 

 little glutathione to protect it, is inhibited by 0.03-0.1 mM iodoacetate, 

 whereas about 5 times this concentration range is necessary in liver slices. 

 The results indicated a direct reaction of iodoacetate with glutathione, and 

 Dickens found that when neutral solutions are mixed a reaction occurs 

 eliminating iodide: 



GSH -I- ICH2COO- -y GS— CH2COO- + I + H+ 



The relative rates of reactions are given as: iodoacetate 15.5., bromoacetate 

 9, and chloroacetate 0.15. Cysteine was also found to react readily with 



