CHEMICAL PROPERTIES 11 



iodoacetate. Quastel (1933 a) pointed out the importance of glutathione 

 for glycolysis and believed it might represent the site of attack for iodo- 

 acetate. 



The SH groups on proteins and enzymes vary widely in reactivity with 

 SH reagents (Chapter II-5), and part of this is not due to steric factors 

 introduced by the complex protein structure since simple thiols also vary 

 in how rapidly they react with iodoacetate. Michaelis and Schubert (1934) 

 reported that under specified conditions the reaction of iodoacetate with 

 cysteine is about 70% complete in 10 min, with thioglycolic anilide in 30 

 min, and with thioglycolate in 3 hr. Smythe (1936) investigated this matter 

 further and determined the half-reaction times at pH 7.1, from which the 

 relative rates in the accompanying tabulation were calculated. The half- 



reaction time for iodoacetate and cysteine is 1.12 min. It appears that, 

 under these conditions, iodoacetamide always reacts somewhat more rapid- 

 ly than iodoacetate, but Hellstrom found iodoacetate to react faster with 

 thioglycolate (see page 10). It may also be noted that no reaction at all 

 occurs with GSSG. Comparison of the rates of reaction of the various 

 halogen acids with cysteine was made by Barron (1951) at pH 7.4 and 28°, 

 indicating that while iodoacetate and bromoacetate are not very different, 



Relative rate of reaction with cysteine 



Iodoacetate 1 . 00 



Bromoacetate . 72 



Chloroacetate . 05 



Fluoroacetate 



as has been found generally, chloroacetate and fluoroacetate are very un- 

 reactive. This has been confirmed more recently by Mayer (1957), who 

 found iodoacetate to react 155 times more rapidly with cysteine than does 



