COMPARISON OF HALOGENATED ACIDS 279 



of iodoacetate the most rapidly and those of chloroacetate the least. In- 

 deed, they could not measure the enzymic hydrolysis of ethyl iodoacetate 

 because of the very rapid spontaneous hydrolysis. Wachtel (1920) noted 

 upon dissolving ethyl bromoacetate in water that decomposition occurs, 

 and considered the possibility of the toxic effects observed being due to the 

 products. In the enzyme inhibition studies shown in Table 1-45 the incuba- 

 tions with the esters are usually 10-90 min, so one wonders how much of 

 the ester is present at any time and how much of the inhibition is due to it. 

 Furthermore, since the bromoacetate esters are hydrolyzed quite rapidly 

 by acetylcholinesterase and liver esterase, this must be considered in work 

 on cellular preparations (Bergmann and Shimoni, 1953). Shirk and Gertler 

 (1958) studied the effect of bromoacetate esters on the growth of fungi, and 

 found many of them to be very potent inhibitors. Ethyl bromoacetate in- 

 hibits Aspergillus niger growth 50% at 0.0025 mM and several others pre- 

 vent growth completely at this concentration. These tests were made over 

 a 96 hr period at 30o so one would expect a good deal of hydrolysis. Bromo- 

 acetate itself is not nearly as potent an inhibitor, so one must assume that 

 the esters either produce the inhibition directly or penetrate readily and 

 release bromoacetate within the cell in much higher concentrations than 

 would be obtained with bromoacetate alone. Very little seems to be known 

 as to the reactivity of the esters with SH groups. Stoppani et al. (1953) 

 stated that cysteine and dimercaprol provide complete protection of pyru- 

 vate decarboxylase against both iodoacetate and its ethyl ester, but this 

 could mean only that the ester is hydrolyzed and acts through the forma- 

 tion of iodoacetate. 



When we turn to the effects of these esters on metabolism, it is found 

 that the actions are fundamentally not those of the unesterified haloace- 

 tates. Mackworth (1948) observed that ethyl iodoacetate inhibits the res- 

 piration of pigeon breast muscle and yeast very potently, in the case of 

 yeast 50% inhibition being given by around 0.04 vaM. Fleckenstein (1948) 

 throughly investigated the effects on yeast and concluded that the esters 

 are quite different in pattern of inhibition from the unesterified compounds. 

 Whereas the latter inhibit glycolysis more than respiration, the esters are 

 potent respiratory inhibitors (see accompanying tabulation). Ethyl iodo- 



