762 6. ARSENICALS 



seen in the resistance to the amino and hydroxy derivatives and goo'd re- 

 sistance to the amide derivatives (King and Strangeways, 1942). It is 

 interesting to compare the susceptibilities of the spontaneously resistant 

 strain of Eagle and Magnuson (1944) (see accompanying tabulation). 



The resistance of this strain is thus primarily against the amide derivatives. 

 The implications of such behavior with regard to the mechanism of action 

 will be dealt with in a later section (page 765). 



Characteristics of Arsenical-Resistant Strains 



Following Ehrlich's suggestion that there is a loss of the specific receptors 

 in resistant organisms, one would like to know if resistants have an altered 

 thiol content, and particulary if there is a change in the functional SH 

 groups. Voegtiin et al. (1924) felt that SH-containing receptors do not 

 disappear but that a shift from SH to SS groups occurs, so that the ability 

 to react with the arsenicals is reduced. They applied this particularly to 

 glutathione. Harvey (1948) was the first to test this hypothesis experimen- 

 tally by determining the SH and SS groups in normal and resistant trypa- 

 nosomes (see accompanying tabulation). There are perhaps differences 

 but they cannot be correlated with resistance, and Harvey preferred to 

 interpret these changes as secondary to the altered cell metabolism; in fact, 



