734 6. ARSENICALS 



for trypanosomiasis, and carbarsone for amebiasis. Some workers believe 

 the common denominator to be the trivalent arsenoxide forms, and the 

 differences in potency and efficacy to be related to the factors of penetration 

 and distribution. There is no question but that various protozoa vary 

 markedly in susceptibility to different arsenicals in vitro, and that a change 

 in the structure may increase activity toward one and decrease it toward 

 another, but such can be explained on the basis of different membrane 

 properties and degrees of penetration (whether it is true or not). One can 

 say, at least, that most protozoa are more susceptible than other microor- 

 ganisms to the arsenoxides. Our principal problem will be to determine as 

 far as possible the mechanisms by which these arsenicals immobilize and 

 kill protozoa. 



Quantities of the Arsenicals Necessary for Inhibition 



The concentrations of arsenicals required for immobilization or the kil- 

 ling of protozoa are often quite low, as is evident from Table 6-11 in which 

 a few examples have been gathered to illustrate this potency. The time 

 factor is of obvious importance; the concentration required to immobilize 

 or kill in a short time being much greater than that required over many 

 hours. This is probably due to the time taken for the organisms to accu- 

 mulate sufficient arsenical, the actual amount of arsenical bound to the 

 organism probably being roughly the same in all cases when immobilization 

 or death occurs. These inhibitions also depend on the density of the trypa- 

 nosome suspension, especially when low concentrations of the arsenicals 

 are used, a high density protecting by distributing the available arsenical 

 between many organisms, so that the behavior is quite similar to the pseudo- 

 irreversible inhibition of enzymes. 



Since 1930 it has been generally believed that the active trypanocidal 

 arsenicals are bound to the trypanosomes while the inactive ones are not, 

 and a large amount of evidence confirming this has been reported. The most 

 convincing study was by Eagle and Magnuson (1944), who tested some 

 30 arsenical derivatives against trypanosomes, and their results have been 

 plotted in Fig. 6-8 to show how the amount of arsenical bound is related 

 to the relative trypanocidal activity. The ratio of the amount of arsenical 

 bound to the trypanosomes to that in the supernate also rises with the 

 activity in a similar manner, and for phenylarsenoxide is 224 under the 

 experimental conditions. It is interesting now to consider how much arsen- 

 ical must be bound to trypanosomes to exert a lethal action. Reiner et al. 

 (1932) found that one trypanosome binds approximately 6 X 10^ molecules 

 of phenylarsenoxide. They calculated the surface area of a cell to be 10~^ 

 cm2 and the area of a phenylarsenoxide molecule to be 3 X 10"^^ cm^; 

 thus, assuming a closely packed monolayer of arsenical on the surface 

 of the cell, 3 X 10' molecules could be adsorbed, which is around 5 times 



