736 



6. ARSENICALS 



the value found. It is quite likely, however, that the binding is not just 

 at the surface and that most of the arsenical is bound within the cell. These 

 figures indicate that a great deal of arsenical is not required to kill these 

 organisms and that specific sites are probably involved. It must be realized 

 in such studies that all of the arsenical bound is probably not responsible 

 for the lethal action and that these figures are undoubtedly higher than 

 the true values for "active binding." Hawking (1938) found that under 

 certain circumstances the ratio between trypanosome-bound arsenical 



25 50 



ARSENICAL eOUNO(« OF TOTAL) - 



Fig. 6-8. Binding of arsenicals by trypanosomes 



as related to the trypanocidal activities. (From 



Eagle and Magnuson, 1944.) 



and arsenical in the medium is 10,000 for phenylarsenoxide, 5000 for try- 

 parsamide oxide, and 30-100 for arsenite. The amount of tryparsamide 

 oxide required to kill in 3 hr is 8 X 10"^^ g per cell, which is about 10' 

 molecules per trypanosome, and this was calculated to be sufficient to cover 

 about 5% of the cell surface; even less would be required for a lethal action 

 in 24 hr. How do these values correspond to the number of SH groups in 

 the trypanosomal cell? Harvey (1948) detected 2.7 X 10~^ meq SH per 

 g protein, or 1.6 X 10^^^ meq SH per cell in Trypanosoma equiperdum, 

 and calculated that this would bind around 4.8 X 10^ molecules of arsenical, 

 a figure of the same order of magnitude as the experimental values given 

 above. There are two difficulties in making such comparisons: (1) Although 

 it is easy to determine the total SH + SS groups in cells, it is difficult to 



