EFFECTS ON PROTOZOA 737 



establish what fraction is in the SH form under normal conditions, where 

 certainly much is in the disulfide form in proteins; and (2) the average ratio 

 arsenical/SH is uncertain, i. e., one does not know exactly the type of bind- 

 ing, whether it is mainly linear (arsenical/SH = 1) or cyclic (arsenical/SH 

 = 0.5). The figures of Harvey are based on the latter assumption, which is 

 probably correct. However, the amount of arsenical bound to functional 

 SH groups (those whose inactivation contributes directly to the trypanocidal 

 action), e. g., to lipoate, may be only a small fraction of this. It nevertheless 

 seems to indicate that the bulk of the arsenical taken up or bound to these 

 cells is attached to SH groups. It is assumed for the less active arsenicals, 

 where less is bound, that less penetrates into the cells. The potent arsenicals 

 would thus fall into the class of very active substances, those for which only 

 a few million or less molecules need be bound to the cell for marked changes 

 to be manifested, the amounts being similar to those for acetylcholine, 

 histamine, and certain hormones. It is worth noting that the metabolic 

 state of the cells is of some importance in the binding of the arsenicals, since 

 Pedlow and Reiner (1935) found the addition of glucose to alter the amounts 

 of oxophenarsine and neoarsphenamine bound to trypanosomes. No dif- 

 ference in binding was observed in vacuum or 100% oxygen, although the 

 permeability of the cells might be changed as well as the number of func- 

 tional SH groups available for reaction. 



Kinetics of Action 



When trypanosomes or spirochaetes are exposed to arsphenamine or pen- 

 tavalent arsenicals, a fairly long lag period is observed, due in part to the 

 time required for these arsenicals to be split and/or reduced. However, there 

 is also a lag period when trivalent arsenicals are used in reasonable concen- 

 trations, and this lag period is longer than the period necessary for the 

 taking up of sufficient arsenical to kill. The trivalent lag period is thus 

 probably the sum of two parts: (1) the period for accumulation or binding 

 of an adequate amount of arsenical, and (2) the time required for the death 

 of the cells following inhibition of the enzyme systems involved. Trypano- 

 somes begin to swell after a certain interval of exposure, become distorted, 

 and eventually lyse (Hawking, 1938). The following equation was used: 



{C - Co)"t = k 



where Cq is the minimal concentration for an effect. The value of n was 

 found to be 0.5-0.8 for arsenite and tryparsamide oxide, and 1.04 for phen- 

 ylarsenoxide.* It was concluded that the total process consists of three 



* An equation with a (t — t^) term might be somewhat better inasmuch as there 

 is a minimal time for any concentration to produce a characteristic effect. 



