194 MECHANISM OF COMPLEMENT ACTION 



The change of surface tension is Hke that accompanying thermo- 

 inactivation/^ and contrary to the hypothesis advanced by Traube,i^' ^^ 

 is probably not the cause of the change in hemolytic power.' ^ 



IV. 



In the following paragraphs there is suggested an hypothesis which 

 has the advantage of explaining many of the known properties of 

 serum complement with more definiteness than current theories and 

 without being in conflict with any well established fact. Definite 

 substances are named only for the sake of making it easier to grasp 

 certain essential ideas, and not with any pretense that complement 

 must be supposed to consist of just these particular substances. The 

 main ideas are these: that there is a hemolytic substance in serum 

 which is constantly breaking down into non-lytic material, and con- 

 stantly renewed from a store of some "precursor" substance; that 

 this lytic substance is so related to the serum proteins that it is more 

 or less permanently inactivated by certain changes in the serum pro- 

 teins; and that the hemolytic substance and all its precursors contain 

 a certain photosensitive molecular grouping whose alteration results 

 in photoinactivation. 



These essential points may be embodied in the following scheme 



A — > changed by light to a 



I I 



B -y " " " " b 



i i 



of , « « (I « L/ 



\ ^ 



c 



in which the different letters represent different chemical individuals; 

 the changes A-^B,a-^b, B'-^C, and b' -^ C represent hydrolyses, 

 and the changes B -^ B' and h ^b' the passage of -S or ^ from solu- 

 tion in fats to solution in water as a result of a change in relative solu- 

 bility produced by the preceding hydrolysis. B {i.e. 'B') is the prin- 

 cipal lysin, and b' , formed by radiation, is probably also hemolytic 

 to a certain extent. 



When complement is heated we need consider only the left-hand 

 series, A -^ B —^ B' —^C. ^isa precursor present in large amount 



1^ Schmidt, H., /. Hyg., 1913, xiii, 314. 



