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X 10~^ which is equivalent to a pH of about 5.4; that is, the point at 

 which complement begins to be affected by acid. The hydrogen ion 

 concentration at which the inactivation of complement becomes 

 complete is about pH = 5.0; this is just on the acid side of the iso- 

 electric point of the serum albumin, which is at pH 4.7.^^ 



Now amphoteric substances, among which we must number the 

 serum proteins, behave as anions on the alkaline side of their isoelec- 

 tric points, and as cations on the acid side of their isoelectric points; 

 while at this point the sign of their charge changes from negative to 

 positive, and there are great changes in their physical properties corre- 

 sponding to slight changes of hydrogen ion concentration.^' 



TABLE III. 



The pH of Samples of the Same Lot of 5 Per Cent Complement Radiated for Different 



Lengths of Time. 



Since the hydrogen ion concentration at which these changes occur 

 is coincident with that at which complement loses its hemolytic power, 

 it is not improbable that there is a connection between the phenomena. 

 We might suppose that the protein cation takes part in an irreversible 

 reaction with the lytic substance or some one of its constituent groups. 

 Other possibilities might be suggested, but the evidence is not ade- 

 quate for distinguishing between one possibility and another, and the 

 essential point remains the same; that there is some close connection 

 between the state of the serum proteins and the effectiveness of the 

 lytic substance. 



^^Michaelis, L., and Davidsohn, H., Biochem. Z., 1911, xxxiii, 456. 

 I'Loeb, J., /. Gen. Physiol, 1918-19, i, 237. 



