780 THERMOLABILITY OF COMPLEMENT 



The mid-piece function is thus better preserved when isolated 

 than when heated in the presence of the other constituents of serum 

 at all the reactions except those in the narrow zone pH 6.1 to 6.4, 

 within which it is almost equally thermostable in both cases. 



If fresh guinea pig serum is heated in dilution in saUne solution 

 instead of in distilled water it is evident from Fig. 7 that the com- 

 plementary function is better preserved and is less affected by change 

 in reaction than when heated in distilled water, at reactions on the 

 alkaline side of pH 6.1 to 6.4. On the acid side of this point the loss 

 in activity with increasing acidity is as great in the presence as in 

 the absence of salt. The determination of the H ion concentration is 

 affected with a greater error in the dilution of the serum in saline 

 than in distilled water, when colorimetric methods are used. Never- 

 theless the turning-point observed is essentially the same as in the 

 experiments in distilled water dilution, and corresponds with the 

 point on the curve given by Brooks^^ at which the destruction of 

 complement begins in saline solution when the temperature is kept 

 at 10°C. 



DISCUSSION. 



The inactivation which complement undergoes when heated in 

 distilled water dilution is closely related to the properties of the eu- 

 globulin fraction since this destruction is least at the reaction 

 at which the euglobulin is least soluble. The chemical reaction 

 involved in thermolability in which the euglobuHn is necessarily 

 one of the reacting substances is therefore determined by the chemical 

 state in which the euglobulin exists. 



It has been found in the case of a number of proteins that solu- 

 bihty and other physical properties are at a minimum at the H ion 

 concentration characteristic of the isoelectric point. At this point the 

 ionization of the protein either as an acid or as a base is at a minimum. 

 If the euglobulin sediment from serum is washed with a large volume 

 of distilled water and brought into solution again by the addition 

 of NaOH to pH 7.4 it becomes least soluble on the addition of HCl be- 

 tween pH 5.1 and 5.7 (depending probably on its purity) and is 

 isoelectric about pH 5.0. The latter determination was made electro- 

 metrically; the value corresponds closely with that given by Rona 



