CALVIN B. COULTER 781; 



and Michaelis^^ for serum euglobulin (about pH 5.2). When examined 

 in whole serum however the euglobulin precipitates best at pH 6.3 

 as noted above and shows no movement in the electric field between 

 pH 6.2 and 6.4. Under these conditions it exists therefore probably 

 not as pure euglobulin but as a compound with some other substance 

 of the serum. 



Different compounds of a protein may differ in the position of the 

 isoelectric point. When the reaction of red blood cell suspensions 

 in saccharose solution is adjusted with NaOH and HCl the isoelectric 

 point as determined by cataphoresis and the optimum for agglutina- 

 tion lie near pH 4.7 ;i^ in the presence of sodium acetate and sodium 

 phthalate this critical point is found near pH 3.8.^^ The compound 

 with a weak acid thus differs from the compound with a strong acid, 

 as pointed out by Professor J. L. R. Morgan ;i^ it is possible then 

 that the value pH 6.1 to 6.4 represents the critical point in the ionization 

 of a compound of the euglobulin with a weak base, in which state 

 it exists in the serum or when dissolved or suspended in distilled 

 water without washing. We may conclude then that it is chiefly 

 or entirely the ionized fraction of the euglobulin which takes part 

 in the reaction involved in thermolability. 



The existence of a point of inflection in the thermostability of 

 the end-piece at pH 6.9 suggests a similar interpretation in the case 

 of this fraction. No other data are available however by which 

 to identify the substance or compound concerned. 



The difference in behavior of the fractions of complement when 

 heated separately and when heated together suggests that during 

 the process of therm oinactivation the ions of the euglobulin compound 

 combine or interact chemically with substances contained in the 

 pseudoglobulin and albumin fraction. 



The protection against destruction afforded by the presence of 

 NaCl on the alkaline side of pH 6.1 to 6.4 maybe explained by the 

 depression in ionization of the Na euglobulin compound existing 



^^Rona, p., and Michaelis, L., Biochem Z., 1910, xxviii, 193. 

 1^ Coulter, C. B., /. Gen. Physiol. 1920-21, iii, 309. 

 ^ ^ Unpublished experiments. 

 ^' Personal suggestion. 



