CHEMICAL COMPOSITION OF BLOOD-PLASMA. 449 



Proteins of the Blood-plasma. The general properties and 

 reactions of proteins and the related compounds, as well as a classi- 

 fication of those occurring in the animal body, are described briefly 

 in the Appendix. This description should be read before attempt- 

 ing to study the proteins of the plasma and the part they take in 

 coagulation. Three proteins are usually described as existing in the 

 plasma of circulating blood, namely, fibrinogen, paraglobulin, 

 or, as it is sometimes called, "serum-globulin," and serum-albumin. 

 The first two of these proteins, fibrinogen and paraglobulin, belong 

 to the group of globulins, and hence have many properties in com- 

 mon. Serum-albumin belongs to the group of albumins, of 

 which egg-albumin constitutes another member. 



Serum-albumin. This substance is a typical protein. It can be 

 obtained readily in crystalline -form from the horse's blood. Its 

 percentage composition, according to Michel, is as follows: C, 53.08; 

 H, 7.10; N, 15.93; S, 1.90; O, 21.96. 



Its molecular composition, according to Schmiedeberg,* may be 

 represented by C 78 H 122 N 20 SO 24 or some multiple of this formula. 

 Serum-albumin shows the general reactions of the native albumins. 

 One of its most useful reactions is its behavior toward magnesium 

 sulphate and ammonium sulphate. Serum-albumin usually occurs in 

 the body-liquids together with the globulins, as is the case in blood. 

 If such a liquid is thoroughly saturated with solid magnesium sul- 

 phate or half saturated with ammonium sulphate, the globulins 

 are precipitated completely, while the albumin is not affected. 

 So far as the blood and similar liquids are concerned, a definition 

 of serum-albumin might be given by saying that it comprises all 

 the proteins not precipitated by saturation with magnesium sul- 

 phate or by half saturation with ammonium sulphate. When its 

 solutions have a neutral or an acid reaction, serum-albumin is 

 precipitated in an insoluble form by heating the solution above a 

 certain degree. Precipitates produced in this way by heating 

 solutions of proteins are spoken of as coagulations heat coagula- 

 tions and the exact temperature at which coagulation occurs 

 is to a certain extent characteristic for each protein. The tem- 

 perature of coagulation of serum-albumin is usually given at from 

 70 to 75 C., but it varies greatly with the conditions, for in- 

 stance, with the reaction of the solution, its concentration in salts, 

 or with the nature of the salts present. It has been asserted, 

 in fact, that careful heating under proper conditions gives separate 

 coagulations at three different temperatures, namely, 73, 77, 

 and 84 C., indicating the possibility that what is called "serum- 

 albumin" may be a mixture of three proteins. Serum-albumin 

 occurs in blood-plasma and blood-serum, in lymph, and in the 

 different normal and pathological exudations found in the body, 



* "Archiv f. exper. Pathol. u. Pharmakol.," 39, 1, 1897. 

 29 



