Arda Alden Green 221 



tionation may be assumed to be relatively pure. Ultimately the most important 

 single factor in the isolation of a given protein is an adequate method of char- 

 acterizing and identifying that protein. 



The Plasma Proteins 



The isolation of individual proteins from plasma is one of the most pressing 

 of present-day problems in biochemistry. Proteins with peculiar physiological 

 roles such as fibrinogen, prothrombin, the recently isolated protein constitu- 

 ents of complement,^" the enzymes choline esterase and phosphatase, and the 

 antibodies to various bacteria are more easily separated, since one has the 

 biochemical yardstick as well as the ordinary means of chemical or physico- 

 chemical characterization. It is no longer adequate to make a separation into 

 "globulin" and "albumin" fractions. 



The application of the new cataphoretic methods of Tiselius" has divided 

 the globulin fraction of serum into at least three components a, ^, and y, and 

 the albumin fraction into two." Extensive studies have been carried out 

 correlating ammonium sulfate and alcohol precipitation methods with elec- 

 trophoretic mobility and with molecular weight as determined by ultracentri- 

 fuge and diffusion methods of both the albumin and globulin fractions."" 

 Cohn's recent review'" divides the plasma proteins into groups based on their 

 electrophoretic mobility. 



It is becoming increasingly evident that each new method of identifying 

 or classifying serum proteins increases the ntunber known. The simple pro- 

 cedure of separating the water-soltible from the water-insoluble fractions of 

 the globulins is an example of such a procedure. Three reproducible iso- 

 electrically precipitable fractions of water-insoluble globulins have been 

 separated from normal horse serum, and their amphoteric properties and 

 viscosities studied.-"'^ They have been called Pj, Pu and Pm. Pn and Pi„ are 

 the more insoluble and have isoelectric points at 6.2 and 5.0 respectively. 

 Reiner and Reiner" had previously found two euglobulin fractions in normal 

 and in antipneumococcus horse serum which correspond to Pu and Pm- 

 Hewitt^ has described two euglobulin fractions obtained from sera from a 

 number of species which he calls euglobulin I and II with isoelectric points 

 at pH 7 and pH 6. Svensson"* has prepared these various fractions and com- 

 pared their electrophoretic mobility with the following results: "The frac- 

 tions euglobulin I and II, described by Hewitt, were found to behave dis- 

 tinctly different in electrophoresis. The former contained principally f3- and 

 y-globulins; the latter was a rather homogeneous a-globulin preparation. . , . 

 The fraction Pi of Green was shown to be identical with euglobulin II of 

 Hewitt and with a-euglobulin of Tiselius. The results indicated that Green's 

 fractions P„ and Pm correspond to y- and /3-euglobulins respectively" (p. 824). 



The blood proteins, apart from their various specific functions and their 

 general function of maintaining the osmotic pressure, apparently combine 

 with smaller organic molecules including phospholipids and carbohydrates. 



