322 PROTEINS 



this course is recommended when both are present on account 

 of the saving of time. 



The proteins isolated by these means may be roughly 

 purified as follows : — 



1. Albumins and globulins. — These will nearly always be 

 contaminated one with the other. A separation may be 

 effected in the following ways : — 



(a) The solution is saturated with magnesium sulphate, 

 whereby the globulins are precipitated and the 

 albumins remain in solution (but see above, under 

 albumins and globulins). 



(6) Dialysis. The extract is placed in a dialyser and 

 floated in water which is continually changed. The 

 precipitated globulins are filtered off from the salt 

 solution, which, of course, is getting weaker and 

 weaker and contains the albumins. The precipitated 

 globulins are re-dissolved in warm saline solution, 

 which may on cooling deposit globulins in a crystal- 

 line form ; if this does not occur, the solution may 

 be saturated with magnesium sulphate. The albumins 

 may be precipitated by saturating the solution with 

 ammonium sulphate. Further purification may be 

 effected by a repetition of the process and by frac- 

 tional precipitation with magnesium sulphate or by 

 ammonium sulphate, according to the protein to be 

 purified. 



2. Glutelins. — The proteins soluble in dilute alkali may 

 be precipitated by very carefully neutralizing the solution and 

 then further adding only just sufficient acid to cause the 

 precipitation of the glutelins. The precipitate may be well 

 washed with a dilute neutral saline solution, in which it is 

 insoluble, in order to remove any globulins which may be 

 present. 



3. Prolamins. — The extract, which is made by treatment 

 with hot alcohol, is either mixed with water sufficient in amount 

 to precipitate the proteins, or the filtered solution may be 

 evaporated under a reduced pressure at a temperature not 

 higher than 50" C. The precipitate is filtered off and may be 

 re-dissolved in as little alcohol as possible. From this solu- 

 tion the protein may be recovered by the addition of absolute 



