46 THE PROTEINS. [CH. II. 



ammonium sulphate if the reaction of the solution be 

 neutral or alkaline. 



They are partially precipitated by solutions of these 

 substances in the presence of acid. 



They are completely precipitated by full saturation 

 with ammonium sulphate from a neutral, acid, or alkaline 

 solution. 



The solubility in water of the globulins of blood serum is apparently 

 modified by the presence of certain " lipines " (see p. 153). If the serum 

 globulins be precipitated by half saturation with ammonium sulphate, the pre- 

 cipitate dissolved in water, as described in Ex. 36, and the solution thoroughly 

 dialysed, it will be found that only a portion of the protein is precipitated by 

 the dialysis. The fraction that remains soluble in water has been called 

 " pseudo-globulin " to distinguish it from the water-insoluble fraction or 

 " eu-globulin." It was formerly believed that " pseudo-globulin " was 

 an albumin and that it was impossible to separate the globulins from the 

 albumins by half saturation with ammonium sulphate. Recent work by 

 Hartley on the globulins of serum and by Raistrick on those of milk have 

 demonstrated, however, that the distribution of nitrogen as mon-amino acids 

 and as bases is practically the same for the two globulin fractions, which differ 

 appreciably from the albumin fraction in this respect. It is probable that 

 the insolubility of the " eu-globulin " in water is due to its association with 

 lipoid. 



32. Dilute 5 cc. of serum with 50 cc. of distilled water. A 

 faint cloud of serum globulin is formed. Cautiously add 0-4 per 

 cent, hydrochloric acid or i per cent, acetic acid until the cloud has 

 reached its maximum density. Divide into two portions A and B. 

 To A add a couple of drops of a saturated solution of ammonium 

 sulphate. The solution becomes quite clear. To B add a couple of 

 drops of strong acid. The cloud disappears. 



NOTE. The globulin of the serum is held in solution both by salts and 

 alkalies. Dilution alone produces a very small precipitate, but if the solution 

 be now treated with just sufficient acid to neutralise the alkali, a much larger 

 fraction of the globulin is thrown down. This globulin is soluble in dilute 

 neutral salts, or in an excess of acid. 



33. Prepare a suspension of globulin by the following method. 

 To 15 cc. of serum in a beaker add 2 cc. (about 30 drops) of I per 

 cent, acetic acid and 100 cc. distilled water. Stir and allow the 

 mixture to stand for about 20 minutes. A precipitate of globulin 

 settles down. Very carefully pour off the supernatant fluid and 

 divide the suspended globulin into two equal portions in clean test- 

 tubes. With these perform the two following exercises. 



