14 THE VEGETABLE PROTEINS 



The usual method employed to isolate the proteins from the aque- 

 ous extract is to saturate the solution with ammonium sulphate, filter 

 out the precipitate produced, which contains all the proteins, dissolve 

 this in the dilute solution of ammonium sulphate which results on 

 treating it with water, and, after filtering clear, to dialyse the solution 

 in running water for several days until no more precipitate is pro- 

 duced. If a precipitate results, this contains the globulins, the salts 

 of such proteins as are soluble in water in the uncombined state but 

 have the properties of globulins when combined with the small 

 quantities of acids which usually develop during the process of ex- 

 traction, and also such insoluble derivatives as frequently result, 

 during dialysis, from changes in the albumins. Such products do not 

 appear to be formed from animal albumins but are not uncommon alter- 

 ation products of vegetable albumins. After filtering out the precipi- 

 tate produced by dialysis, the solution should be tested for proteins 

 which can be precipitated as acid compounds from very dilute saline 

 solutions. This is done by adding to a part of the solution about 0*5 

 per cent, of sodium chloride and passing carbonic acid through it for 

 some time. The small concentration in hydrogen ions thus produced 

 enables such proteins, if present, to unite with hydrochloric acid and 

 thereby form insoluble compounds. [Cf. Osborne (3 1 5)]. If a precipi- 

 tate is thus produced the entire solution is treated in the same way, 

 the resulting precipitate filtered out, and the filtrate again dialysed 

 until no more precipitate separates. From the solution, thus freed 

 from globulins, the albumins are separated by fractional coagulation 

 by heating to definite temperatures in a water bath, or by fractional 

 precipitation with ammonium sulphate, at definite concentrations; the 

 procedure, in either case, being determined by preliminary experi- 

 ments, whereby the proper temperatures or concentrations in am- 

 monium sulphate are determined. When the albumins have been 

 separated from the solution by heat, the filtered solution 'is dialysed 

 into about twice its volume of alcohol, whereby it is rapidly reduced 

 in volume, and at the same time becomes mixed with a considerable 

 proportion of alcohol. By repeating the dialysis in a fresh quantity 

 of alcohol, or by adding alcohol to the solution, the concentration in 

 alcohol is made sufficiently great to precipitate all the dissolved pro- 

 teose. If the latter does not separate readily from the solution it can 

 be made to do so by adding a drop or two of strong ammonium acet- 

 ate solution. The precipitate is digested with absolute alcohol, dis- 

 solved in water, filtered from any insoluble residue which may be 

 present if globulins or albumins have not been completely separated 



