THE CHEMISTEY OF ALBUMINS 191 



pepsin and trypsin alone, while in the case of the mixture of 

 pepsin and acid, and of trypsin and alkali, digestion will be 

 almost complete. 



This demonstrates the fact that pepsin is most active in 

 presence of dilute acid, while trypsin is most active in the 

 presence of dilute alkali. In order to investigate the products 

 of decomposition in each of these cases larger quantities of 

 egg-white must of course be taken ; if this is done, the products 

 present in solution can then be investigated in the manner to be 

 described. 



In following the reaction it will be advisable to make 

 observations from time to time, as the reaction is progressive, 

 products of decreasing complexity being obtained as it 

 proceeds. If to a portion of the solution shortly after the 

 beginning of the reaction strong alcohol or a saturated solution 

 of ammonium sulphate is added, a precipitate is formed ; the 

 substances thus precipitated are known as albumoses. At 

 a further stage ammonium sulphate is added ; no precipitate 

 will be obtained, but a precipitate will still be formed if alcohol 

 is added. These products of decomposition of albumin, which 

 are soluble in water and precipitated by alcohol but not by 

 ammonium sulphate, are known as peptones. It will be found 

 on testing that they still give the biuret reaction, showing 

 that a complex residue containing amino (NH 2 ) and imino 

 (NH) groups is still present. The red substance of the biuret 

 reaction is believed by Schiff to be a copper potassium com- 

 pound having the following constitution : 



OH OH 



II I I II 



C-NH 2 -Cu NH 2 -C 



NH/ ^ 



X C-NH,-K K-NH 2 -C 



II I I II 



OH OH 



