328 PROTEINS 



proteoses obtained from animal proteins have been de- 

 scribed ; but in all cases it is difficult to say whether 

 these substances were not produced by some secondary 

 action of enzymes upon the protein, during the process of 

 isolation. 



3. Peptones. Substances belonging to this class still 

 give the biuret reaction, but unlike all other proteins 

 they are not precipitated from solution by saturation 

 with ammonium sulphate. 



4. Polypeptides, which include such substances as 

 leucyl glutamic acid, obtained by Fischer and Abder- 

 halden from gliadin by hydrolysis with 70 per cent 

 sulphuric acid, and glycyl tyrosine and glycyl leucine, ob- 

 tained by the same authors from silk fibroin and elastin 

 respectively. 



COMPARISON BETWEEN VEGETABLE AND ANIMAL PROTEINS. 



From the foregoing it will be seen that, in the main, the 

 animal and vegetable proteins conform sufficiently well with 

 regard to their general properties and solubilities that they 

 may be included in the same scheme of classification. The 

 greatest irregularities are exhibited in the groups of albumins 

 and globulins, but even these are not sufficiently serious to 

 suggest any fundamental difference between the proteins de- 

 rived from animal and vegetable sources. These views are 

 confirmed by chemical evidence : with the single exception of 

 di-amino trihydroxy-dodecanic acid, a substance as yet only 

 obtained from casein, all the known products of hydrolysis of 

 animal proteins have been obtained from vegetable proteins, 

 and there is no real reason for assuming that there is any fun- 

 damental difference in the structure of the protein molecule 

 from the two sources. 



On the whole, vegetable proteins yield more glutamic 

 acid, and many also yield rather more proline, arginine and 

 ammonia than do animal proteins. 



The comparatively large quantities of proline and arginine 

 which occur in some cases may be responsible for the slightly 

 higher nitrogen content which characterizes proteins of vege- 

 table origin. 



Further, it should be noted that the prolamins, or alco- 



