96 PHYSIOLOGICAL CHEMISTRY 



III. PRODUCTS OF PROTEIN HYDROLYSIS 



1. Infraproteins, e.g., acid infra-protein (acid albuminate), alkali 

 infraprotein (alkali albuminate). 



2. Proteoses, e.g., protoproteose, netero protease, deutero protease. 



3. Peptones, e.g., ampho peptone, antipeptone. 



4. Polypep tides, e.g., dipeptides, tripeptides, tetrapeptides. 



CONSIDERATIONS OF THE VARIOUS CLASSES 

 OF PROTEINS 



SIMPLE PROTEINS 



The simple proteins are true protein substances which, upon hy- 

 drolysis, yield only a-amino acids or their derivatives. " Although 

 no means are at present available whereby the chemical individuality of 

 any protein can be established, a number of simple proteins have been 

 isolated from animal and vegetable tissues which have been so well 

 characterized by constancy of ultimate composition and uniformity of 

 physical properties that they may be treated as chemical individuals 

 until further knowledge makes it possible to characterize them more 

 definitely." Under simple proteins we may class albumins, globulins, 

 glutelins, prolamins, albuminoids, histones and protamines. 



ALBUMINS 



Albumins constitute the first class of simple proteins and may be 

 defined as simple proteins which are coagulable by heat and soluble 

 in pure (salt-free) water. Those of animal origin are not precipitated 

 upon saturating their neutral solutions at 30C. with sodium chloride 

 or magnesium sulphate, but if a saturated solution of this character 

 be acidified with acetic acid the albumin precipitates. All albumins 

 of animal origin may be precipitated by saturating their solutions with 

 ammonium sulphate. 1 They may be thrown out of solution by the 

 addition of a sufficient quantity of a mineral acid, whereas a weak 

 acidity produces a slight precipitate which dissolves upon agitating the 

 solution. Metallic salts also possess the property of precipitating al- 

 bumins, some of the precipitates being soluble in excess of the reagent, 

 whereas others are insoluble in such an excess. Of those proteins 

 which occur native the albumins contain the highest percentage of sul- 

 phur, ranging from 1.6 to 2.5 per cent. Some albumins have been 



^ In this connection, Osborne's observation that there are certain vegetable albumins 

 which are precipitated by saturating their solutions with sodium chloride or magnesium 

 sulphate or by half-saturating with ammonium sulphate, is of interest. 



