22 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



The decompositions that occur in the body are, moreover, different 

 from those which can be made to occur in the laboratory ; hence the 

 conclusion that living protoplasm differs somewhat from the non- 

 living proteid material obtainable from it. 



(1) In the body. Carbonic acid, water, and urea' are the chief 

 final products. Glycocine, leucine, creatine, uric acid, ammonia, &c., 

 are probably intermediate products. Carbohydrates (glycogen) 

 and fats may also originate from proteids. 



(2) Outside the body. Various strong reagents break up proteids 

 into ammonia, carbonic acid, amines, fatty acids, amido-acids like 

 leucine, arginine, and glycocine, and aromatic compounds like 

 tyrosine. 



TESTS FOR PROTEIDS 



Solubilities. — All proteids are insoluble in alcohol and ether. 

 Some are soluble in water, others insoluble. Many of the latter are 

 soluble in weak saline solutions. Some are insoluble, others soluble 

 in concentrated saline solutions. It is on these varying solubilities 

 that proteids are classified. 



All proteids are soluble with the aid of heat in concentrated 

 mineral acids and alkalis. Such treatment, however, decomposes as 

 well as dissolves the proteid. Proteids are also soluble in gastric and 

 pancreatic juices ; but here, again, they undergo a change, being 

 converted into a hydrated variety of proteid of smaller molecular 

 weight called peptone. The intermediate substances formed in this 

 process are called proteoses or albumoses. Commercial peptone con- 

 tains a mixture of proteoses and true peptone. 



Heat Coagulation. — Many of the proteids which are soluble in 

 water or saline solutions are rendered insoluble when those solutions 

 are heated. This is true for most of the proteids that occur in nature. 

 The solidifying of white of egg when heated is a familiar instance of 

 this. The temperature of heat coagulation differs in different pro- 

 teids : thus myosinogen and fibrinogen coagulate at about 56° C. ; 

 serum albumin and serum globulin at about 75° C. 



The proteids which are coagulated by heating their solutions come 

 for the most part into two classes — the albumins and the globulins. 

 The full distinction between these we shall see immediately. We 

 may, however, state here that the albumins are soluble in distilled 

 water; the globulins are not, but require salts to hold them in 

 solution. 



' Becent research has shown that urea can be also obtained from proteids by 

 analytical methods outside the body (see under Ubine). 



