396 THE CHEMICAL COMPOSITION OF THE BODY [CH. XXV. 



bufc practically all that is known with certainty is that many different 

 substances may be obtained by the decomposition of proteids. How 

 they are built up into the proteid molecule is unknown. The decom- 

 positions 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 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, etc., 

 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, hexone bases, fatty acids, amido- 

 acids like leucine and arginine, and aromatic compounds like tyrosine. 



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 haat in concentrated 

 mineral acids and alkalies. 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 con- 

 verted into a hydrated variety of proteid, of smaller molecular weight, 

 called peptone. The intermediate substances formed in this process 

 are called proteases or albumoses. Commercial peptone contains a 

 mixture of proteoses and true peptone. 



Heat Coagulation. Most native proteids, like white of egg, are 

 rendered insoluble when their solutions are heated. The temperature 

 of heat coagulation differs in different proteids ; thus myosinogen and 

 fibrinogen coagulate at 56 C., serum albumin and serum globulin at 

 about 75 C. 



The proteids which are coagulated by heat come under two 

 classes : the albumins and the globulins. These differ in solubility ; 

 the albumins are soluble in distilled water, the globulins require salts 

 to hold them in solution. 



Indiffusibility. The proteids (peptones excepted) belong to the 

 class of substances called colloids by Thomas Graham ; that is, they 

 pass with difficulty, or not at all, through animal membranes. In the 

 construction of dialysers, vegetable parchment is largely used. 



Proteids may thus be separated from diffusible (crystalloid) sub- 

 stances like salts, but the process is a tedious one. If some serum or 

 white of egg is placed in a dialyser (fig. 341) and distilled water 

 outside, the greater amount of the salts passes into the water through 

 the membrane and is replaced by water ; the two proteids albumin 



