CH. XXV.] THE PROTEIDS. 393 



occur without their presence ; and though it is impossible to state 

 positively that they occur as such in living protoplasm, they are 

 invariably obtained by subjecting living structures to analysis. 



Proteids are highly complex compounds of carbon, hydrogen, 

 oxygen, nitrogen, and sulphur occurring in a solid viscous condition 

 or in solution in nearly all the liquids and solids of the body. The 

 different members of the group present differences in chemical and 

 physical properties. They all possess, however, certain common 

 chemical reactions, and are united by a close genetic relationship. 



The various proteids differ a good deal in elementary composi- 

 tion. Hoppe-Seyler gives the following percentages : 



C H N S 



From 51-5 6-9 15-2 0-3 20-9 



To 54-5 7-3 17-0 2-0 23-5 



We are, however, not acquainted with the constitutional formula 

 of proteid substances. There have been many theories on the 

 subject, but practically all that is known with certainty is that 

 many different substances may be obtained by the decomposi- 

 tion of proteids. How they are built up into the proteid mole- 

 cule is unknown 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 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, 

 Ac., are probably intermediate products. Carbohydrates (glycogen) 

 and fats may also originate from proteids. 



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

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

 like leucine and arginiue, 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 

 sol u llc 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 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 converted into a hydrated variety of proteid, of smaller 

 molecular weight, called peptone. The intermediate substances 

 formed in this process are called proteoses or allmmoses. Commer- 

 cial peptone contains a mixture of proteoses and tnie peptone. 



