80 THE CHEMICAL COMPOSITION OF THE BODY 



the compounds differing according to the method employed. In the body 

 it seems clear that living proteid is built up by the food supplied to it, which 

 necessarily contains proteid derived from either a vegetable or an animal 

 source; how this process takes place we are yet unable to say. Recently 

 Taylor has been able to synthesize proteid, protamin, by the reversible 

 action of trypsin on the amido-acids which were previously obtained by the 

 digestion of protamin. The reaction is indicated by the equation: 



Protein -[- Water + Amido-acids. 



Robertson has demonstrated a similar reversible reaction of pepsin on para- 

 nuclein derived from the digestion of casein. These experiments lend a new 

 stimulus to the efforts to build up proteids in the chemical laboratory along 

 the lines of catalytic action of enzymes. 



In the course of later chapters in this book we shall endeavor to trace 

 the steps of the breaking up of proteid in the body, but we may anticipate 

 by mentioning that it is now generally believed that the chief ultimate prod- 

 ucts of this decomposition are urea, a body the formula of which is 

 CO (NH a ) a> carbon dioxide and water, while the intermediate substances or 

 by-products are chiefly ammonia compounds. When proteid material is 

 decomposed by putrefaction, by the action of chemical reagents, acids, alka- 

 lies, or by heat, various bodies are produced, of which amido-acids (acids 

 in which one or more of the hydrogen atoms of the radical of the acid are 

 replaced by amidogen, NH 2 ) and bodies belonging to the aromatic or benzene 

 series predominate. Hence it comes that various theories of the way in 

 which proteids are built up have arisen. The one which has appeared to 

 have received the greatest support is that of Latham. This observer has 

 suggested that proteid may be considered as made up of a series of cyan- 

 alcohols (bodies obtained by the union of any aldehyde with hydrocyanic 

 acid) with a benzene nucleus. Taking ordinary ethyl alcohol, CH 3 CH 2 OH, 

 as the type, the aldehyde of which is CH 3 CHO, the corresponding cyan- 

 alcohol would be CHsCHCNOH. 



CLASSES OF PROTEIDS. 

 Simple Proteids. 



Native Albumins. 



Albumins; serum albumins, egg albumins, lactalbumin. 



Globulins; serum globulin, myosinogen, cytoglobulin, etc. 

 Derived Albumins. 



Albuminates; acid and alkali albumins. 



Coagulated proteids ; heat coagulated and enzyme coagulated proteid. 



Proteose, Peptones, Polypeptids; all derived as cleavage-products 

 of enzyme action on other proteids. 



