CHEMIC COMPOSITION OF THE HUMAN BODY. " 13 



When any one of the typical proteins, found in animal or vegetable 

 tissues, is hydrolyzed by acids, alkalies and animal ferments under appro- 

 priate conditions, it can be resolved through a series of descending stages 

 into relatively simple nitrogen-holding bodies termed amino-acids and 

 diamino-arids, of which somewhat more than twenty have been isolated 

 and their properties determined. The principal amino-acids are as follows: 

 Glycocoll, alanin, leucin, isoleucin, amino-isovalerianic acid, serin, aspartic 

 acid, glutamic acid, phenylalanin, tyrosin, prolin, tryptophan. The principal 

 diamino-acids are as follows: Ornithin, lysin, histidin, arginin, cystin. 



The protein molecule is therefore structurally complex. The manner 

 in which these elementary compounds are arranged, united or grouped 

 in any given protein, is practically unknown. More or less successful 

 attempts have been made at the reconstruction of the protein molecule by 

 synthetic methods, by the union of two or more of the amino-acids. A 

 number of such compounds have been formed by the union of from two to 

 ten or more amino-acids, all of them exhibiting many of the protein reac- 

 tions. Such bodies are termed, according to their complexity, peptids and 

 polypeptids. 



Physical Properties. As a class the proteins are characterized by the 

 following properties: 



1. Indiffusibility. None of the proteins normally assume the crystalline 



form, and hence they are not capable of diffusing through parchment 

 or an animal membrane. Peptone, a product of the digestion of 

 proteins, is an exception as regards its diffusibility. As met with in 

 the body, all proteins are amorphous, but vary in consistence from the 

 liquid to the solid state. The colloid character of the proteins permits 

 of their separation and purification from crystalloid diffusible com- 

 pounds by the process of dialysis. 



2. Solubility. Some of the proteins are soluble in water, others in solutions 



of the neutral salts of varying degrees of concentration, in strong acids 

 and alkalies. All are insoluble in alcohol and ether. 



3. Coagulability. Under the influence of heat and animal ferments, 



some of the proteins readily pass from the soluble liquid state to the 

 insoluble solid state, attended by a permanent alteration in their chemic 

 composition. To this change the term coagulation has been given. 

 The various proteins, however, coagulate at different temperatures. 

 Proteins are capable of precipitation without losing their solubility 

 by ammonium sulphate, sodium chlorid, and magnesium sulphate. 



4. Fermentability. In the presence of specific microorganisms bacteria 



the proteins, owing to their complexity and instability, are prone 

 to undergo disintegration and reduction to simpler compounds. This 

 decomposition or putrefaction occurs most readily when the conditions 

 most favorable to the growth of bacteria are present viz., a temperature 

 varying from 25 C. to 40 C., moisture, and oxygen. The intermediate 

 as well as the terminal products of the decomposition of the proteins are 

 numerous, and vary with the composition of the protein and the specific 

 physiologic action of the bacteria. Among the intermediate products 

 is a series of alkaloid bodies, some of which possess marked toxic 



