CHEMIC COMPOSITION OF THE HUMAN BODY. 15 



The average percentage composition of several proteins is shown in the 

 following analyses: 



C. H. N. O. S. 



Egg-albumin. .. 52.9 7.2 15.6 23.9 0.4 (Wiirtz). 



Serum-albumin 53.0 6.8 16.0 22.29 1.77 (Hammersten). 



Casein 53.3 7.07 15.91 22.03 0.82 (Chi ttenden and Painter). 



Myosin 52.82 7.11 16.77 2I -9 I - 2 7 (Chi ttenden and Cummins). 



The molecular composition of the proteins is not definitely known, and 

 the formulae which have been suggested are therefore only approximative. 

 Leow assigns to albumin the formula C 72 H 112 N 18 O 22 S, while Schiitzen- 

 berger raises the numbers to C 240 H 392 N e5 O 75 S 3 , either of which shows that 

 the protein molecule is extremely complex. 



Structure of the Protein Molecule. From the large size of the protein 

 molecule as indicated by its chemic composition it might be inferred that its 

 structure was equally complex. This modern investigation has shown to 

 be the case. 



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

 is hydrolyzed by acids, alkalies and animal ferments under appropriate con- 

 ditions, it can be resolved through a series of descending stages into relatively 

 simple nitrogen-holding bodies termed amino-acids and diamino-acids, 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 reactions. Such 

 bodies are termed polypeptids. 



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

 lowing properties: 



I. Indiffusibility. None of the proteins normally assumes 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 



