PROTEINS. 625 



I. Simple proteins. 



These are protein substances which yield only a-amino-acids or 

 their derivatives by hydrolysis. 



The simple proteins occur in all animal and vegetable organisms. 

 In the animal body they are the most prominent solid constituents 

 of the muscles, glands, and blood-serum, and are found to a greater 

 or less extent in all tissues, secretions, and excretions. The percent- 

 age composition of simple proteins is as follows : 



Carbon 50.0 to 55.0 per cent. 



Hydrogen 6.5 " 7.3 " 



Nitrogen 15.0 " 18.0 " 



Oxygen 21.0 " 24.0 



Sulphur 0.3 " 2.5 " 



A few simple proteins contain phosphorus to the extent of 0.42 

 0.85 per cent., and a few contain also a trace of iron. 



The nitrogen of proteins is split off in four forms, viz., as ammo- 

 nia, as diamino-acids, as monami no-acids, and as a guanidine residue. 

 Part of the nitrogen is easily split off as ammonia by the action of 

 alkalies. 



By boiling proteins with alkalies, part of the sulphur is split off 

 as sulphide, while the remainder can be obtained as sulphate, after 

 fusing the residue with potassium nitrate and sodium carbonate. As 

 about one-half of the total sulphur present is obtained by each opera- 

 tion, it is assumed that there are at least two atoms of sulphur in the 

 protein molecule. 



When acted upon by enzymes or other hydrolytic agents, the 

 simple proteins form first proteins of lower molecular weight, which 

 are diffusible and not coagulated by heat. By the prolonged action 

 of certain ferments (trypsin, bacteria, etc.), or by long boiling with 

 acids, they give rise to the formation of amino-acids (tyrosine, leucine, 

 aspartic, and glutamic acids), of the hexone bases (lysine, arginine, 

 histidine), and a number of undefined bodies. 



In solubility the simple proteins vary ; some are soluble in water, 

 others only in water containing either acids, alkalies, or certain 

 neutral salts, while yet others are insoluble. The soluble proteins 

 are converted into insoluble modifications by the action of heat or 

 certain reagents. This change is called coagulation, and is distin- 

 guished from precipitation by the fact that proteins when once coagu- 

 lated cannot return to their original condition. The temperature at 

 which coagulation takes place depends on the nature of the protein 

 present, the reaction of the solution, and the presence of neutral salts. 

 40 



