PROTEINS 



1. The proteins form the chief organic constituents of 

 the animal body, and occur in greater or less quantity in 

 plants. Their composition is very complex and but little is 

 known of their structure. All proteins contain, carbon, hydro- 

 gen, oxygen and nitrogen, most of them also contain sulphur, 

 several phosphorus, and some, iron. 



Proteins are almost all amorphous, non-volatile, non- 

 diffusible, colorless, odorless and nearly tasteless solids. 

 They vary in solubility. When burned or subject to dry dis- 

 tillation, they give off a disagreeable odor due to amraoniacal 

 derivatives. Proteins are also distinguished by the ease with 

 which they undergo chemical change uiider the influence of 

 reagents, ferments, or variations in temperature. They all 

 undergo the process of putrefaction. By boiling with dilute 

 acids or alkalies, and also by the action of certain ferments, 

 the proteins undergo hydrolysis, forming simpler compounds. 



Classification. Because of incomplete knowledge of their 

 structure, an accurate classification is difficult. The simplest 

 method, perhaps, is according to their source. 1. Native pro- 

 teids, which may be isolated from the organism without loss of 

 their properties. 2. Derived proteids, which are obtained by 

 the action of heat and reagents on native proteids. 



A classification more commonly in use, however, is 

 according to their composition. 1. Simple proteids. 2. Com- 

 pound prokids. 3. Albuminoids. 



Simple proteids are the most prominent solid constituent in 

 muscle, glands, and blood serum, and to a greater or less 

 extent in all tissues. The average percentage composition of 

 simple proteids is: carbon, 50%-5S% ; hydrogen, 6.3%-7.3%; 

 nitrogen, 15%-18%; oxygen, 21%-24%; sulphur, 0.3%-2.5%; 

 Some contain phosphorus, 0.85%, and a few a trace of iron. 



By the action gf heat or certain reagents, soluble simple pro- 

 teids become insoluble modifications by coagulation. A coagu- 



