PROTEINS. 629 



constituent of cartilage, is collagen mixed with a small quantity of 

 other material. 



On boiling with water (more readily with acidified water) collagen 

 is converted into gelatin, while the latter, when heated to 130 C. 

 (266 F.), is converted into collagen. (Collagen may, therefore, be 

 considered an anhydride of gelatin.) 



Gelatin yields no tryptophane, no tyrosine, and contains a rather 

 small percentage of sulphur. 



Reticulin, occurring in reticular tissue, and skelatins, forming the skeletal 

 tissues of invertebrates, are classed with the albuminoids. 



(/) Histones. Soluble in water and insoluble in very dilute 

 ammonia, and, in the absence of ammonium salts, insoluble even in 

 an excess of ammonia ; yield precipitates with solutions of other pro- 

 teins. On hydrolysis they yield a large number of amino-acids, 

 among which the basic ones predominate, such as arginine and histi- 

 dine, while others of them are absent (cystine, tyrosine). 



(g) Protamines. Simpler polypeptides than the proteins included 

 in the preceding groups. They are soluble in water, uncoagulable by 

 heat, have the property of precipitating aqueous solutions of other 

 proteins, possess strong basic properties, and form stable salts with 

 strong mineral acids. They are the simplest natural proteins. 



II. Conjugated proteins. 



Substances which contain the protein molecule united to some 

 other molecule or molecules otherwise than as a salt. 



(a) Nucleoproteins. These are compounds of one or more pro- 

 tein molecules with nucleic acid. They occur chiefly in the cell 

 nuclei, but are found also in the protoplasm. Nucleoproteins yield, 

 on digestion with pepsin, a simple protein, usually a histone or a pro- 

 tamine, and nuclein. Nuclein is generally but not always resistant 

 to peptic digestion. On treatment with caustic alkali it is split into 

 protein and nucleic acid, which is the important portion of the 

 nucleoproteins. This nucleic acid consists of a carbohydrate group 

 linking together nitrogenous bases and phosphoric acid. As there 

 are many different nucleic acids these constituent groups vary within 

 certain limits. The carbohydrates may be pentose or hexose. The 

 nitrogenous bases may be one or more purine bodies (guanine, adenine, 

 etc.) or pyrimidine derivatives (thymine, cytosine, uracil). The phos- 

 phoric acid is said to be metaphosphoric acid. The purine bodies in 

 nucleins are the origin of the uric acid of human urine. 



