34 NUTRITION OF FARM ANIMALS 



proteins but are characterized by great insolubility in all 

 neutral solvents. They form the principal organic constituents 

 of the skeletal structures of animals and of their external cover- 

 ing and its appendages and hence have also been called sclero- 

 proteins. This definition does not provide for gelatin, which 

 is, however, an artificial derivative of collagen. Besides gela- 

 tin the more important members of this group are chondrin, 

 or collagen, which constitutes the organic basis of cartilage and 

 bone; elastin, the characteristic component of the ligaments; 

 and the keratins of the epidermal tissues such as hair, wool, 

 feathers, horns, hoofs, etc. 



The conjugated proteins 



52. Nucleoproteins. In the scheme of classification here 

 followed (41), the nucleoproteins are defined as follows : " These 

 proteins are especially characteristic of the nucleus of the 

 vegetable and animal cell (74). They consist of protein mole- 

 cules united with one or more of the compounds known as 

 nucleic acids. These are complex organic compounds contain- 

 ing a phosphoric acid radicle and also a xanthin group." 



The simple proteins of the nucleoproteins apparently may be 

 of quite diverse nature and belong to various groups of the 

 simple proteins. The special interest of the nucleoproteins 

 attaches to the nucleic acids entering into their composition. 



53. Nucleic acids. These compounds contain in addition 

 to carbon, hydrogen, nitrogen and oxygen the element phos- 

 phorus. Their constitution has not yet been fully worked out, 

 but their cleavage yields four classes of products, viz., 



1. Xanthin, or purin, bases 



2. Pyrimidin bases 



3. A pentose carbohydrate 



4. Phosphoric acid 



According to the recent investigations of Levene and others, 

 the nucleic acid molecule may be regarded as built up from 

 nucleosids, or glucosid-like combinations of a pentose carbohy- 

 drate with a purin or pyrimidin base. By the union of such a 

 nucleosid with phosphoric acid there is formed a nucleotid. 

 Finally, the most common nucleic acids are tetranucleotids. 



