THE CHEMISTRY OF THE BODY. II 



is, it contains phosphorus combined in its molecule in addition to the 

 five elements common to proteins generally. It is insoluble in water, 

 but soluble in weak alkalies, and is precipitated from its alkaline solution 

 by acetic acid, the precipitate being soluble in excess of the acid. The 

 alkaline solution is not coagulated by heat. 



(6) The Scleroproteins are distinguished by their relative insolu- 

 bility. They form the chief constituents of the fibrous and horny 

 structures of the body. Thus, white fibres are mainly composed of 

 collagen, yellow fibres of elastin, and hair, horn, and hoofs of keratin. 

 When collagen is boiled with water it yields gelatin, a substance 

 which is soluble in boiling water, the solution setting to a jelly on 

 cooling. Elastin and keratin are both insoluble in hot or cold water, 

 dilute acids or alkalies, or in alcohol or ether. Keratin is remarkable 

 for the amount of sulphur contained in its molecule. 



(7) The Conjugated Proteins. These are nucleoprotein, gluco- 

 protein, and chromoprotein, and each consists of a protein combined with 

 another body called the prosthetic group. Nucleoproteins are a constant 

 constituent of cell nuclei. They are soluble in water, weak salt solution, 

 or dilate alkalies. They possess acid characters, hence the affinity of 

 nuclear chromatin, which contains nucleoprotein, for basic dyes. A 

 nucleoprotein is a compound of a protein with nuclein, and the latter 

 consists of protein combined with nucleic acid, an organic acid contain- 

 ing phosphorus. If a nucleoprotein is subjected to digestion by gastric 

 juice, an insoluble brownish residue is -obtained. This residue consists 

 of nuclein. If the nucleic acid obtained from nuclein is hydrolysed, the 

 two substances, adenine and guanine, are constantly found among the 

 products of disintegration. These two bodies belong to the purin 

 group, that is, they may be regarded as derivatives of purin, which has 

 the formula 



N = CH 



HC C NH 



II II >H. 



N C N 



Adenine has the formula C 5 H 5 N 5 , while guanine is C 5 H 5 N 5 0. Both 

 these substances, when oxidised, yield uric acid, C 5 H 4 N 4 3 . 



In glucoprotein the combined body is a carbohydrate radical, often 

 glucosamine (C 6 H 13 N0 5 ), and therefore containing nitrogen; the 

 glucosamine is split off when the glucoprotein is boiled with mineral 

 acid. It reduces alkaline solutions of cupric salts. Most glucoproteins 

 belong to the group of mucins. These are soluble in weak alkalies, 

 they are not coagulated by heat, and they are precipitated by acetic 



