CHEMIC COMPOSITION OF THE HUMAN BODY 19 



PROTEOSES, PEPTONES AND POLYPEPTIDS. 



During the progress of the digestive process, as it takes place in the stom- 

 ach and intestines, there is produced by the action of the gastric and pan- 

 creatic juices, out of the proteins of the food, a series of new proteins, 

 known as proteoses, peptones and polypeptids. The chemic properties of 

 these substances will be considered in connection with the process of digestion. 



COAGULATED PROTEINS. 



Although these proteins are not found as constituents of the animal 

 organism, they possess much interest on account of their relation to prepared 

 foods and to the digestive process. They are produced when solutions of 

 egg-albumin, serum-albumin, or globulins are subjected to a temperature 

 of iooC. or to the prolonged action of alcohol. They are insoluble in 

 water, in dilute acids, and in neutral saline solutions. 



In this same group may be included also those coagulated proteins 

 which are produced by the action of animal ferments on soluble proteins 

 e.g., fibrin, myosin, casein. 



(a) Fibrin. Fibrin is derived from one of the blood proteins viz., 

 fibrinogen. It is not present under normal circumstances in the 

 circulating blood, but makes its appearance after the blood is with- 

 drawn from the vessels and at the time of coagulation. It can also be 

 obtained by whipping the blood with a bundle of twigs, on which it 

 accumulates. When freed from blood by washing under water, it is 

 seen to consist of bundles of white elastic fibers or threads. It is in- 

 soluble in water, in alcohol, and ether. In dilute acids it swells, be- 

 comes transparent, and finally is converted into acid albumin. In 

 dilute alkalies a similar change takes place, but the resulting 

 product is an alkali-albumin. Fibrin possesses the property of 

 decomposing hydrogen dioxid, H 2 O 2 i.e., liberating oxygen, which 

 accumulates in the form of bubbles on the fibrin. On incinera- 

 tion fibrin yields an ash which contains calcium phosphate and mag- 

 nesium phosphate. 



Two views are held as to the origin of fibrin: first that it is the result 

 of the action of a special enzyme, termed thrombin on fibrinogen, 

 though the nature of the action is not very clear; second that it is 

 the result of a definite combination, physio-chemic in character, of 

 fibrinogen with thrombin which, however, is not regarded as an enzyme, 

 inasmuch as it is not destroyed by boiling, but a definite compound 

 partaking of the nature of an organic colloid. The amount of fibrin 

 formed from fibrinogen will be proportional to the amount of thrombin 

 present. 



(b) Myosin fibrin and myogen fibrin are two insoluble proteins developed 

 out of the two chief proteins of muscle-plasma. Their develop- 

 ment after death is believed to be the cause of the stiffening of the 

 muscles. It is not definitely known whether this is the result of the 

 action of a special enzyme or not. 



(c) Casein. Casein is derived from the chief protein of milk caseinogen 

 by the action of a special ferment known as rennin or chymosin. 

 This ferment is a constituent of gastric juice. 



