CHEMIC COMPOSITION OF THE HUMAN BODY 19 



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. 



The Color Reactions of Proteins. When proteins are present in 

 solution, they may be detected by the following color reactions viz., 



1. Xanthoproteic. The solution is boiled with nitric acid for several 



minutes, when the protein assumes a light yellow color. After 

 the solution has cooled, the addition of ammonia changes the color 

 to an orange or amber-red, due to the presence of phenylalanin and 

 tyrosin. 



2. The rose-red reaction. The solution is boiled with acid nitrate of 



mercury (Millon's reagent) for a few minutes, when the coagulated 

 protein turns a purple-red color. This color is attributed to the 

 presence of tyrosin. 



3. The blue-violet reaction. To the solution is added an excess of sodium 



hydroxide, at least an equal volume, and then drop by drop, a very 

 dilute solution of copper sulphate. A blue-violet color is produced, 

 which deepens somewhat on heating, but no further change ensues. 



