8 THE AMINO ACIDS 



Conspicuous foods containing phosphoproteins are 

 milk with its caseinogen and egg yolk with its vitellin. 

 A trace of iron is also evident in these proteins and 

 although it is possibly present as an impurity there 

 is no evidence that it does not exist in combination 

 with the protein. 



Hemoglobins are compounds of the protein mole- 

 cule with hematin or some similar substance. The 

 coloring matter of the blood is hemoglobin which 

 acts as oxygen carrier for the tissues and is charac- 

 terized by holding iron as a constituent part in organic 

 combination. Globin is the protein portion of hemo- 

 globin. In certain of the lower animal forms copper 

 enters into combination with protein forming haemo- 

 cyanin imparting a blue color to the blood. 



Lecitho proteins are compounds of the protein mole- 

 cule with lecithins. Lecithins are complexes charac- 

 terized by yielding glycerol, phosphoric acid, fatty 

 acid radicles, and a nitrogenous base, choline. The 

 lecithins are present in all plant and animal cells but 

 are especially abundant in the nervous tissues. They 

 belong to the group of essential cell constituents. 



C. Derived Proteins 



Certain of the native soluble proteins upon con- 

 tinued contact with water, or the influence of enzymes 

 or acid change their character and become insoluble. 

 Such insoluble substances are called proteans. After 

 repeated reprecipitation globulins may become insolu- 



