12 THE HUMAN BODY 



1. Boiled, either in the solid state or in solution, with strong nitric acid 

 they give a yellow liquid which becomes orange on neutralization with am- 

 monia. This is the xanthoproteic test. 



2. Boiled with a solution containing subnitrate and pernitrate of mercury 

 they give a pink precipitate, or, if in very small quantity, a pink colored 

 solution. This is known as Millon's test. 



3. If a solution containing a protein be strongly acidulated with acetic 

 acid and be boiled with the addition of an equal bulk of a saturated watery 

 solution of sodium sulphate, the protein will be precipitated. 



The simple proteins which are found in the bodies of man and 

 the lower animals fall into several groups as follows : 



1. Albumins. Several proteins of this group are found in the Body; serum 

 albumin, one of the proteins of the blood, myogen, a muscle protein, and cell 

 albumin, found in the cellular tissues, are examples. Egg albumin (white of 

 egg) is perhaps the most familiar example of an albumin. 



The albumins are characterized by being coagulated by heat (illustrated 

 by boiled white of egg) ; in this respect they are similar to the proteins of the 

 next group, from which they differ by being soluble in pure water. 



2. Globulins. These proteins, as indicated above, do not differ greatly 

 from albumins. Like them they are coagulated by heat, but unlike them, are 

 not soluble in pure water. If a small amount of an inorganic salt is added to 

 the water they will go into solution. Two blood proteins, serum globulin or 

 paraglobulin, and fibrinogen belong to this group; also myosin, one of the 

 muscle proteins, and cell globulin, found in cellular tissues. 



3. Albuminoids. In chemical structure these simple proteins are closely 

 similar to those already described. They are found, however, exclusively in 

 the supporting and protective tissues of the body, bone, connective tissue, 

 epidermis, and hair, and evidently have some important structural difference 

 as compared with the proteins of the cellular tissues since the Body cannot 

 make use of them in building up its cell proteins in the way it uses other pro- 

 tein foods. 



4. Protamins. These are the simplest proteins known. They have thus 

 far been found only in the spermatozoa of fishes. Their molecules consist 

 of a relatively small number of amino acid groupings and contain no sulphur. 



5. Histons are intermediate in complexity between protamins and proteins 

 of the albumin class. The one of chief importance in the body is globin, which 

 is combined with a pigment to form hemoglobin, the red coloring matter of 

 the blood. 



Conjugated Proteins. In addition to the simple proteins de- 

 scribed above there are present in the Body certain groups of com- 

 pounds consisting of proteins combined with non-protein sub- 

 stances. The most important of these are : 



