48 A TEXTBOOK OF PHYSIOLOGY 



.The albumins, of which serum albumin, lactalbumin, and egg 

 albumin, are examples, are soluble in distilled water and in saline, 

 but not in saturated solutions of ammonium sulphate and anhydrous 

 sodium sulphate. 



The globulins, on the other hand, are insoluble in distilled water 

 and in saturated solutions of all neutral salts. Furthermore, they 

 are insoluble in half-saturated solutions of ammonium sulphate and 

 anhydrous sodium sulphate. They are soluble in weak saline solutions. 

 The most important are serum globulins, egg globulin, the myosinogen 

 of muscle, and the fibrinogen of blood. 



The phosphoproteins derive their name from the large amount of 

 phosphorus which they contain. Phosphorus is easily split off by 

 prolonged treatment with 1 per cent, caustic soda at 37 C., a fact 

 which distinguishes the phosphoproteins from nucleoproteins. They 

 differ, too, from the latter in containing no purin bodies. They 

 resemble the globulins in many of their properties, but they are not 

 coagulable by heat. The chief membors of the groups are the case- 

 indgen of milk and the vitellin of egg yolk. 



The scleroproteins comprise a heterogeneous group of proteins 

 formerly known as the albuminoids. They are obtained mainly from 

 the " hard " or supporting structures of the body. Among, the better 

 known are collagen, from white fibrous tissue, and its hydride gelatin. 

 Gelatin is remarkable in yielding little or no aromatic bodies on de- 

 composition. Keratin, characterized by containing a large amount 

 of sulphur, occurs in the skin and its appendages, such as hair and 

 horn. Elastin is found in elastic tissue, ossein in bone. 



The compound proteins consist of proteins to which groups other 

 than protein are united to form a complex molecule. The groups 

 usually classified are (1) chromoproteins, (2) glucoproteins, (3) nucleo- 

 proteins. Some authorities add lecitho-proteins that is, compounds 

 of lecithin and protein (see Lecithin). 



The chromoproteins, as their name signifies, are the coloured 

 proteins, the chief member being haemoglobin, a compound of a 

 globin, and an iron-containing portion, hsematin. 



The glucoproteins contain a carbohydrate nucleus attached to 

 the protein. Several proteins not contained in this class, such as 

 egg albumin and iiucleoprotein, contain carbohydrate, but not in 

 such large amounts. The chief members of this class are the mucins. 

 The carbohydrate in them is usually glucosamine, of which there is 

 often as much as 30 per cent. When glucoprotein is treated with 

 dilute mineral acid, a sugar is split off which gives the " reducing " 

 tests for sugar, but will not ferment with yeast. 



The nucleoproteins form the chief constituent of the nuclei of cells. 

 They consist of protein in combination with nuclein, itself a compound 

 of protein with nucleic acid. There are several nucleic acids. The 

 simplest (so-called guanylic acid), found in the pancreas, yields, on 

 decomposition, phosphoric acid, guanin, and a pentose; nucleic acid 

 proper yields the purin bodies, guanin and adenin, a hexose, and 



