PROTEINS 95 



Individual Groups. Simple Proteins 



The important characteristics of the proteins as a class have 

 been considered. Let us now review the properties of groups 

 and of individual proteins. 



Albumins. The albumins are found widely distributed in 

 nature. Serum albumin in the blood, ovalbumin in egg-white 

 and lactalbumin in milk are the best known members of this 

 group. Compounds resembling the albumins have been found 

 in plants, although they differ in some of their properties from 

 the animal albumins. Albumins contain no glycocoll, and rela- 

 tively much sulphur (about 1.6-2.2%). They are soluble in 

 water and dilute salt solutions, are precipitated by strong min- 

 eral acids, by saturation with ammonium sulphate in neutral 

 solution and by many other precipitants. They are not pre- 

 cipitated by saturating a neutral solution with magnesium sul- 

 phate or sodium chloride, but if such a solution is acidified, the 

 albumins precipitate. Albumins in solution coagulate on boil- 

 ing, if salts are present and if the solution is faintly acid. 



Globulins. Globulins also are widely distributed in nature, 

 both in animals and in plants. They often are associated with 

 albumins. Important members of the group are serum globu- 

 lin, fibrinogen and its derivative fibrin of the blood, myosinogen 

 and myosin of the muscles, ovoglobulin in eggs, lactoglobulin in 

 milk, neuroglobulin in nerve tissue, and several plant globulins 

 such as edestin from hemp seed, legumin from peas, lentils, etc., 

 and various others from nuts or other materials. 



The globulins are insoluble in pure water, and they may be 

 precipitated by pouring a solution of a globulin into a large 

 volume of pure water, or by dialyzing out the salts against dis- 

 tilled water through a parchment membrane. Globulins also 

 differ from albumins in containing glycocoll, and in the greater 

 ease with which they precipitate on the addition of a neutral 

 salt. Thus they are precipitated by half saturation with am- 

 monium sulphate or by saturation with magnesium sulphate or 

 sodium chloride in neutral solution. 



