THE INDIVIDUAL GROUPS OF PROTEINS 435 



COAGULABLE PROTEINS. ALBUMINS. GLOBULINS. 



These proteins generally occur together in most tissues and 

 fluids of animals, e.g. in egg-white, blood and muscular tissue. They 

 are also present in various parts of plants, especially in the fruits and 

 seeds. 



The coagulable proteins are the most typical proteins and are often 

 called native proteins. They have the common property of being 

 changed into insoluble modifications when their solutions are heated to 

 boiling in the presence of a little acetic acid. The insoluble form is 

 also present in the various animal tissues such as muscle. 



The chief distinction between albumins and globulins is their 

 behaviour towards water and solutions of salts. They can be separ- 

 ated from solution in an unchanged condition in this way. A large 

 number of salts have been used for this purpose, the principal ones 

 being sodium chloride, magnesium sulphate and ammonium sulphate. 



Albumins are soluble in water and in dilute salt solutions. 

 Albumins are not precipitated by saturating their aqueous solutions 

 with sodium chloride or magnesium sulphate nor by half-saturation 

 with ammonium sulphate (i.e. adding an equal volume of saturated 

 ammonium sulphate), but they are precipitated by saturation of the 

 solution with ammonium sulphate. 



Globulins are insoluble in water, but soluble in dilute salt solu- 

 tions. Globulins are precipitated from dilute salt solutions by 

 saturation with sodium chloride or magnesium sulphate, or by half- 

 saturation with ammonium sulphate. 1 



There are, however, several globulins which behave slightly dif- 

 ferently by being soluble in water, or by being precipitated with less 

 salt than is required for complete saturation. These are atypical 

 globulins. 



1 The amount of salt required to saturate an aqueous solution is 

 3*6 gm. of sodium chloride for every 10 c.c. 

 10*2 gm. of cryst. magnesium sulphate for every 10 c.c. 

 4'o gm. of ammonium sulphate for every 10 c.c of half-saturated solution. 

 It is better to weigh out the requisite amount of salt than to add it until no more 

 dissolves, as an excess is in this way avoided and does not interfere with further operations. 



