436 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



THE COAGULABLE PROTEINS OF EGG-WHITE. 



White of egg is made up of a pale yellow fluid contained in a net- 

 work of a fibrinous material, which is broken up by beating the egg 

 and the egg-white obtained by straining it through calico. 



Egg-white has a faintly alkaline reaction to litmus and a specific 

 gravity of 1*045. It contains 13*3 per cent, of solid matter, 85-88 

 percent being water. Of the solid matter 12-2 per cent, is protein; 

 the remainder is glucose -5 per cent, ash -66 per cent and traces of 

 soaps, fat and cholesterol. Of the protein 67 per cent, is globulin, 

 10 per cent is ovomucoid or ovomucin, a glycoprotein (p. 471). 



Globulin (Ovoglobulin). 



Preparation. 



The globulin is precipitated from egg-white or a solution of egg- 

 white by saturation with sodium chloride or magnesium sulphate, or 

 by half-saturation with ammonium sulphate. 



An equal volume of saturated ammonium sulphate solution is 

 added slowly to egg-white, or a solution of egg-white in water, with 

 constant stirring. After standing the precipitate is filtered off, dis- 

 solved in water l and precipitated again with ammonium sulphate. 



This process is repeated several times. The final solution is dialysed to 

 remove salt and the protein can be obtained by evaporation of the solution at 

 a low temperature. 



In a coagulated state it may be separated by acidifying and boil- 

 ing (p. 368), washing the coagulum with water, alcohol and ether, and 

 drying in the air, or by precipitating with alcohol, drying with alcohol, 

 washing with ether and exposing to the air. 



Properties. 



In the uncoagulated state, globulin is an amorphous yellow mass, 

 insoluble in water but soluble in dilute salt solutions ; it is precipit- 

 ated from solution by saturation with sodium chloride, magnesium 

 sulphate, or by half-saturation with ammonium sulphate. 



The solution in salt solutions shows all the general reactions for 

 proteins. 



In the coagulated state, globulin forms an amorphous white powder 

 which is insoluble in water and dilute salt solutions. It dissolves 

 slowly on warming in dilute acids and alkalies, undergoing hydrolysis 

 into derivatives (metaprotein). 



A suspension in water will show most of the colour reactions for 

 proteins. 



This substance has been termed ovomucin by Osborne and 

 Campbell. It is not certain if it is a single protein. 



1 Sufficient salt is still present to make the solution a dilute salt solution, 



