THE INDIVIDUAL GROUPS OF PROTEINS 463 



EGG-YOLK. 



Egg-yolk contains fat, lecithin, lutein (the yellow colouring matter), 

 cholesterol, the phosphoprotein vitellin and small quantities of a co- 

 agulable protein livetin. The vitellin and lecithin appear to be in a 

 kind of loose combination and this combination was formerly termed 

 vitellin. It is preferable to term the protein vitellin and the combina- 

 tion lecitho-vitellin. 



Lecitho-vitellin. 



Preparation. 



(a) Egg-yolks, are broken in a bottle and shaken several times with ether, 

 until the ether has only a faint yellow colour. The residue is dissolved in 10 

 per cent, sodium chloride and poured into a large volume of water or dialysed. 

 Lecitho-vitellin is precipitated. This compound cannot be dried with alcohol 

 and ether as it separates into lecithin and vitellin, but it can be filtered off, 

 washed with water, dried by pressure between sheets of paper and in vacua over 

 sulphuric acid. , 



(b) Egg-yolk is mixed with 10 per cent, sodium chloride solution and the 

 mixiure is extracted several times with ether. The salt solution is dialysed 

 or poured into 20 volumes of water. It is purified by solution in sodium 

 chloride and precipitation with water. 



Livetin is contained in the water into which the egg-yolk solution in 

 sodium chloride is poured. It coagulates when the solution is acidified and 

 boiled. It d ffers in this way from vitellin as well as in its phosphorus content. 



Properties. 



A solution of lecitho-vitellin in 10 per cent, sodium chloride pre- 

 pared from egg-yolk as above coagulates on heating at 7-75' The 

 compound in solution behaves like a globulin in being insoluble in 

 water and in being precipitated by saturation with magnesium sul- 

 phate, or half-saturation with ammonium sulphate. It gradually 

 breaks up into lecithin and vitellin by repetition of precipitations or 

 on long contact with water. It gives the colour reactions and most 

 of the precipitation reactions for proteins. It is converted by alcohol 

 into lecithin and vitellin. 



