88 GENERAL BIOCHEMISTRY 



a tissue together with the fats and other lipides. They may be 

 separated from the fats by treating the ether extract with acetone. 

 The lecithins and the cephalins precipitate, while the other lipides 

 will remain in solution. Sphingomyelins differ from the other phos- 

 pholipides in that these compounds are soluble in neither acetone nor 

 ether. Conseqtiently, the sphingomyelins are not removed from a 

 tissue by the customary ether extraction, but rather by extraction with 

 hot alcohol. On cooling, the sphingomyelins crystallize from the 

 alcohol as a white solid. 



On complete hydrolysis, lecithins yield two molecules of fatty acids, 

 glycerol, phosphoric acid, and the nitrogenous base, choline. At one 

 time it was generally believed that every lecithin contained one sat- 

 urated and one unsaturated fatty acid radical in the molecule. Recent 

 investigation has shown that both acids may be saturated, or both 

 unsaturated, or one saturated and one luisatinated. It is probable 

 that a large number of lecithins exist and that the number is limited 

 only by the various binary combinations of the fatty acids. The 

 following formiUa is representative of a' typical lecithin: 



O 



II 



O 

 CH— O— C— R^ 



O 



II 

 CH.— O— P— O— CH2— CHoN(CH3)3 



I 



o- 



a-lecithin 



The name of the above compound is derived from the fact that the 

 phosphoric acid-choline group is attached to a terminal (a) carbon 

 atom of the glycerol. If the phosphoric acid-choline group were 

 attached to the central (/?) carbon atom, the compound would be 

 described as a /3-lecithin. The naturally occurring lecithins that have 

 been isolated thus far have been shown to be of the a. variety. 

 Although this indicates that a-lecithins are strongly predominant 

 in nature, it does not completely exclude the possibility of the exist- 

 ence of /^-lecithins. 



Hydrolysis of lecithins can be accomplished in the laboratory by the 

 use of moderately strong acids or bases to yield fatty acids, choline, 



