THE CEPHALINS 441 



of 1:1, was postulated 1)}^ Thudichum^ as earl}^ as 1874. This phospliolipid 

 was first recognized as differing from lecithin because of its slight solubility 

 in alcohol. Thiidichum suggested that cephalin contained colamine 

 (ethanolaniine or aminoethyl alcohol) as a component, in place of the cho- 

 line in the lecithin molecule. 



Phosphatidylethanolamine, which represents the sole structure assumed 

 for cephalin up to 1942, exists in two forms, the a- and /3-isomers. Their 

 formulas are known to be identical \^'ith those for the a- and /3-lecithins, ex- 

 cept that ethanolaniine is present in place of choline: 



H,CO-CR H,CO-CR 



HCO-CR' HCO-R-OCHjCHjNHi 



1 ^0 I O^^OH 



HjCOP-GCHjCHjNHj HjCO-CR' 



OH 



a-Cephalin ;3-Cephalin 



As in the case of lecithin, the two component fatty acids (R and R') are 

 different. There are therefore many chances for isomerism and, in fact, 

 many different cephalins are known. 



b. Preparation of Phosphatidylethanolamine. Several procedures have 

 been emploj^d for the preparation of cephalin from brain. ^^^ -256 



Renall'-^^ first treated the hashed brain with 3 volumes of acetone for 24 

 hours; the operation was repeated with fresh acetone for a second 24-hour 

 period. This was followed by treatment with 1.5 volumes of alcohol. 

 Since cephalin is insoluble both in acetone and in alcohol, these solvents 

 serve the double purpose of acting as dehydrating agents and for the re- 

 moval of soluble lipids. Cephalin is then extracted with 3 volumes of low- 

 boiling petroleum ether (b.p., 30-60°C.) This operation is repeated. The 

 water-clear ether centrifugate, concentrated in a stream of CO2, is purified 

 by drop-by-drop addition to alcohol. Since this precipitate contains cere- 

 broside, cephalin is further purified by redissolving in ether, centrifuging, 

 and reprecipitation, by the addition of the ether solution to alcohol. After 

 filtration, the impure cephalin is suspended in water (500 ml. to 30 g.) and 

 treated \\-ith 10 ml. of 1 A^ HCl several times (usually 80 ml. is required), un- 

 til a cheesy precipitate forms. By this treatment alkalies and alkaline 

 earth metals (Ca, K, and traces of NH4 and Mg) are removed. The precipi- 

 tate is centrifuged, washed with acetone, and dried in a vacuum desiccator. 

 The final purification involves solution in ether with the removal by centri- 

 fugation of the insoluble cerebroside. The ether solution of cephalin is con- 

 centrated to a small volume, is precipitated with acetone, and dried in vacuo. 

 During the whole procedure, contact with air and light must be avoided. 



25* W. Koch, Z. physiol Chem., 86, 134-140 (1902). 



MS M. H. Renall, Biochem. Z., 55, 296-300 (1913). 



**« P. A. Levene and I. P. Rolf, J. Biol Chem., 74, 713-714 (1927). 



