454 V. CHEMISTRY OF PHOSPHATIDES AND CEREBROSIDES 



tion method. The per cent of total hpid in this form reported by Artom 

 and calculated on the figures of Chargaff et al. are, respectively: liver, 27.2- 

 32.2 and 27.2; heart, 48.6-51.2 and 38.6; brain, 40.2 and 44.4. The phos- 

 phatidylserine (in per cent of total lipid) compares with lipid amino nitro- 

 gen determined by Chargaff et al. as follows: liver, 7.4-11.3 and 6.7; heart, 

 7.2-11.3 and 7.0; lung, 7.8-12.0 and 15.7; brain, 21.5 and 21.6. 



5. The Sphingomyelins 



Sphingomyelin was first discovered by Thudichum^^ in 1884. It is 

 readily distinguished from other phospholipids because it contains two ni- 

 trogen atoms to each phosphorus atom present in the molecule. Moreover, 

 it is insoluble in diethyl ether. Sphmgomyelins are also unique in con- 

 taining no glycerol in the molecule, although the recent elucidation of the 

 composition of lipositol has shown that sphingomyelin is not the only phos- 

 phatide from which glycerol is absent. Not only has sphingomyelin been 

 found in the free state in the tissues, but it may also occur as a fatty acid 

 ester. ^^^ 



(1) Structure of the Sphingomyelins 



The best insight as to the structure of sphingomyelin is gained from an 

 examination of its hydrolysis products, and especially of the intermediates 

 which can be isolated when only a partial hydrolysis obtains. Thus, a fatty 

 acid, two nitrogenous bases — choline and sphingosine — and phosphoric 

 acid have long been recognized as the ultimate hydrolysis products. The 

 fatty acid has most frequently been identified as lignoceric acid. The iden- 

 tification of lignocerjdsphingosine as a product of partial hydrolysis offers 

 considerable confirmation of the fact that the fatty acid and sphingosine 

 are joined by a NH — ^CO linkage through the carboxyl and amino groups, 

 respectively. It is believed that choline phosphate is joined to sphingosine 

 through a hydroxyl group in ester linkage. The original structure for 

 sphingomyelin was suggested by Levene,^" but this must now be modified 

 to agree with the present conception of the structure of sphingosine which 

 has recently been demonstrated by Carter et al.^^^ 



CH3(CH2),2 CH=CHC-C-CH20P-OCHjCH2N-CH3 

 H NH OH OH^^' 



C=0 



I 



(CH2)22 



CHj 



Sphingomyelin 



310 S. J. Thannhauser and M. Reichel, J. Biol. Chem., 135, 1-13 (1940). 



311 P. A. Levene, J. Biol. Chem., 24, 69-89 (1916). 



312 H. E. Carter, F. J. Glick, W. P. Norris, and G. E. Phillips, /. Biol. Chem., H2, 

 449-450 (1942). 



