PHOSPHOLIPIDS 67 



and the transfer of the /3-carbon of serine into both moieties of chohne was 

 also curtailed. These results do not necessarily prove that folic acid, vita- 

 min B]2, or pyridoxine is involved in the enzymatic systems concerned with 

 transmethylation. It is suggested that a decreased synthesis of the ap- 

 propriate methyl acceptors may be the causative factor in the decrease in 

 transmethylation. 



c. Sphingosine. Sphingosine (CH3 (CH2)i2 • CH,CH • CHOH • CH- 

 (NH2) • CH2OH) is the normal nitrogenous component in sphingomyelin 

 molecules. Zabin and Mead^*"-**^ demonstrated that this compound is 

 synthesized in the rat mainly from the acetate molecule. When carboxyl- 

 labeled acetate was administered to weanling rats, the sphingosine iso- 

 lated from the brain and carcass had the same specific activity as the 

 fatty acids in the sphingosine fraction. These authors believe that, since 

 carbons 1 and 2 contained no C'^, the results rule out a biosynthesis of 

 sphingosine by condensation of a Cie acid with ethanolamine; it is more 

 probable that serine is utilized in this biosynthesis.*^" 



(5) Intermediates in Phospholipid Synthesis 



In the synthesis of the phospholipid molecule, Chaikoff^^* suggested 

 several possibilities. Although it is not possible to identify the inter- 

 mediate compounds in the transformation of inorganic phosphorus to leci- 

 thin, glycerophosphate and phosphorylcholine might be expected to act as 

 intermediates; in the case of cephalin, glycerophosphate and phosphoryl- 

 ethanolamine would be the logical intermediates. Chaikoff^-* further 

 stated that it is improbable that a single pathway exists for the conversion 

 of inorganic phosphate to phosphohpid. It is suggested that compounds 

 such as glycerophosphate, phosphorylcholine, phosphorylaminoethanol, 

 diglycerides, and triglycerides may all be present; moreover, the sequence 

 in which the several intermediates combine with each other need not be 

 orderly. 22* Chargaff^^^ has indicated that phosphoproteins may serve as 

 intermediates in phospholipid synthesis. 



According to Taurog (cited by Chaikoff--*), glycerophosphate and phos- 

 phorylcholine were found to be incorporated into the liver and kidney 

 phospholipids, both in vitro as slices and in vivo in the intact animal. 

 Zilversmit et al.,^^- on the basis of data from the same laboratory, later 

 concluded that, of the two metabolites cited above, only glycerophosphoric 



'« I. Zabin and J. F. Mead, Federation Proc, 18, 294-295 (1953). 

 "" I. Zabin and J. F. Mead, /. Biol. Chem., 205, 271-277 (1953). 

 "2 D. B. ZUversmit, C. Entenman, and I. L. Chaikoff, /. Biol. Chem., 176, 193-208 

 (1948). 



