PHOSPHOLIPIDS 69 



Phosphorylethanolamine (ethanolamine phosphoric acid), hkewise, is 

 believed not to be in the direct pathway in the synthesis of phosphatidyl- 

 ethanolamine (cephahn). Norman and Dawson^" found that the rate 

 of incorporation of P^' into ethanolamine phosphoric acid in rat brain is 

 100 times greater than that into phosphatidylethanolamine. Unless the 

 turnover of the latter fraction is exceedingly heterogeneous, one may as- 

 sume that the ethanolamine phosphoric acid is formed by a pathway which 

 is independent of phosphatidylethanolamine catabolism. 



Another suggestion is that both glycerophosphate and phosphoryl- 

 choline are first hydrolyzed into their component units and then recon- 

 structed into a new phospholipid molecule. In fact, Popjak and Muir'*' 

 suggest that the synthetic reaction is reversible. However, there are 

 several facts which argue against the hydrolysis and resynthesis theory. 

 In the first place, ^-glycerophosphate is hydrolyzed extremely slowly by 

 animal phosphatase. ^^^ Moreover, phosphorylcholine has also been shown 

 by Welch and Welch^^* to be resistant to hver phosphatases. They are 

 thus protected from destruction by the liver. 



In another communication, Ansell and Norman*^* postulated that glyc- 

 erylphosphorylethanolamine (GPE) may be an intermediate in the in vivo 

 synthesis of phosphatidylethanolamine in the brain. Ducet^^* likewise 

 suggested that glycerylphosphorylcholine (GPC) may be intermediate in 

 the synthesis of phosphatidylchohne in plants; he based this assumption 

 upon the widespread distribution of GPC in the plant kingdom. More- 

 over, Schmidt and co-workers*^^ demonstrated the presence of free GPC in 

 aqueous extracts of animal tissues, and Campbell and Work,*^^ Walker,^^' 

 and Ansell and Norman^^^ isolated GPE from similar sources. These 

 findings would appear to render this hypothesis plausible. However, 

 Dawson, ^^^ on the basis of a comparison of the specific radioactivity of liver 

 phosphatidylcholine with that of GPC, and of the radioactivity of phos- 

 phatidylethanolamine with that of GPE, concluded that GPC and GPE 

 cannot act as intermediates in the synthesis of their respective phospho- 

 lipids. On the other hand, it is postulated that they may serve as products 

 of catabolism of these phospholipids, since liver phosphatidylcholine 



3" J. M. Norman and R. M. C. Dawson, Biochem. J., 54, 396-399 (1953). 

 3" J. J. Rae, H. D. Kay, and E. J. King, Biochem. J., 28, 143-151 (1934). 

 3" G. B. Ansell and J. M. Norman, Biochem., J., 54, viii-ix (1953); 55, 768-774 (1953). 

 3S4 G. Ducet, Bull. sac. chim. bioL, 31, 52-56 (1949). 



356 G. Schmidt, L. Hecht, P. Fallot, L. Greenbaum, and S. J. Thannhauser, /. Biol. 

 Chem., 197, 601-609 (1952). 



366 P. N. Campbell and T. S. Work, Biochem. J., 50, 449-454 (1952). 



3"D. M. Walker, Biochem. J., 52, 679-683 (1952). 



358 R. M. C. Dawson, Biochem. J., 57, xv (1954); 59, 5-8 (1955). 



