168 S, S, COHEN 



P 



H2C-O-CR 

 p HC-O-CR + CMP 



HC-O-CR ^T""^ fl- Lecithin ■*" 



H,COH '"'"'oe . _ ^ ^^ 



H,C-0-C— R 



Diglyceride P 



HC— O— C— R + CMP 



I / 



H^C— O— P-O-CH2CH2NH2 



O" 



Cephalin 



(XXIII) 



Sphingomyelin is spithesized in a similar reaction involving CPP-choline 

 (Kennedy, 1957): 



i\r-acylsphingosme + CPP choline ^ sphingomyelin -f CMP 



Althongli a-glycerophosphoryl choline has been found in significant 

 amount in liver (Schmidt et al., 1955) and in seminal vesicle (Williams- 

 Ashman and Banks, 1956), there is no evidence that this compound is other 

 than a degradation product of lecithin, rather than an intermediate of 

 lecithin biosynthesis (Dawson, 1955). A number of additional cytidine 

 diphosjDhate derivatives have been recognized in bacteria. In general, it may 

 be stated that a chemical understanding of the intermediary metabolism of 

 the more complex hpids and phosjoholipids has fairly begun. Nevertheless, 

 the chemical description of these compounds, an essential prerequisite to 

 mictabohc studies, has tended to lag behind the study of many other natural 

 products. 



D. Mechanisms of Peptide Synthesis 



Previous sections have discussed the distribution of protein-synthesizing 

 enzyme systems and their general properties and operation, with particular 

 reference to the interrelations with the nucleic acids and nucleic acid-syn- 

 thesizing systems. We shall be concerned in this section with problems of the 



