236 V. OXIDATION AND METABOLISM OF PHOSPHOLIPIDS 



radioactive phosphate. On the basis of the assumption of Zilversmit and 

 co-workers'' that a mathematical relationship exists between the specific 

 radioactivity of the precursor and it« immediate metabolic product, after 

 the injection of a single dose of the isotope, Dawson^ concluded that GPC 

 and GPE were metabolic products of phosphatidylcholine and phosphati- 

 dylethanolamine, respectively; however, it was likewise indicated that 

 they could not serve as major precursors in the synthesis of these phospho- 

 lipids. 



It is believed that GPC and GPE originate from lecithin as the result 

 of the action of specific enzymes, called "lecitholipases" ; these are distinct 

 from the usual pancreatic lipases, which act chiefly upon the ester linkages 

 in neutral fat. 



The second hydrolysis of the partially decomposed lecithin molecule is 

 believed to involve the release of choline from a-glycerylphosphorylcholine, 

 which would leave a-glycerophosphoric acid. This reasoning is based 

 upon the fact that the ester linkage to choline is readily susceptible to hy- 

 drolysis by acid in the a-glycerylphosphorylcholine molecule, just as it is in 

 lecithin; on the other hand, it is very difficult to hydrolyze phosphoryl- 

 choline. This suggests that the removal of choline from phosphorylcho- 

 line after a prior hydrolysis of the glycerylphosphate linkage would appear 

 to be quite unlikely. 



Some hght has been thrown on the hydrolysis of lecithin by the investiga- 

 tion of the action of enzyme preparations obtained from a strain of Serratia 

 plymuthicum (an aerobic bacterium from water which forms CO2 and H2 

 from glucose, lactose, and sucrose), by Hayaishi and Romberg.^ The fol- 

 lowing course of metabolism was demonstrated: 



Lecithin 



T^L-a-glycerylphosphorylcholine > L-a-glj'cerylphosphate + Choline 



Lysolecithin 



The Postulated Course of the MetaboUsm of Lecithin by Enzyme Preparations of 



Serratia plymuthicum^ 



When cells were cultured on lecithin, the concentration of the lecithin- 

 hydrolyzing enzyme was found to be five times or more greater than that ob- 

 tained when the cells were grown on ordinary broth. 



The pathway for the hydrolysis of the classical cephalin (phosphatidyl- 

 ethanolamine) is considered to be as follows: The hydrolysis of sphin- 

 gomyelin presents new problems, since this substance does not con- 



"> D. B. Zilversmit, C. Entenman, and M. C. Fishier, /. Gen. Physiol, 26, 325-331 

 (1943). 



8 O. Hayaishi and A. Romberg, J. Biol. Chem., 206, 647-663 (1954). 



