PHOSPHOLIPIDS 51 



phosphate containing radioactive phosphate (P^-) has been most widely 

 employed. Hevesy and Hahn,-*^ and, three years earlier, Artom et al.,^^^ re- 

 ported that P*- was incorporated into the phosphohpids m both intestine 

 and liver, and possibly also in the kidney. 



In addition to phosphorus as a tag, the nitrogen isotope has been em- 

 ployed to trace choline synthesis. Thus Stetten^*^"^'*^ used N^^ in in- 

 vestigating the fate of choline and ethanolamine. The incorporation of 

 labeled choline into lecithin, and of tagged ethanolamine into phospha- 

 tidylethanolamine, could readily be demonstrated. Arsenocholine, which 

 contains arsenic in place of the nitrogen in ordinary choline, was used by 

 Welch^^^'^'** to follow the in vivo synthesis of liver and brain lecithin. 

 Phospholipid synthesis from methionine and choUne-'*'* has been followed by 

 using compounds in which the methyl group is tagged with C^*. 



(2) The Site of Biosynthesis of Phospholipids 



a. Liver. The liver has long been considered to be the main organ in 

 which phospholipid synthesis takes place. Since London^^s g^g^ demon- 

 strated this fact in 1928, several different tests have been employed. For 

 example, many workers-^-^^^-^*^"^*^ found that P^-, given as inorganic phos- 

 phate (Na2HP04), is incorporated into phospholipid in the Uver. Thus, 

 it was observed that a rapid increase of phospholipid-P^- occurred in the 

 liver after the feedmg of inorganic salt, and that this was followed by a 

 decrease during the later periods. According to Fishier et al.,"^^^ no syn- 

 thesis of phospholipid takes place in the hepatectomized dog. Maximum 

 concentrations of phospholipids were found in the livers of normal rats 

 and mice^^^ within ten hours after the injection of Na2HP^-04. Phospho- 



2" G. Hevesy and L. Hahn, Kgl. Danske Videnskah. Selskab, Biol. Medd., 15, No. 5, 

 1-60 (1940). 



^^ C. Artom, G. Sarzana, C. Perrier, M. Santangelo, and E. Segre, Arch, intern, physiol., 

 4-5, 32-39 (1937). 



"9 De W. Stetten, Jr., J. Biol. Chem., 138, 437-438 (1941). 



2« De W. Stetten, Jr., J. Biol Chem., HO, 143-152 (1941). 



2^1 De W. Stetten, Jr., /. Biol. Chem., U2, 629-633 (1942). 



2« A. D. Welch, Proc. Soc. Exptl. Biol. Med., 35, 107-108 (1936). 



2" A. D. Welch and M. S. Welch, Proc. Soc. Exptl. Biol. Med., 39, 7-9 (1938). 



"* J. A. Stekol, S. Weiss, P. T. Hsu, and P. Smith, Federation Proc, 12, 274-275 (1953). 



"s C. Artom, G. Sarzana, and E. Segre, Arch, intern, physiol., 47, 245-276 (1938). 



»« I. Perlman, S. Ruben, and I. L. Chaikoff, /. Biol. Chem., 122, 169-182 (1937-1938). 



"^ C. Entenman, S. Ruben, I. Perlman, F. W. Lorenz, and I. L. Chaikoff, J. Biol. 

 Chem., 124, 795-802 (1938). 



"* M. C. Fishier, C. Entenman, M. L. Montgomery, and I. L. Chaikoff, J. Biol. Chem., 

 /50, 47-55(1953). 



"9 H. B. Jones, I. L. Chaikoff, and J. H. Lawrence, /. Biol. Chem., 128, 631-644 (1939); 

 133, 319-327 (1940). 



