PHOSPHOLIPIDS 57 



in the brain is accelerated if glucose, mannose, galactose or fructose is 

 present under aerobic conditions.-^" Peripheral nerves such as the sciatic 

 nerve of the dog are also able to convert phosphate to phospholipids. ^^^ 



f. Blood Plasma and Cells. According to Hevesy and collabora- 

 ^Qj.g 230,274 ii^Q newly formed phospholipid present in plasma after the 

 administration of P^^ is actually formed in the liver and possibly in other 

 organs. According to an unpublished study of Fishier referred to by 

 Chaikoff,"* the liver was found to be the tissue which first assumed the 

 highest level of phospholipid-P^- following the administration of the iso- 

 tope. However, ninety-eight hours thereafter practically identical values 

 were noted for liver and plasma. Moreover, when P^^ was injected intra- 

 venously immediately after hepatectomy, no phospholipid-P^- could be 

 recovered from the plasma as late as six hours after the operation, although 

 the kidney and small intestine presented values for newly formed phos- 

 pholipid at the same level as that in unoperated dogs. These data strongly 

 suggest that the liver is the sole site for the synthesis of plasma phospholip- 

 ids, although they do not absolutely exclude the possibility that other 

 organs may serve to a hmited extent in this capacity. Entenman et al.~^^ 

 also interpreted their data on the effect of ingested chohne on the synthesis 

 of choline-containing phospholipids in the liver as evidence for the fact 

 that plasma phospholipids are formed in this organ. 



The incorporation of P*- into blood cell phospholipids has been shown by 

 Fishier (cited by Chaikoff^^*) and by others'^-^^^ to be much slower than 

 into plasma. Fishier noted that, whereas it required thirty-six hours to 

 reach a maximum P^- incorporation into the phospholipids of plasma, 

 forty-eight hours elapsed before measurable amounts of P^^ were present 

 in the cells; from 200 to 300 hours were required before maximum levels of 

 radiophospholipids were obtained in the cells. The highest level of phos- 

 pholipid-P^^ present in the formed elements of the blood never exceeds 

 25% of the amount found in the plasma. Hevesy and Aten-^^ reported 

 that, in the case of the laying bird, the renewal of the corpuscle phospholipid 

 was only one-third of that in the plasma. The specific activity of phos- 

 pholipid phosphorus in the blood cells of man^^^ was shown to reach a level 

 of 50% of that in the plasma eight days after the injection of P^-. 



g. Fetus. According to Popjak,^^^'^^' phospholipids can readil}^ be 

 synthesized in the liver of the fetus. It was found that the specific activi- 



*«i G. Hevesy and A. H. W. Aten, Jr., Kgl. Danske Videnskab. Selskab, Biol. Medd.. 14, 

 No. 5, 1-38 (1939). 



»2 G. Popjdk, Nature, 160, 841-842 (1947). 

 »' G. Popjdk, Biochem. J., 42, xi (1948). 



