26 METABOLISM IN THE NORMAL FUNCTIONAL STATE 



Of these, metabolic heterogeneity of the phosphoHpids is Ukely to 

 be the major factor, but owing to a lack of suitable fractionation 

 techniques there is still little information available regarding the 

 relative rates at which most of the individual phospholipids 

 exchange their phosphorus. 



The turnover of phosphorus in brain phospholipids is known 

 not to be uniform (Sloane-Stanley, 1952). Early studies had 

 revealed that the cephalin fraction of the lipids of rabbit brain 

 became radioactive to a markedly greater degree than did the 

 lecithin fraction following an intraperitoneal injection of radio- 

 active phosphorus (Hevesy and Hahn, 1940; Chargaff^^ al., 1940). 

 Cephalin was then thought to be a single entity but was later 

 shown (I'olch, 1949) to be a mixture of at least three components. 

 Following this Dawson (1953, 1954) showed both in vivo and 

 in vitro that the greater exchange of phosphorus in the cephalin 

 fraction was due to a rapid and almost exclusive incorporation of 

 phosphate into the diphosphoinositide component. The cephalin 

 diphosphoinositide fraction was isolated and hydrolysed to release 

 inositol diphosphate. This was found to have a specific radio- 

 activity more than ten times that of any other phospholipid 

 estimated. The time course of the incorporation of radioactivity 

 into the cephalin components and into the lecithins has been 

 followed by Ansell and Dohmen (1957, Fig. 4). Within 3 hr of 

 an intraperitoneal injection of radioactive phosphate to the rat, the 

 relative specific radioactivity of phosphoinositide phosphorus was 

 30-100 times as great as that in other phospholipids measured. 

 The exchange of phosphorus with phosphatidyl serine and 

 sphingomyelin was so low as to be scarcely detectable (cf. also 

 Dawson, 1954). The recognition that phospholipids in the adult 

 brain exchange their phosphorus at widely differing rates raises 

 many questions regarding the significance of both the rapid and 

 slow exchange processes. 



The high rate of exchange in the inosilide phospholipids is 

 apparently peculiar to nervous tissue. In the liver of the rat the 

 uptake of phosphate into phosphatidyl choline (lecithins) is much 

 greater than into the inositol containing phospholipids (Dawson, 

 1955). In liver the inositol lipids contain a monophosphoinositide 

 (McKibben, 1956) and it has recently been found that the cephalin 

 diphosphoinositide fraction from brain also contains a mono- 



