METABOLISM IN THE NORMAL FUNCTIONAL STATE 23 



only slowly when compared with the ^- and y-phosphorus atoms 

 (Zetterstrom et al.y 1950), though the extent of the renewal was 

 still considerable. Since the adenylic acid moiety of the diphos- 

 phopyridine nucleotide was found to incorporate radioactive 

 phosphorus to the same extent as the a-phosphorus of adenosine 

 triphosphate (Zetterstrom and Ljungren, 1951) it appears that the 

 pyridine nucleotide is split and resynthesized at an appreciable 

 rate. In a study relating to the mechanisms of phospholipid syn- 

 thesis incorporation of radioactive phosphorus into phosphoryl- 

 ethanolamine (Ansell and Dawson, 1950) was found to precede 

 incorporation into glycerylphosphorylethanolamine (Ansell and 

 Norman, 1953), suggesting a product-precursor relationship. It 

 was shown that neither of these two phosphates arose by break- 

 down of the corresponding phospholipids. As with other acid- 

 soluble phosphates incorporation was rapid, the specific radio- 

 activity of phosphorylethanolamine being 50-70% of the specific 

 radioactivity of the inorganic and acid-hydrolysable phosphorus 

 within 3 hr of a subcutaneous injection of ^sp. 



Incorporation into Acid-insoluble Phosphates 



Phospholipids. At an early stage in the study of radioactive phos- 

 phate metabolism in the body it was recognized that the phospho- 

 lipids of brain became only feebly radioactive when animals were 

 either injected or fed with radioactive phosphate. The experiments of 

 Hevesy and Hahn (1940), Changus et al (1938), Fries et al (1940) 

 and others (Table 5) showed that the quantities of radioactive 

 phosphorus entering the brain phospholipids constituted only a 

 minute fraction of the total dose administered. Measurements 

 extended over a period of 30 days showed that the maximum 

 percentage incorporation in the phospholipids of adult rat brain 

 was not attained until 8-10 days after a dose of radioactive phos- 

 phate and thereafter declined at a slow rate (Fig. 2). Experiments 

 of this type were interpreted as showing that the phosphorus 

 metabolism of the phospholipids in brain was extremely slow. 



In the light of subsequent developments it would seem that this 

 concept arose through the methods chosen to express the results. 

 Since the passage of phosphate from the blood to the brain is slow 

 the " acid-soluble " phosphates of brain do not exchange rapidly 

 with the blood phosphate and do not become highly radioactive 



