24 METABOLISM IN THE NORMAL FUNCTIONAL STATE 



relative to the initial dose of phosphorus (Fig. 1). Since these 

 phosphates are the precursors of the phospholipid phosphorus, the 

 phospholipids themselves are likely to become radioactive to a 

 similar relatively slight degree. When the radioactivity of the total 

 phospholipids was related to the radioactivity of brain inorganic 

 phosphate or acid-soluble phosphates, the relative radioactivity of 

 the phospholipids was found to be appreciable within a few hours 

 of giving the initial dose (Fig. 3). Even so the system is very slow 



10 20 



Days after injection 



Fig. 2. The percentage of radioactive phosphate found in the 

 total phospholipids of adult rat brain at different intervals after 

 administration by stomach tube. Data from Changus et al., 



(1938). 



to attain an equilibrium although the initial rise in the relative 

 specific activities of the phospholipids is rapid. Comparison of the 

 rates of rise, calculated from data given for the mouse and cat 

 (Fig. 3), shows that equilibrium is not achieved even after 17 days. 

 It is perhaps of interest that the rates of rise showed little difference 

 whether the phosphorus was administered by stomach tube or 

 injected intracisternally, results which tend to support conclusions 

 arrived at when considering the entry of phosphate into the brain 

 (pp. 18-19). 



In spite of the relatively slow increase in radioactivity the 

 quantities of phosphorus turned over are likely to be considerable. 

 Thus from the relative specific activities of the total phospholipid 



