GENERAL METABOLISM in vittO 93 



probably much higher. The enzyme is uniformly distributed 

 throughout the brain but, with the exception of the kidney, occurs 

 to a lesser extent in other tissues. The high rate of splitting of this 

 phospholipid is in keeping with the high rate of phosphorus 

 incorporation though the role of the enzyme is not known. It has 

 been suggested (Sloane-Stanley, 1952) that it may be concerned 

 with phosphoprotein metabolism and with the relatively slow 

 extrusion of sodium from nerve fibres, similar to that taking place 

 in resting peripheral nerve. 



Although the hydrolysis of other phospholipids was found to be 

 slow, hydrolysis of the phosphorylated bases and glycerol esters 

 can proceed at an appreciable rate (Strickland et al., \956a, b). 

 Under conditions at which the alkaline phosphatase activity was 

 greatest (about pH 9-0) the rates of hydrolysis of the phosphate 

 esters of serine, ethanolamine, choline, and tyrosine were 20-25 

 jLtmoles/g wet wt. hr~^. Under acidic conditions (pH 5-5) hydrolysis 

 was extremely small. At pH 7-4 glycerylphosphorylcholine was 

 hydrolysed at a rate of 30 /xmoles/g wet wt. hr~^ (Thompson, 1957). 

 Hydrolyses under these conditions appear to be performed large ly 

 by non-specific phosphatases the functions of which are not under- 

 stood. 



The work on the degradation and synthesis of the phospholipids 

 of brain emphasizes that their metabolism is an active process 

 maintaining many of the lipids in a constant state of renewal. 

 Though the mechanisms of synthesis are becoming clearer, the 

 significance of the lipases and related enzymes is still obscure. 

 Evidence previously presented (p. 48) indicates that the lipids or 

 their hydrolysis products may be oxidized by the brain (see also 

 Rossiter, 1957) in the absence of additional substrates under 

 conditions of high metabolic demand. It is possible that the 

 hydrolytic enzymes are involved in providing the substrates for 

 such oxidations and thus may also contribute to the degradative 

 changes occurring for example irt myelin, as a result of tissue 

 damage. Whether they are also involved in the synthesis is a 

 matter for future work. 



Phosphoproteins 



It was noted (p. 28) that the exchange of radioactive phosphorus 

 in the brain in vivo with the phosphoprotein fraction was more 

 rapid than with the phosphorus of phospholipids or any other 



