90 GENERAL METABOLISM ifl Vttro 



compound and it has been shown by means of an elegant technique 

 (McMurray ^/«/., 1957a; see also Berry and McMurray, 1957), that 

 in both mitochondrial preparations and water dispersions, adeno- 

 sine triphosphate is a closer precursor of total phospholipid 

 phosphorus than is inorganic phosphate. 



It now seems well established that the pathways involved in the 

 synthesis of a phospholipid, such as lecithin, in brain are likely to 

 be similar to those described in liver by Kennedy (1956) and 

 Kennedy and Weiss (1956). Thus it has been shown (Rossiter, 

 McLeod and Strickland, 1957; Rossiter, McMurray and Strick- 

 land, 1957, McMurray et al., \9S7b) that the reactions involved in 

 the synthesis of lecithin in the brain are: 



AT32P + choline ^ 32p choline + ADP (1) 



CT32P 4- 32p choline > CMP~32p choline + PP (2j 



AT32P -f glycerol > L-a-glycerol=^2p _^ aDP (3^ 



Acyl-CoA + L-a-glycerol32p > 



L-a-32p phosphatidic acid + CoA (4) 



L-a-^2p phosphatidic acid >■ D-a^-diglyceride + ^^p (5) 



D-a^-diglyceride+ CMP^32p choline->L-a-=^2p lecithin + CMP (6) 

 Fatty acid + CoA + ATP->Acyl-CoA + AMP + PP (7) 



CMP + ATP > CTP + AMP (8) 



In such a synthesis adenosine triphosphate has a dual role. It can 

 act as a source of radioactive phosphate in the labelling of inter- 

 mediates (reactions (1) and (3)) cf. the synthesis of phosphoryl 

 ethanolamine (Ansell and Dawson, 1951), and is also essential for 

 the incorporation of these intermediates into the phospholipid 

 molecule (reactions (4) and (7), and (8) and (9)). The require- 

 ment for cytidine triphosphate in the synthesis of lecithin is 

 absolute for it cannot be replaced by other nucleotide triphosphates 

 (McMurray et al., 1957a). The activation of phosphorylcholine via 

 the cytidine derivative may prove to be a general pattern in 

 phospholipid synthesis since the intermediate, cytidine diphospho- 



