92 GENERAL METABOLISM in vitro 



the knowledge of phospholipid synthesis in brain and it is not 

 unlikely that the synthesis of other phospholipids will be found to 

 follow a similar general pattern. 



Hydrolysis of Phospholipids 



By injecting rats with radioactive phosphate a few hours before 

 removal of the brain, Schachner et al. (1942) obtained cerebral 

 tissues in which the phospholipids were labelled with radioactive 

 phosphorus. Incubation of dispersions under anaerobic conditions 

 showed that phosphorus equivalent to 10-15% of the total intrinsic 

 phospholipids was released within 4 hr. Direct estimation of the 

 phospholipid phosphorus under similar conditions confirmed the 

 extent of the breakdown (Sperry, 1947). The process was shown 

 to be due to cerebral enzymes since heating the tissue markedly 

 reduced the degree of splitting while the addition of blood, 

 normally a contaminant of brain dispersions, had no additional 

 effects. 



Analyses of the changes taking place in various crude phospho- 

 lipid fractions (Tyrell, 1950) revealed that the most significant 

 decrease in phosphorus occurred in the cephalin fraction, sphingo- 

 myelin showing no change. Parallel investigations (Sloane- 

 Stanley, 1951, 1953) in which various cerebral phospholipid 

 fractions were added to dispersions of whole brain in a bicarbonate 

 saline and incubated anaerobically for 3-4 hr, showed that the 

 cephalin diphosphoinositide fraction was rapidly degraded at a 

 rate of some 60-80 /xmoles/g wet wt. hr"^ to form inositol mono- 

 phosphate and inorganic phosphate. All other lipid preparations 

 such as lecithin, sphingomyelin and phosphatidyl ethanolamine 

 and serine were hydrolysed at rates varying from 1-0-7-0/xmoles/g 

 wet wt. hr~^. Analysis of the products formed suggested that at 

 least two enzymes were involved, one rapidly releasing inositol 

 diphosphate and the second hydrolysing the product to form 

 inositol monophosphate and inorganic phosphate. The residual 

 parts were considered to remain intact either as monoglycerides 

 or phosphatidic acids. In these experiments the diphosphoinositide 

 was added as an aqueous suspension of the solid lipid and, in such 

 a form, contact with the enzymes would be limited. Using the 

 water-soluble potassium salt it has been shown (Rodnight, 1956) 

 that the splitting of diphosphoinositide by cerebral tissues proceeds 

 at a rate of 300/>tmoles/g wet wt. hr"^ and that the maximal rate is 



