GENERAL METABOLISM in vitro 89 



5% of the total radioactivity. However it was noted that a glycero- 

 phosphate fraction was also obtained which was 4-5 times more 

 radioactive than diphosphoinositide. Dawson considered this to 

 arise from a phosphatidic acid formed from glycerolphosphate and 

 a fatty acid. Recently phosphatidic acids have been isolated from 

 brain tissues where they have been found to be present in the 

 greatest concentrations in the microsomal fractions (Hokin and 

 Hokin, 1958«). These acids were the only phospholipids present in 

 the microsomes which incorporated radioactive phosphate rapidly 

 and to a marked extent. The same workers (Hokin and Hokin, 

 19586) have also demonstrated the presence of a monophosphoino- 

 sitide, accompanying cephalin diphosphoinositide, which becomes 

 highly radioactive when slices are incubated with radioactive 

 phosphate. It seems probable that the hydrolytic products of 

 monophosphoinositide would be a contaminant of the hydrolytic 

 products of diphosphoinositide and thus contribute greatly to 

 the radioactivity of the latter. 



Labelling of phospholipids in simple saline dispersions of brain 

 is extremely poor (Fries et al., 1942) and requires the presence of 

 adenylic acid and pyruvate (Dawson, 1953). Under these latter 

 conditions labelling takes place under aerobic conditions. Anaero- 

 biosis abolished labelling. However it has been shown that brain 

 contains two systems capable of supporting the labelling of 

 phospholipids, a glycolytic system demonstrable in dispersions 

 made in water, and an oxidative system dernonstrable in mito- 

 chondrial preparations (McMurray et al., 19S7a; McMurray et al., 

 19576). Under anaerobic conditions a source of energy-rich 

 phosphate was required which could be supplied by adding 

 adenosine triphosphate, hexose diphosphate and adenylic acid, or a 

 triphosphate such as guanosine triphosphate and adenosine 

 diphosphate. A specific requirement for adenosine triphosphate 

 was found, the triphosphates of uridine, cytidine, guanine, and 

 inosine being ineffective when added alone. They become effective 

 only in the presence of adenosine diphosphate which was pre- 

 sumably phosphorylated to adenosine triphosphate by nucleotide 

 transphosphorylases. Under aerobic conditions, with mito- 

 chondrial preparations it was shown that optimum labelling of the 

 phospholipids was obtained only under conditions optimal for 

 oxidative phosphorylation. The requirement for adenosine 

 triphosphate is indicative of the primary place occupied by this 



7— PMB 



