GENERAL METABOLISM hi Vttro 81 



wet wt. (Glock and McLean, 1955). It exists predominantly in the 

 reduced form in contrast to diphosphopyridine nucleotide which 

 was found to be present mainly in the oxidized form. Unlike other 

 nucleotides the quantity of diphosphopyridine nucleotide is not 

 markedly altered on removal and slicing of the brain (Gore et al., 

 1950) though the relative proportions of oxidized and reduced 

 forms may well be changed. No increases in quantity were observed 

 when a variety of possible precursors were added to the incubation 

 medium. Nevertheless, it has been found (Heald, 19566) that 

 diphosphopyridine nucleotide in guinea pig cerebral slices incor- 

 porates radioactive phosphate at an appreciable rate, suggesting 

 that it is split and resynthesized. Possible mechanisms include 

 that described by Romberg (1948, 1950) whereby nicotinamide 

 mononucleotide is phosphorylated by adenosine triphosphate to 

 form diphosphopyridine nucleotide. Other nucleotides such as 

 those of cytidine and uridine have not yet been isolated from 

 cerebral slices though there is good reason to suppose that they are 

 present (see p. 90). 



The total acid-soluble phosphorus of guinea pig cerebral slices 

 after incubation varies from 10-15 /xmoles phosphorus/g wet wt. 

 according to whether slices were incubated in saline free from or 

 containing added phosphate. Thus the phosphates listed in 

 Table 12 comprise some 80% of the total present. Of those 

 remaining little is known. In a fraction comprising l-Sjitmoles 

 phosphorus/g wet wt., the barium salts of which were soluble in 

 80% ethanol, only ethanolamine phosphate could be detected 

 chromatographically. 



Mechanisms of synthesis of the acid-soluble phosphates, 

 principally adenosine triphosphate and creatine phosphate, have 

 been studied mainly in disintegrated preparations and more 

 recently in tissue slices. It had been shown quite early, that both 

 inorganic phosphate and adenylic acid or adenosine triphosphate 

 were essential for the oxidation of substrates, such as pyruvic acid, 

 by dialysed preparations of pigeon brain (Banga et al, 1939). 

 Further examination (Ochoa, 1941) showed that the oxidation of 

 pyruvate in the presence of adenylic acid and inorganic phosphate 

 yielded considerable quantities of an acid-hydrolysable phosphate 

 presumably adenosine triphosphate, while at the same time 

 inorganic phosphate was removed from the incubation medium. 

 Adenosine triphosphate as such was not directly estimated but was 



