122 FACTORS AFFECTING METABOLISM in vitro 



ATP < — > GTP < — > Phosphoprotein (1) 



ATP ^-^ Phosphoprotein < — > GTP (2) 



In experiment 2 (Table 19) the radioactivity of the precursors was 

 identical, and identical quantities of each were present in both 

 systems. At the end of the experiment with adenosine triphos- 

 phate as the labelled precursor the specific radioactivity of adeno- 

 sine triphosphate was higher than that of guanosine triphosphate, 

 while with guanosine triphosphate as the labelled precursor the 

 specific activity of the nucleotides was identical ; in both experi- 

 ments the specific activity of the guanosine nucleotide was similar 

 at the end. Nevertheless, the specific activity of the phospho- 

 protein was lower with adenosine triphosphate as the precursor 

 than with guanosine triphosphate as the precursor. If phospho- 

 protein was an intermediate between adenosine triphosphate and 

 guanosine triphosphate it seems reasonable to expect that its 

 specific radioactivity, with adenosine triphosphate as the labelled 

 precursor, would be equal to or higher than that when guanosine 

 triphosphate was the precursor. The data is thus more strongly in 

 conformity with reaction (1) than reaction (2). 



The existence of a nucleotide diphosphokinase of the type 

 necessary for the exchange of phosphate between adenosine 

 triphosphate and guanosine triphosphate might be inferred from 

 the data given in Table 19, but for the reasons given above the 

 reaction in the intact slice is a phosphorylation of guanosine 

 triphosphate by adenosine triphosphate and not the reverse. In 

 this respect the system differs from that described by Ayengar et 

 al. (1956). Here guanosine nucleotides were found to be inter- 

 mediates in the phosphorylation of adenosine diphosphate to the 

 triphosphate which occurred during the metabolism of succinyl- 

 coenzyme A. It also differs in direction from the reaction described 

 by McMurray et al. (1957) whereby guanosine triphosphate 

 phosphorylated adenosine diphosphate to the triphosphate in 

 aqueous dispersions of rat brain under anaerobic conditions. 

 Of the nucleoside diphosphokinases, that mentioned above 

 together with the enzyme catalysing a reaction between uridine 

 triphosphate and adenosine diphosphate are the only ones de- 

 scribed in brain which appear to possess an appreciable activity. 

 Thus, although extracts of sheep brain can catalyse the phos- 



