FACTORS AFFECTING METABOLISM Itl Vltro 



121 



made on the basis of a principle described by McMurray et al. 

 (1957) which enables some distinction to be made as to which of 

 two precursors, A or B, is the one more immediate in the phos- 

 phorylation of a compound C. Radioactive A is added to a system 

 containing inactive B, and radioactive B to a system containing 

 inactive A, inactive C being present in each system. At the end of 

 the incubation the specific activities of A, B and C are determined 

 and the specific activity of C relative to A is calculated. If B is 



Table 19. — The Incorporation of Phosphate from ^^P Labelled 



Intermediates in the Phosphoprotein Fraction 



(Data from Heald and Stancer, 1960) 



Phosphoprotein was estimated as phosphorylserine. Carrier phosphoryl- 

 serine was included in each experiment. 



In experiment (1) relative specific radioactivity = (s.a. of phospho- 

 protein/s.a. of inorganic phosphate at end of experiment) x 10^. In 

 experiment (2) relative specific radioactivity = (s.a. of phosphoprotein/s.a. 

 of ATP at end of experiment) x lO'^. 



converted to A before forming C then the relative activity of C in 

 both experiments would be equal. If B is a more immediate 

 precursor than A then the relative specific activity of C should be 

 higher than in the case where A is the precursor. Experiments 

 based on this principle (Heald and Stancer, 1960; Table 19) 

 showed that adenosine triphosphate was a more immediate pre- 

 cursor of phosphoprotein than was inorganic phosphate and that 

 guanosine triphosphate was a more immediate precursor than was 

 adenosine triphosphate. However, it can be argued that the latter 

 experiment does not exclude the possibility that the phospho- 

 protein was acting as an intermediate between the adenosine and 

 guanosine nucleotides. The possible reactions are: 



9— PMB 



