174 ^- A- KREBS, R. HEMS VOL. 12 (1953) 



The results of an experiment are summarised in Table I. During the first 20 mins 

 the concentrations of inorganic phosphate, ATP and ITP remained fairty constant. A 

 small rise in the concentration of ITP was probably due to the presence of I DP in the 

 starting material. Both ATP and ITP rapidly incorporated ^^P. In the case of ATP the 

 rate of incorporation slowed down when the specific activity of the ATP phosphorus 

 equalled two-thirds of the specific activity of inorganic phosphorus. In the case of ITP 

 the rate fell when the specific activity of ITP phosphorus equalled one-third of the 

 specific activity of inorganic phosphates. The last horizontal column of Table I shows 

 that the specific activity of ATP phosphorus was throughout about twice that of the 

 specific activity of the ITP phosphorus. 



The results of this experiment show that two phosphate groups of ATP and one of 

 ITP readily interchange with inorganic phosphate and that the rates at which the three 

 different phosphate groups concerned react are about equal. 



TABLE I 



INCORPORATION OF INORGANIC PHOSPHATE INTO ATP AND ITP IN PIGEON BREAST MUSCLE SUSPENSIONS 



(For experimental conditions see text) 

 Period of incubation (mins) o 3 6 g 20 60 



Inorganic phosphate 



amount (//g/40 /nl) 



specific activity (counts/min//fg P) 



ATP-phospliorus 



amount {/iigl^o /il) 



specific activity (counts/min//ig P) 



ITP-phosphorus 



amount (/ig/40 /il) 



specific activity (counts/min/^g P) 



Ratio specific activity of ATP-phosphorus 0.382 0.431 0.485 0.655 0-777 



specific activity of inorganic phosphate 



Ratio specific activity of ITP-phosphorus 0.173 0.227 0-253 o.^iy 0.372 



specific activity of inorganic phosphate 



Ratio specific activity of ATP-phosphorus 2.21 1.90 1.92 2.07 2.09 



specific activity of ITP phosphorus 



Action of myokinase {adenylate phosphokinase) on ITP 



It seemed feasible that an incorporation of ^-P into ITP was brought about bj' 

 myokina.se if IDP could replace ADP. Mixtures of ITP, IDP and IMP, and of ITP, IDP 

 and AMP were added to myokinase, prepared from pigeon breast muscle according to 

 Kalckar^. Under the conditions used, as described by Eggleston and Hems\ adenosine 

 phosphates reached equilibrium concentrations in less than 45 mins. In contrast the 

 concentrations of inosine phosphates did not change. Thus myokinase is not responsible 

 for the reactions of ITP. 



Hydrolysis of ITP by muscle tissue 



The incorporation of ^^p into ITP requires probably two separate steps, the first 

 being the fission of ITP into IDP, the second the reversal of this reaction. The first step 

 was studied by investigating the decomposition of ITP in various enzyme preparations. 



References p. 180. 



