VOL. 12 (1953) REACTIONS OF ADENOSINE AND INOSINE PHOSPHATES 



175 



TABLE II 



HYDROLYSIS OF ITP AND ATP BY PIGEON BREAST MUSCLE SUSPENSIONS 



(For experimental conditions see text. The initial value for inorganic P was mainly derived from the 

 saline medium. No AMP was detectable among the products.) 



Whilst it is well known that both ITP and ATP are hydrolysed by animal tissues 

 (Kleinzeller^; Spicer and Bowen^) it was thought desirable to obtain data on the 

 relative rates of hydrolysis in the enzyme preparations used in the present work. Pigeon 

 breast muscle was disintegrated in calcium-free saline as described before. The final 

 suspension contained i% tissue, 1.25-10-^ M MgClg and inosine and adenosine phos- 

 phates in the quantities stated under zero time in Table II. The solutions were incubated 

 at 20° in Thunberg tubes in vacuo. The results are given in Table 11. The rates of dis- 

 appearance of ATP and ITP were of the same order; initially ITP reacted a little faster 

 than ATP. The main product of the decomposition of ITP was IDP. No IMP was detect- 

 able in the early stages but small amounts appeared after 20 and 60 mins. The main end 

 product of the decomposition of ATP was IMP. The yield of this substance and of the 

 inorganic phosphate indicated a partial hydrolysis of IMP. ADP did not accumulate; 

 the ADP initially present gradually decreased. No AMP appeared. Muscle tissue is 

 known to possess at least two different enzymes capable of hydrolysing ATP - myosin- 

 ATPase and a "soluble" ATPase. Myosin was prepared according to Bailey^'' from pigeon 

 breast muscle. 20 grams of muscle yielded 50 ml solution of myosin in 0.5 M KCl. The 

 soluble ATPase was prepared from rat leg muscle according to Kielley and Meyerhof^^. 

 The details of the experimental conditions and the results of the experiments are given 

 in Tables III and IV. Both enzymes behaved similarly: when the substrates were added 

 separately they were both split. As measured by the liberation of inorganic phosphate 

 the rate of hydrolysis of ITP was a little slower than that of ATP. But when added 

 together the rates were not additive; on the contrary the formation of inorganic phosphate 

 was slower in a mixture of ATP and ITP than with ATP alone. The observations may 



TABLE III 



HYDROLYSIS OF ATP AND ITP BY MYOSIN 



Each test tube contained i ml myosin solution, o.i ml o.ii M CaClj, 0.5 ml 0.5 M triethanolamine 



buffer of pH 7.4, substrate as indicated below, water to 5.0 ml, 25°. Inorganic P determined according 



to LowRY AND LoPEzis. Substrate (i ml each o.oi M): I: ATP; II: ITP; III: ATP and ITP 



Time 

 {mins) 



20 

 40 



l-ig inorganic P formed in o.i ml solution 

 I II III 



2.18 

 4.40 



1-53 



2.52 



1.69 

 3.10 



References p. 180. 



