D. \V. VAN' BEKKUM 



irradiation. In 1952 Ashvvell and Hickman' reported a threefold increase 

 of ATP dephosphorylating activity (to be denominated ATP-ase activity 

 hereafter) of mouse spleen homogenates at 1 to 1 1 days after total body 

 irradiation with lethal doses of X-rays and in 1953 more details on this 

 subject were published (Ashwell and Hickman^). The authors conclude 

 that this rise could be best explained by the premise that a large amount of 

 inert cellular material had been destroyed after irradiation, while the par- 

 ticular enzyme systems remained unaffected. More recently Dubois et aP 

 showed that the increase of ATP-ase activity could be observed in rat spleen 

 and thymus homogenates even after total body irradiation with doses as 

 small as 25-50 r and they suggested the possible significance of this effect 

 with regard to the maintenance of energy requiring reactions after irradia- 

 tion. Previously it had been pointed out by Maxwell et al'^ that the increase 



Figure 7. ATP-ase activity of rat spleen homogenates 

 at various periods after total body irradiation with a dose 

 o/ 1 , 1 00 r . Values are expressed as a percentage of corre- 

 sponding controls 



% 

 250 



200 



150 



100 



SO - 



12 



Zf 



2¥K 



of ATP-ase activity cannot be the cause of the depressed phosphorylation of 

 spleen mitochondria, because ATP-ase activity had been blocked by NaF 

 in the system used for the measurement of oxidative phosphorylation. 

 However in our hands the amount of NaF employed by Maxwell et al left 

 a small part of the ATP-ase activity uninhibited and we found this remaining 

 activity in homogenates to be proportional to the values obtained in the 

 absence of NaF. Therefore a more extensive investigation of the possible 

 role of ATP-ase in the disturbance of oxidative phosphorylation has been 

 carried out, using mouse and rat spleen preparations. The results confirm 

 the conclusion of Maxwell et al and may be summarized as follows : 



(7) The ATP-ase activity of rat spleen homogenates has been estimated 

 at various intervals after irradiation and the results have been compared 

 with the time-effect curve obtained in the experiments on oxidative phos- 

 phorylation. It has been found that the increase of ATP-ase activity of the 

 homogenates is not apparent within a few hours after irradiation, when the 

 disturbance of oxidative phosphorylation is already well developed {Figure 7). 



207 



