Oxidative Phosphorylation in Irradiated Cells 85 



and after a suitable interval their spleens were excised under 

 Nembutal anaesthesia and immediately frozen in liquid air. 

 The frozen tissue was homogenized in perchloric acid and the 

 nucleotides were adsorbed from the neutralized perchloric acid 

 extract on charcoal. Elution was carried out with pyridine 

 solutions and AMP, ADP and ATP fractions were separated 

 by ion exchange on Amberlite IRA 400 according to a 

 modification of the method described by Cohn and Carter 

 (1950). 



The specific activity of the ATP of the tissue can thus be 

 estimated, but the turnover rate of the phosphate groups of 

 ATP cannot be measured because no method is available to 

 determine the specific activity of the intracellular inorganic 

 phosphate which is supposedly the immediate precursor of 

 these groups. Therefore the specific activity of the total 

 inorganic phosphate was measured, and the specific activity 

 of the labile phosphate groups of ATP was expressed relative 

 to the former value (relative specific activity). Some of 

 the results obtained 4 hours after irradiation with a dose of 

 700 r are summarized in Table II. 



Table II 



The Effect of Total. Body Irradiation on the Incorporation of 

 Labelled Inorganic Phosphate in Rat Spleen ATP in vivo* 



ATP specific activity: ATP relative specific 

 arbitrary units activity 



Controls 10-9±l-5 0-66±006 



4 hours after 700 r total 



body dose 7-9±l-3 0-56±0-04 



P of difference < • 001 < • 001 



* The figures in the table represent means of 10 observations ± s.D. 



The specific activity of ATP as well as the relative specific 

 activity of its labile phosphate groups were found to be 

 significantly depressed. Although this cannot be considered 

 to prove the existence of a disturbance of phosphorylating 

 reactions in vivo the data are at least in complete agree- 

 ment with the observations on isolated mitochondria. It is 

 noteworthy that no difference was observed in ATP content 



