84 D. W. VAN Bekkum 



irradiation in various tissues. The postirradiation interval 

 was 4 hours in all cases because after longer periods secondary 

 effects may be expected. So far, the disturbance has only been 

 found in spleen and thymus mitochondria. Results with 

 liver, regenerating liver and a number of transplantable 

 mouse tumours were negative even after doses of several 

 thousands of r. Intestinal mucosa and bone marrow did not 

 yield satisfactory mitochondrial preparations so that these 

 tissues could not be studied. 



To investigate whether the effect in the mitochondria 

 occurs also after local irradiation of the spleen, the exterior- 

 ized spleen was irradiated with the rest of the body of the 

 rat shielded. A depression of oxidative phosphorylation was 

 found, which was comparable to that observed after total 

 body irradiation, and therefore the effect is at least chiefly 

 due to the action of X-rays on the spleen cells directly. As to 

 the nature of the cells containing these sensitive mitochondria, 

 evidence has been presented that the mitochondria from both 

 lymphoid and erythropoietic cells are susceptible to irradia- 

 tion (van Bekkum, 1955a). 



It has not been possible to reproduce the effect of radia- 

 tion on the mitochondria in vitro, even with doses as large as 

 20,000 r. Similar negative findings have been reported by 

 Potter and Bethel (1952) and by Ord and Stocken (1955). In 

 :)ur opinion these negative in vitro findings carry little weight, 

 jince it is not possible to imitate even remotely the conditions 

 which exist inside the cell. 



On the nature of the mitochondrial defect 



The possibility has been considered that decreased phos- 

 phorylation after irradiation might be artificially induced 

 during the isolation process, as a result of some unknown and 

 completely unrelated change of the irradiated tissues, e.g. an 

 alteration of the viscosity of the homogenates. Therefore an 

 attempt was made to measure the rate of phosphorylation 

 in vivo by the use of radioactive phosphate. Rats were 

 injected intravenously with labelled inorganic phosphate 



