80 D. W. VAN Bekkum 



was used as a substrate, because washing causes a deficiency 

 of DPN in these mitochondria. Incubation was usually carried 

 out at 37°C for 24 minutes in oxygen. In recent experiments 

 the gas phase was air, which did not make any difference. For 

 a detailed description of these methods reference is made to 

 previous papers (van Bekkum et ah, 1954; van Bekkum and 

 Vos, 1955). 



It should be pointed out that the properties of spleen and 

 thymus mitochondria are not identical with those of liver 

 mitochondria. Morphological differences have been observed: 

 spleen mitochondria are smaller and of less uniform size. 

 The number of mitochondria per cell is distinctly smaller in 

 spleen and thymus than in liver, and accordingly the yield of 

 mitochondria from the former tissues is much smaller. The 

 P/0 ratio of spleen mitochondria is about 1 when succinate is 

 oxidized. Under the same conditions the P/0 ratio of liver 

 mitochondria is usually about double that value. The P/0 

 ratio of thymus mitochondria was found to be well below 1. 

 Finally the phosphorylating activity of spleen mitochondria 

 may vary considerably between individual rats. This is 

 probably partly due to the marked variation of the cellular 

 composition of the spleen in rats. It has in fact been shown 

 that mitochondria isolated from spleens, in which the erythro- 

 poiesis has been greatly stimulated, display an increased 

 phosphorylation as well as elevated P/0 ratios (van Bekkum, 

 1955a). In order to minimize the effect of these variations, 

 the mitochondria were prepared from the pooled spleens or 

 thymus glands of 2-4 rats. Furthermore, in every experiment, 

 control and irradiated tissues were handled simultaneously. 



Results 



In our earlier experiments the relatively large dose of 1100 r 

 of total body irradiation was administered ; 4 hours thereafter 

 the rats were killed and the isolation of the spleen mitochon- 

 dria was started. A significant depression of the phosphate 

 uptake was present both when succinate and a-ketoglutarate 



