97 



hibition but an increase in the rate of hydrolysis of ATP. After these exposures, 

 which are sublethal, the effect is reversible. You will note that at 14 days the 

 activity is again approaching the normal level with the higher doses and is back 

 to normal with the lowest. This represents a change that is detectible at about 

 1/20 of the X-ray LD50 for this species. 



JONES: When was the first observation? 



DUBOIS: At three hours after X-ray exposure. 



CARTER: This is done on a homogenate of spleen? 



DUBOIS: Yes. 



CARTER: Are the nuclei intact or disrupted? 



DUBOIS: Disrupted. The maximum increase in activity of this enzyme 

 occurs after 400 r. After 600 or 800 r there is no further increase in enzyme 

 activity. The only difference when a lethal dose is given is that there is no re- 

 versal of the effect during the survival time. 



This change in not restricted to the spleen. It also occurs in the thy- 

 mus (Figure 4). In the thymus, the dose required to produce an equivalent 



amount of increase in terms of percentage 

 is somewhat higher than in spleen. A very 

 small effect is observed at 50 r, a little 

 greater effect at 100, and a pronounced in- 

 crease at 200 and 400 r. 



ROENTGENS 



50 



a 100 



400 

 • 200 



5 7 10 



DAYS AFTER X-RAY 



Figure 4. Adenosine triphosphatase ac- 

 tivity of the thymus glands of rats at inter- 

 vals after various exposure to radiation. 

 (This chart appeared in the paper by K. P. 

 DuBois and D. F. Petersen, American 

 Journ. of Physio. 176, 282-286, 1954). 



It may be of interest to note that 

 there is no increase in enzyme activity in 

 the thymus after 20,000 r, although activity 

 does increase in the spleen. Nor is there 

 any decrease in thymus weight 24 hours 

 after 20, 000 r. There is a case in which 

 the response to high dosage is strikingly 

 different from that at low dosage, which 

 calls to mind the previous comment along 

 these lines. In other words, if the initial 

 test had been done using 20, 000 r, we would 

 have concluded that the ATP-ase activity is 

 not changed in this tissue, whereas at the 

 lower dose there is a marked increase. 



CARTER: Do you have any data that would indicate that the substrate 

 for this reaction existed at higher or lower levels than normal? Does this ex- 

 pression of enzymatic activity have some counterpart in the concentration of the 

 substrate? 



DUBOIS: No, not on the basis of the in vitro system in which ATP was 

 added in excess. 



CARTER: Or in the cell? 



DUBOIS: In the cell the ATP concentration after 400 r of X-ray is re- 

 duced to i of normal. 



