300 RADIATION BIOLOGY 



ciency of a rays and X rays in the inhibition of sulfhydryl enzymes as 

 contrasted with the greatly diminished efficiency of a rays in the inhibition 

 of carboxypeptidase is probably due to the different mechanisms of action. 

 The sulfhydryl enzymes are inhibited by oxidation of the sulfhydryl 

 groups of the protein, whereas inhibition of carboxypeptidase seems to be 

 due to protein denaturation alone. In fact, addition of hydrogen perox- 

 ide has no effect on the activity of enzymes (Dale, Gray, and Meredith, 

 1949). 



Carter (1949) has studied the activity of some enzymes after total-body 

 irradiation of rats. The spleen was removed at various intervals follow- 

 ing exposure to X rays, and enzyme activity was measured in the diluted 

 homogenates. Irradiation of ribonuclease with 500 r produces no imme- 

 diate effect on the activity of the enzyme. There is, however, a marked 

 decrease 24 to 48 hours later, and recovery does not come until 12 days 

 after irradiation. Significant decrease in activity is found on irradiation 

 .with X rays in doses as low as 100 r. The activity of cathepsin II is 

 increased 24 hours after irradiation and approaches normal values 72 

 hours later. The activity of peptidase is also slightly increased. When 

 rats are irradiated with 500 r, the activity of catalase, esterase, arginase, 

 and phosphatase (non-sulfhydryl enzymes) is not altered when measured 

 soon after irradiation (Ludewig and Chanutin, 1950). 



A demonstration of enzyme inhibition by OH radicals has been given 

 by Collinson et at. (1950), who report inhibition of ribonuclease in aqueous 

 solutions by OH radicals produced chemically. 



Ultraviolet irradiation of Fe+++ salts produces the following reactions: 



Fe+++ -I- H2O aq + /iv ^ Fe++ aq + H+ aq + OH aq 

 H2O2 aq + Fe++ aq -^ Fe+++ aq + OH" aq + OH aq 

 H2O2 2,0^ -\- hv -^ 20H aq 



These results are a strong indication that enzyme inhibition by ionizing 

 radiations may be due to the irradiation products in water, even in those 

 cases in which there is no oxidation of sulfhydryl groups (ribonuclease 

 requires no sulfhydryl groups for activity). 



EFFECT OF IONIZING RADIATIONS ON METABOLISM OF SINGLE CELLS 



It has been maintained by a number of investigators (see reviews by 

 Packard, 1931; Fricke, 1934; Scott, 1937) that the respiration of single 

 cells is strikingly resistant to the action of ionizing radiations. In fact, 

 Chesley (1934) reported that the respiration of sea urchin eggs, both fer- 

 tilized and unfertilized, was unaffected by X-ray irradiation with as much 

 as 43,000 r. However, the respiration of sea urchin sperm (dilute suspen- 



