INDIRECT EFFECTS 197 



version of the — SH groups to the R-S-Hg-benzoate. Urease with gluta- 

 thione was inhibited by gamma rays to the same extent as the enzyme 

 alone. When glutathione was added to the irradiated enzyme containing 

 p-Cl-Hg-benzoate, the enzyme activity was restored completely. Pro- 

 tection of the — SH groups by formation of the reversible mercaptile 

 compound protected the enzyme from the inhibiting action of gamma 

 rays. 



That H2O2 plays an important role in the process of inhibiting — SH 

 groups by radiation was suggested by the protective action of small 

 amounts of catalase which frustrate the inactivation of phosphoglycer- 

 aldehyde dehydrogenase, for example, by alpha rays (31, 31a). 



Inactivation of enzymes, may, however, be more difficult to produce 

 in vivo than in vitro because numerous tissue components may compete 

 (32, 33) for the radiation products, such as OH, O2H, and H2O2 (34) 

 which are formed from water, or for other inactivating radicals. 



The hydrogen peroxide concentration is reduced to half its initial 

 value by the action of catalase of 1 X 10~^ molar concentration, as 

 found in the liver in as short a time as approximately 0.02 sec (37). 

 The localization of catalase within the various types of cells is not suf- 

 ficiently understood, and in some cells or cell districts the lifetime of 

 hydrogen peroxide may be much longer than that which follows from 

 the figure stated above. Also, organic peroxides of long lifetime may be 

 formed under the effect of radiation. Organic peroxides have recently 

 been shown to be mutagenic and to induce chromosome abnormalities 

 in Vicia faba (37a). Furthermore, morphology on the molecular scale 

 may have much influence on radiosensitivity (116). Radiosensitivity 

 may be very pronounced in vivo as well; a dose of 0.01 r suffices to in- 

 fluence the growth rate of a single cell of the mold Phycomyces (36). 



Many of the decomposition products of water have a lifetime of only 

 a fraction of 1 sec. Indications are not lacking, however, for the exist- 

 ence of inactivating products of longer and even of much longer life- 

 time. 



Indirect Effects 



When one sarcoma of a rat was irradiated while the other was shielded, 

 the rate of incorporation of the administered labeled phosphate not 

 only into the desoxyribonucleic acid of the irradiated but also into 

 that of the shielded sarcoma was found to be reduced (23a, 25). The 

 lifetime of the inactivation products, presumably responsible for the 

 above-mentioned indirect radiation effect, was estimated to be about 1 

 hr (17). Attempts were made to demonstrate the existence of a circu- 

 lating substance produced by irradiation in the rabbit. The existence of 



