298 



RADIATION BIOLOGY 



A large number of enzymes essential for the metabolism of proteins, 

 fatty acids, and carbohydrates and for the formation of high-energy bond 

 P require the presence of sulfhydryl groups for enzymatic activity (Bar- 

 ron, 1951). The great reactivity of thiol groups toward the oxidizing 

 agents produced on irradiation of water makes these sulfhydryl enzymes 

 extremely sensitive to the action of ionizing radiation. The oxidation of 

 sulfhydryl groups by ionizing radiations has been studied with phospho- 

 glyceraldehyde dehydrogenase from rabbit muscle, an enzyme which is 

 readily crystallized. On irradiation with X rays of 70 ng of the enzyme 

 per milliliter of buffer solution, there is half inhibition with 200 r and 

 almost complete inhibition with 500 r. This inhibition may be produced 

 by oxidation of the sulfhydryl groups or by protein denaturation. Since 

 the first process is reversible, it can be distinguished by the addition of a 

 thiol compound after irradiation. The oxidized R — S — S — R groups are 

 then converted into the active RSH — RSH groups. When the enzyme 

 is 21 per cent inhibited there is complete reactivation on addition of gluta- 

 thione, an indication that under those conditions X irradiation produces 

 only oxidation of the sulfhydryl groups ; when the enzyme is half inhibited 

 there is 62 per cent reactivation; finally, when the enzyme is completely 

 inhibited, the reactivation is only 10 per cent. Lack of reactivation may 

 be interpreted as protein denaturation by X irradiation. Similar results 

 are obtainea on irradiation of another sulfhydryl enzyme, adenosinetri- 

 phosphatase (myosin) (Table 5-3). Not all sulfhydryl enzymes, however, 



Table 5-3. Inhibition of Phosphoglyceraldehyde Dehydrogenase and Adeno- 

 sine Triphosphatase by X Rays; Reactivation with Glutathione, Added after 



X Irradiation 

 (Barron, Dickman, Muntz, and Singer, 1949) 



are as sensitive to ionizing radiations as those just mentioned. Crystal- 

 line yeast hexokinase has sulfhydryl groups not very reactive toward 

 oxidizing agents, and is also rather resistant to the reaction of X rays. 

 Inhibition of succinoxidase by X rays can also be reversed on addition of 

 glutathione (Barron, Dickman, Muntz, and Singer, 1949). The part 

 played by H2O2 in these inhibitions may be measured by the addition of 



