196 CELLULAR METABOLISM 



In this manner evidence was presented of the reversible inhibition of 

 sulfhydryl enzymes on irradiation with x-rays and further irreversible 

 inhibition with increasing doses of radiation (30). Immediately after 

 irradiation the formation of desoxyribonucleic acid both in vitro and in 

 surviving tissue slices (126) is found to be depressed. However, Rich- 

 mond (127) observed recently that, shortly after irradiation with a 

 similar x-ray dose, hemin formation in bone marrow or spleen homoge- 

 nate increases three-fold, as does oxygen uptake (128). After the lapse 

 of 2 days hemin formation in the bone marrow homogenate (not, how- 

 ever, in spleen homogenate) is found to be markedly depressed below 

 the value found in controls. 



One hundred roentgens of x-rays produced a 21 per cent and 500 r a 

 94 per cent inhibition of phosphoglyceraldehyde dehydrogenase. When 

 the enzyme was 21 per cent inhibited, there was complete reactivation 

 on addition of glutathione. Presumably, the inhibition was due to the 

 following or similar reversible oxidation: 



2RSH + 20H ;^ RS + 20H + 2H 



Yeast hexokinase, which catalyzes the reaction glucose + ATP ;:± 

 glucose-6-phosphate + ADP, was inhibited only 13 per cent by irradi- 

 ation with 1000 r. 



Adenosinetriphosphatase at pH 9.1 could be 10 per cent inactivated 

 by the remarkably low dose of 10 r and 95 per cent inactivated with 

 1000 r. Enzyme inhibition is almost completely reversed on addition 

 of glutathione. 



There are two kinds of — SH groups of proteins: the easily oxidizing, 

 and the sluggish groups which are not oxidized by mild agents but which 

 do react with mercaptide-forming groups (Hg, As, etc.). Evidence has 

 been presented that only the first-mentioned groups are oxidized by a 

 moderate dose of ionizing radiation. In the case of hexokinase evidence 

 came from the fact that iodosobenzoate inhibited only 15 per cent of 

 the total — SH groups. It was concluded that the rest of the — SH 

 groups are of the sluggish type. Inhibition of the enzyme with x-rays 

 was low, as was expected, viz., 12-18 per cent only. 



The specific action of irradiation on — SH groups was also shown in 

 experiments in which urease was inhibited by gamma-ray doses of 25- 

 200 r. If enzyme inhibition by gamma rays were due only to oxidation 

 of the — SH groups by the oxidizing products or irradiated water, there 

 would be no inhibition on irradiation of urease if the — SH groups were 

 protected by a p-chloromercuribenzoate. If inhibition were due to de- 

 struction or denaturation of the enzyme, it would occur even after con- 



