BASIC RADIATION BIOCHEMISTRY 



267 



depresses deamination. This effect is masked in the case of histidine, 

 where the hydrochloric acid combines mainly with the basic group of the 

 glyoxaline. 



The extent of the pH dependence of these reactions is still not firmly 

 established, partly in view of lack of agreement in experimental results of 

 different investigators (Stein and Weiss, 1949; Dale, unpubhshed). We 

 shall also encounter a pH dependence in the case of the liberation of H2S 

 from cysteine. 



LIBERATION OF HYDROGEN SULFIDE 



FROM CYSTEINE HYDROCHLORIDE AND 



FROM REDUCED GLUTATHIONE 



When cysteine hydrochloride or glutathione in its reduced form is 

 irradiated by X rays, liberation of H2S is observed, and the dependence of 

 this reaction on concentration of solute and on pH has been measured 

 quantitatively (Dale and Davies, 1951). The results are given in Figs. 

 4-5 and 6, and the yield-concentration relation is discussed in the follow- 



I 10 io2 10' 10" 10^ 10" 



CONCENTRATION OF CYSTEINE • HCI. /ig/ml 



Fig. 4-5. Yield-concentration curve for H2S production from aqueous solutions of 

 cysteine • HCl; X-ray dose = 33 X 10^ r; pH « 2. {Dale and Davies, 1951. Repro- 

 duced by permission of the editors of the Biochemical Journal.) 



0.6 



_ 0.4 

 in 



0.2 



6 

 pH 



Fig. 4-6. Yield-pH relation for H2S production from aqueous solutions of cysteine • HCl 

 (5 X 10^ Mg/ml); X-ray dose = 9.5 X 10^ r. {Dale and Davies, 1951. Reproduced 

 by permission of the editors of the Biochemical Journal.) 



