loS 



E. S. GUZMAN BARRON 



moiety. The protein-substrate complex formation resembles thus strik- 

 ingly the antibody-antigen complex formation in the specificity. 



Some proteins require the presence of — SH groups for enzymatic 

 activity. Furthermore, in some cases only a portion of the — SH groups 



TABLE II 



Effect of Fluoroacetate on Citrate Formation by Baker's Yeast. 



Yeast cells, 8.5 mgs. ; Mg acetate, 0.077 ^ I fluoroacetate, 0.005 ^- Duration 

 of experiments, 4 hours. Blank values for oxygen uptake and citric acid 

 have been subtracted. 



present in the protein are necessary for enzyme activity, as demonstrated 

 with urease by Hellerman and coworkers (35). A survey made by Bar- 

 ron and Singer (13, 78) showed that a large number of enzymes which 

 catalyze the metabolism of carbohydrates, fats and protein derivatives 

 need the electronegative — SH groups for activity (Table III). Doubt- 

 less there are still other enzymes, not yet studied, which belong to this 

 class. An indication in favor of this assumption is given by the inhibi- 

 tion of mitosis produced by small doses of x-rays and by nitrogen 

 mustards. Up to the time of Dale's experiments (26) little attention was 

 paid to enzyme inhibition by x-rays. In 1940 Dale demonstrated that 

 enzyme inhibition could be produced by small doses of x-rays provided 

 protein concentration was kept low. Barron, Dickman, and Singer (8) 

 have shown that x-rays, a, and fi particles and y radiations inhibit sulf- 

 hydryl enzymes at small doses by oxidation of the — SH groups of the 

 protein (Tables IV and V). Inhibitions produced by x-rays and a 

 particles could be released on addition of glutathione. This inhibition 

 is undoubtedly produced by the OH and OH2 radicals and H2O2 formed 

 by the ionization of water during irradiation. In fact, protection of the 

 — SH groups by mercaptide formation with Cl-Hg benzoic acid abol- 



