642 4. SULFHYDRYL REAGENTS 



groups, but they could be applied to the determination of changes in the 

 SH content after treatment of the enzymes with the usual inhibiting re- 

 agents. 



TYPES OF SH REACTION IMPORTANT IN INHIBITION 



The reactions of most of the important SH reagents have usually been 

 classified into four types. The SH groups have been written as un-ionized 

 in all cases, not implying that this is necessarily the only reactive form. 

 The mechanisms of these reactions will be discussed in the chapters devot- 

 ed to the individual inhibitors. 



(I) Oxidation of SH growps 



2R-SH ^ X • * > R-S-S-^R ^ XH, 



R = (SH), + X •* » R^l + XH. 



S 



(X may be an acceptor of either hydrogen atoms or electrons.) 



Examples: o-iodosobenzoate, porphyrindin, porphyrexide, iodine, alloxan 

 (not the only mechanism), ferricyanide, oxidized glutathione, tetrathionate, 

 sulfite, performic acid, and oxygen (catalyzed by metal ions). 



(II) Mercaptide formation 



R-SH * X" ^ jT R-S-X - H 



2R— SH + X^ • -^ ^ R-S— X-S— R ■ 2H 



. /S^ 



R=(SH), + X ' "* > : R^ X + 2H 



Examples: HgClg, organic mercurials, arsenite, organic arsenicals, and 

 various heavy metal ions (Cu++, Pb++, Cd++, Ag+, etc.). 



(III) Alkylation of SH groups [alkyl transfer) 



R— SH + X— R' ±? R— S— R' + X 



Examples: iodoacetate, iodoacetamide, S- and N-mustards, chloraceto- 

 phenone, chloropicrin, bromobenzylcyanide, and fluoropjTuvate. 



