INHIBITION OF ENZYMES 787 



dehydrogenase at 5°, an observation confirmed by Velick (1958). However, 

 Winer et al. (1959) find a slow dissociation of NADH from heart lactate 

 dehydrogenase, complete release occurring after 1 hr at 26° and pH 7 with 

 0.135 vciM 2?-MB. The L(+)-lactate dehydrogenase of yeast (cytochrome 

 bj) possesses a flavin prosthetic group and this is readily dissociated by 

 p-MPS (Armstrong et al, 1960, 1963). The binding of NADH and NADPH 

 to cytochrome 65 aporeductase is blocked by p-MB (Strittmatter, 1961 b) 

 and there is some evidence that pyridoxal-P may be split from L-threonine 

 deliydrase by the same mercurial (Nishimura and Greenberg, 1961). The 

 evidence for the displacement of Fe++ from homogentisate oxidase by p-MB 

 has already been discussed, and the reactions of inhibition and reactivation 

 (Crandall, 1955) may be written: 



Inhibition: E— S— Fe+ + R— Hg— X -> E— S— Hg— R + Fe++ + X- 



E— S— Hg— R + GSH -> E— SH + GS— Hg— R 

 Reactivation: „ „ „ 



E— SH + Fe++ -> E— S— Fe+ + H+ 



The nonheme Fe of succinate dehydrogenase is lost more rapidly by dialysis 

 after treatment with p-MPS, and this may be related to the marked spectral 

 changes observed upon reaction with the mercurial (Massey, 1958). The 

 iron of the photosynthetic pyridine nucleotide reductase is released as Fe+++ 

 by p-MB with proportional loss of activity (Katoh and Takamiya, 1963). 

 The inhibition of aminopeptidase by EDTA is made irreversible by simul- 

 taneous treatment with p-MB and it was concluded that the Mn++ is bound 

 to an SH group (Bryce and Rabin, 1964). A final type of experiment will 

 be mentioned. Mn++ activates the hydroxylamine reductase of P. aeruginosa 

 and this activation is prevented by p-MB, "suggesting that SH groups may 

 be involved in binding the metal to the enzyme" (Walker and Nicholas, 

 1961). It seems to me that such conclusions are unjustified, inasmuch as 

 any mechanism of inhibition would presumably abolish activation by Mn++, 

 whether it affected the binding or not. 



The results on coenzyme displacement may be summarized by stating 

 that the same difficulties are encountered as in protection experiments. 

 There are three general mechanisms by which a mercurial could dissociate 

 an enzyme-coenzyme complex: (1) compete with the coenzyme for the SH 

 group, (2) sterically or electrostatically interfere with coenzyme binding by 

 reacting at an adjacent site, and (3) alter the enzyme configuration in such 

 a way as to disrupt secondarily the coenzyme binding. In no case have these 

 mechanisms been distinguished. 



Changes in Enzyme Structure Brought About by Mercurials 



Evidence has accumulated during the past several years that mercurials 

 occasionally initiate configurational changes in enzymes; certain aspects of 



