750 7. MERCURIALS 



If we designate the equilibrium with X by ^^=(R— Hg+) (X-)/(R— Hg— X), 

 this taken in conjunction with the expressions for K^, K„, and K leads to: 



pA' = pAg - pA„ - vK, (7-3) 



Thus in any experimental situation the observable p^ will be less than the 

 true p/Cg for the reaction of R — Hg+ and S~ by the sum of ^K„ and \>K^, 

 each of these expressing the competitions involved. Thus in the work of 

 Hughes (1957) on the reaction of human seralbumin with MM, a p^ of 4.6 

 was found; if piif^ = 8.7 and ^^K^ — 8.7 (Table 7-5), one may calculate p^g 

 to be 22.0, X being I" in this case. 



The rates of mercaptide formation increase with pH as would be expected 

 if the reactive form of the thiol is the ionized R' — S~. It is not so easy to 

 decide on the reactive forms of the mercurial. It seems unlikely that the 

 Hg++ or R — Hg+ ions are the only reactive species because of their ex- 

 tremely low concentrations in most physiological media, and it is possible 

 that the S^ group makes a sideways attack on the Hg atom utilizing a 

 pair of the six .s electron pairs to displace the X~ ligand. 



It is generally considered that mercurials do not react with disulfide 

 (S— S) bonds, and there is sufficient evidence that this is true for many 

 proteins and enzymes at physiological conditions. However, Cunningham et 

 al. (1957) have shown that p-MB catalyzes the splitting of S — S bonds in 

 cystine, insulin, and ribonuclease at pH 7 if incubation is carried out at 80°. 

 At this temperature, this may well be a matter of equilibria between S — S 

 and SH groups, i.e., between native and denatured forms of the proteins, 

 with p-MB shifting the equilibria by reacting with the SH groups. It is 

 quite possible that in certain enzymes the S — S groups exist in a state 

 where reaction with mercurials is significant, and such a reaction should 

 not be completely ignored. 



Certain metabolically important cofactors, such as coenzyme A and li- 

 poate, are thiols, and it is of some interest to inquire into whether the 

 mercurials react readily with them. Surprisingly little quantitative work 

 has been done and most of the evidence is indirect. For example, Galston 

 et al. (1955) found that p-MB increases the yield of peroxidase, catalase, 

 and tyrosinase in plant breis when added to the preparation medium; 

 since coenzyme A inactivates these enzymes, it was assumed that p-MB 

 protects the enzymes by forming a mercaptide with the coenzyme A. In 

 coenzyme A-deficient rats, the toxicity of mercurials is increased, and the 

 mercurials inhibit the coenzyme A-dependent acetylation of sulfanilamide 

 (Leuschner et al., 1957). Mersalyl and HgClg reduce the coenzyme A level 

 of yeast 25% at 22.5 mM and 0.3 mM, respectively (Estler et al, 1960). 

 Sanner and Pihl (1962) followed the reaction between 2?-MB and coenzyme 

 A by changes in the absorption at 255 m/^ and showed that the thiol could 

 be titrated by the mercurial. Turning to DL-a-lipoate, one finds that its 



