774 7. MERCURIALS 



vicinal to the active center and interference with the catalysis sterically or 

 electrostatically, and (3) secondary altering of the protein structure to dis- 

 rupt the normal configuration of the active center. In the last case, which 

 is probably fairly common (see page 787), the inhibition may be formally 

 competitive (if the substrate stabilizes the enzyme structure), noncompeti- 

 tive, or quite complex. Reaction imth the substrate must often occur, espe- 

 cially when the substrate is protein, nucleic acid, nucleotide, or thiol, but 

 in most cases this possibility seems to have been ignored. It is obvious for 

 glutathione reductase and this complicates the analysis of the inhibition 

 (Mapson and Isherwood, 1963), but it may also be an important mechanism 

 when thioesters are involved, e.g., acetoacetyl-CoA in fatty acid synthesis 

 (Stern, 1956) or malonyl semialdehyde pantetheine in propionate metabol- 

 ism (Vagelos and Earl, 1959). The inhibition of NADPH: methemoglobin 

 oxidoreductase by p-MB occurs when either the enzyme or the methemo- 

 globin is incubated with the mercurial (Bide and Collier, 1964). Sometimes 

 one finds indirect evidence for reaction with the substrate, as with 5'-aden- 

 ylate deaminase (Lee, 1957). Here the inhibition by p-MPS is much greater 

 when it is preincubated with adenylate and the reaction started by adding 

 the enzyme than when preincubation is with the enzyme and reaction start- 

 ed by adding the substrate. Reaction with coenzymes is evident when lipoate 

 or coenzyme A is involved, but may be more general than is usually sup- 

 posed. A reaction of -p-MB with NAD was detected spectrophotometrically 

 by Palmer and Massey (1962) and this was considered to be significant in 

 titrations of certain dehydrogenases. Hill (1956) had previously established 

 a 1 : 1 complex of Hg++ with NADH, but had found no complex with p-MB. 

 Onrust et al. (1954) considered the possibility that at least part of the inhi- 

 bition of pyruvate oxidase by p-MB might be due to reaction with the sul- 

 fur of thiamine-diP, but excluded this when they found that thiamine-diP 

 does not reverse the inhibition. However, Pershin and Shcherbakova (1958) 

 observed that thiamine is able to reduce the bacteriostatic action of Hg++, 

 although this could be by a mechanism other than reaction of the thiamine 

 with Hg++. Kuratomi (1959), on the basis of preincubation experiments 

 with components of the pyruvate oxidase system, postulated that j'-MB 

 can react with thiamine-diP. This problem remains to be settled and possibly 

 is an important one. It would be interesting to know if mercurials can open 

 the thiazole ring under physiological conditions (which is not likely) or 

 react with the SH groups after ring opening, in which case the state of the 

 thiamine-diP in the preparation would be important. Another possibility 

 is that a complex is formed with groups other than the sulfur since oppor- 

 tunities for chelation exist. 



It may be suggested that in all studies of enzyme inihibition, in which 

 substrates or coenzymes capable of reacting with mercurials are involved, 

 the appropriate preincubations with the inhibitor be carried out, as pre- 



