860 7. MERCURIALS 



inhibitions constitute mutual depletion systems and are in zones B or C 

 (see page 1-66). The inhibition is independent of K^ in zone C — i.e., i = 

 (I;)/(E^) — and so the degree of inhibition indicates, not the affinity of the 

 enzyme for the mercurial, but only the concentration of enzyme (or of other 

 substances binding the mercurials). When we see in the table that 0.001 raM 

 p-MB inhibits an enzyme 50%, does this mean that K^ is approximately 

 0.001 vciM, or that the enzyme concentration is 0.002 mM (assuming pure 

 enzyme)? It is actually more likely to be the latter, and this is one reason 

 why the values in the table should not be taken too seriously. These ques- 

 tions will be considered in the following section. 



It is rather difficult to find many enzymes which are insensitive to the 

 mercurials. The following enzymes might be considered as relatively resis- 

 tant ( < 10% inhibition at 1 mM or above): adenylate kinase, most alkaline 

 phosphatases, a-amylase, potato apyrase, cellobiase, coagulases, copropor- 

 phyrinogen oxidase, dinitrophenol reductase, elastase, /5-glycerophospha- 

 tase, glycolate oxidase, kynurenine formamidase, certain lipases, maltase, 

 neuraminidase, nitrite reductase, oxalate decarboxylase, pepsin, peroxid- 

 ases, phospholipases, many proteases and peptidases (especially bacterial, 

 fungal, and venom), some pyrophosphatases, most RNases and DNases, 

 certain sulfatases, thiamine diphosphatase, and uricase. These certainly do 

 not constitute as a whole a very extensive or particularly important group 

 of enzymes. Actually, the majority of enzymes are inhibited in an inter- 

 mediate fashion between these and the examples in Table 7-13, i.e., 50- 

 100% by concentrations in the range 0.05-1 mM, although some of these 

 would undoubtedly exhibit a much greater sensitivity if examined under 

 appropriate conditions (in a pure form, at physiological temperatures and 

 pH, and in the absence of high concentrations of ligands).* 



Comparison of Mercurials 



Since the introduction of p-MB some 30 years ago, it and the related p- 

 MPS have been used for enzyme inhibition more and more frequently at 

 the expense of Hg++. It is interesting, therefore, to look into the results 

 which have been obtained with the inorganic and organic mercurials. Of 

 the total of 160 reports on enzyme inhibition using both Hg++ and p-MB, 

 25% do not allow an accurate comparison, due mainly to different concen- 



* A word should perhaps be said against the common practice of reporting only 

 an inhibition of 100% with a single mercurial concentration, particularly if this is 

 relatively high, since such results are not very meaningful and the enzymes cannot 

 be accurately classified as to sensitivity. For example, to state that 1 mM p-MB 

 inhibits 100% is bad on two counts: One does not know the true sensitivity of the 

 enzyme, since 0.01 mil/ might also inhibit 100%, and such a result does not provide 

 much evidence for the importance of SH groups at the active center, although it 

 is often so interpreted. 



