778 7. MERCURIALS 



in which the substrate can slow down spontaneous or induced enzyme de- 

 naturation. However, neostigmine, which can protect chohnesterase against 

 thermal denaturation, does not protect at all against Hg++. There is ac- 

 tually some doubt as to whether the inhibition is related to SH groups, 

 since p-MB and MM up to 1 raM do not inhibit even after 2 hr at 37^, or it 

 might mean that the reacting SH groups are not at the active center and 

 the inhibition by Hg++ is due to a dimerization or polymerization. If all 

 enzymes subjected to mercurials were studied in as much detail as in this 

 work, there would probably be many more interesting examples of devia- 

 tions from classic theory; as long as one tests an enzyme under standard 

 conditions with one concentration of a mercurial, as is done in most reports, 

 interpretation presents no problems. 



Protection of Enzymes against Mercurials 



Enzymes may be protected against mercurials by (1) substrates, (2) co- 

 enzymes, (3) metal ion cofactors, (4) reversible inhibitors, and (5) thiols or 

 other mercurial complexers. Various conclusions have been drawn from 

 such experiments, mainly regarding the relation of SH groups to the bind- 

 ing of the protector, but there are many pitfalls; the discussion of protection 

 with respect to iodoacetate (page 47 and Fig. 1-5 in Volume III) applies 

 equally well to the mercurials. Protection may occur by two general mech- 

 anisms: reaction of the protector with the enzyme to block off the mer- 

 curial, or reaction of the protect&r with the mercurial. The latter mechanism 

 applies to the thiols such as cysteine or glutathione, which have been widely 

 used for this purpose, but, as has been pointed out several times, such pro- 

 tection does not provide much useful information, since in reality all one 

 does is to reduce the effective mercurial concentration. It also applies to 

 other complexers and perhaps is involved in the following: the protection 

 of fumarate hydratase (Mello Ayres and Lara, 1962) and fumarase (Fave- 

 lukes and Stoppani, 1958) by phosphate, of ascorbate oxidase by amino 

 acids and RNA (Frieden and Maggiolo, 1957), of acid phosphatase by EDTA 

 (Macdonald, 1961), of urease by ascorbate (Mapson, 1946), and of thyroxine 

 delahogenase by FMN (Tata, 1960). However, in these cases it is often 

 difficult to interpret the mechanism of the protection. We shall not be con- 

 cerned with this type of protection, but only with those protectors presum- 

 ably reacting with the enzyme. 



Some examples of protection are summarized in Table 7-9 along with 

 instances in which protection does not occur (or at least is not observed 

 under the conditions used). The + sign does not indicate that complete 

 protection can be achieved; indeed, in most cases only partial protection 

 has been reported, and this is what we would expect. The degree of pro- 

 tection may depend on the concentrations of mercurial and protector; e.g., 

 protection may be complete with low mercurial concentrations, whereas 



