818 7. MERCURIALS 



procedures might easily solve this problem. There is no other instance where 

 this mechanism has been postulated, but it is a possibility that must be 

 borne in mind. 



(C) The mercurial reacts with a SH group to create a more favorable electric 

 field at the active center. Since the binding of the substrate and the break- 

 down of the ES complex are influenced by the electric field arising from 

 charged groups vicinal to the active center, it is possible that the binding 

 of a charged mercurial could so alter this field as to facilitate the catalysis. 

 There is no evidence for this and it would be difficult to obtain. 



(D) The mercurial increases the active form of the enzyme. Certain enzymes 

 within the cell or as extracted may be in an inactive form, possibly with 

 the active center not in the proper configuration. Reaction with a mercurial 

 might so alter the protein structure as to release the activity, much as 

 some enzymes can be activated by heat. This would probably apply only 

 to true activations, i.e., where the initial state of the enzyme is inactive, 

 as in the work of Hilz and Klempien (1959) on ascites tumor ribonuclease, 

 Hg++ at 0.005 and 0.05 uvM increasing the enzyme rate from to 12 and 

 24 ^moles TCA-soluble phosphate/hr, respectively, but here it may be only 

 that the enzyme is completely inactivated by a natural inhibitor. 



(E) The Hg^+ ion may replace a normal metal ion activator. Carboxypepti- 

 dase which is normally activated by Zn++ can be activated with respect to 

 the esterase activity with Hg++ (Coleman and Vallee, 1961). This situation 

 is probably very uncommon. Rapoport et al. (1955) reported that Hg++ 

 stimulates glycerate-2,3-diphosphatase only in the presence of certain or- 

 ganic nitrogen substances, and the activation was assumed to be due to 

 the complex formed. Substances which are necessary for the Hg++ effect 

 include nonenzyme proteins and certain amino acids, of w^hich histidine is 

 the most effective (Sauer and Rapoport, 1959). It was concluded that in 

 addition to an SH group, activity requires the presence of some metal ion 

 and some complexer bound in a cylic resonance system: 



with the enzyme. In this case, Hg++ would simply function as a metal ion 

 for linking the resonating system to the enzyme. 



(F) The mercurial may disrupt water structure. The structure of water 

 around the active center may be such as to retard somewhat the access of 



