Reactions of Electrophilic Reagents That Target the Thiolate Groups of Metallothionein Clusters:  Preferential Reaction of the α-Domain with 5,5‘-Dithio-bis(2-nitrobenzoate) (DTNB) and Aurothiomalate (AuSTm)

Kinetic studies of the reactions of isolated α- and β-domains of rabbit liver MT-II with DTNB (5,5‘-dithio-2,2‘-dinitrobenzoic acid) and AuSTm (aurothiomalate, Myochrysine) were carried out in 5 mM Tris·HCl/0.1 M KCl at pH 7.4 and 25 °C. These results demonstrate that the kinetics of the DTNB reaction with the β-domain are monophasic, with observed rates similar to those of the slow step of the reaction of the holo-protein, Cd 7 MT, which confirms previous findings that the α-domain is the site of the kinetically fast step. DTNB concentration dependence studies resulted in the following rate law, rate = { k 1s + k 2s [DTNB]}[MT], that corresponds to two of the four terms in the holoprotein rate law, those of the slow step. The reaction of aurothiomalate with the β-domain is independent of the AuSTm concentration and described by a rate function with a single rate constant, rate = k 1s [MT]. The α-domain reaction with AuSTm, also AuSTm-concentration independent, involves slow and fast phases with rate = { k 1s + k 1f }[MT]. The latter dominates 87 ± 3% of the reaction. Comparison of these results with previous studies of other electrophiles demonstrate that the kinetically preferred reaction of MT with an electrophile may be localized in either the α- or the β-domain, depending on the specific attacking reagent.