BIPHASIC KINETICS OF AUROTHIONEIN FORMATION FROM GOLD SODIUM THIOMALATE - A NOVEL METALLOCHROMIC TECHNIQUE TO PROBE ZN-2+ AND CD-2+ DISPLACEMENT FROM METALLOTHIONEIN

A method has been developed to exploit the metallochromic dyes pyridylazoresorcinol and Zincon as monitors of the rate of zinc and cadmium displacement from Zn− Zn,Cd−, Zn,Cd,Cu− and Cd-thioneins. In this report, the antiarthritic drug gold sodium thiomalate (AuSTm) is the displacing agent. In 5 mM Tris-HCl/100 mM NaC1O4buffer, pH 7.4, at 25 °C, the reactions are biphasic. The fast and slow components are first order and independent of the choice of dye and its concentration. The reaction kinetics for each of the thionein preparations (except one Zn,Cd,Cu-thionein preparation) were also independent of the gold concentration. Thus, the rate law for aurothionein formation from AuSTm is rate = kf[MT] + ks[MT]. The averaged rate constants of the fast and slow steps obtained by using PAR with six different protein preparations were kf= (2.7 ± 1.2) × 10−2s−1and ks= (6.9 ± 0.9) × 10−s−1and by using Zincon, were kf= 2.4 ± 0.6 × 10−2s−1and ks= 9.6 ± 1.7 × 10−4s−1. The same rate laws for metal displacement describe the reactions generating Au,Cd-Th or Au,Zn,Cd-Th (for which gold is the limiting reagent) and those forming (TmSAu)20−Th with complete loss of protein-bound zinc and cadmium (for which the protein is the limiting reagent). Differences in the kinetics due to the source of the metallothioneins and their metal contents were all within the experimental error. The biological implications of the kinetics for aurothionein formation during chrysotherapy are discussed. © 1990, American Chemical Society. All rights reserved.