Characterization of calf liver Cu,Zn-metallothionein: naturally variable Cu and Zn stoichiometries

Abstract

Cu,Zn-metallothioneins were purified from bovine calf liver in order to examine the stoichiometry of metal binding to the protein. Copper and zinc analyses were carried out by atomic absorption spectrophotometry. Consistent quantitative thiolate analyses were obtained spectrophotometrically with Ellman's reagent and amperometrically with phenylmercuric acetate. These were used to define protein concentration. A complementary method to assess the sum of the thiol and Cu(I) content of metallothionein involved titration of the reducing equivalents of the protein with ferricyanide. The stoichiometry of reaction was consistent with the oxidation of all the sulphydryl groups to disulphides and all of the bound Cu from the cuprous to the cupric oxidation state. According to these methods, total numbers of zinc plus copper ions bound to metallothionein isolated from a number of calf livers centred on about 7, 10–12, or 15 g-atoms of metal per mol of protein. The reactivity of ferricyanide and 4,7-phenylsulphonyl-2,9-dimethyl-1,10-phenanthroline (BCS) with Cu,Zn-metallothioneins of various metal ratios was assessed. Zinc metallothionein reacted almost entirely in two slow steps with ferricyanide. As the Cu content of the protein increased, the fraction of reaction occurring in the time of mixing increased in parallel. BCS was able to remove 70–80% of metallothionein-bound Cu as Cu(I). The rest was resistant to reaction.