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J. Biol. Chem., Vol. 263, Issue 13, 6128-6133, 05, 1988

Domain specificity in metal binding to metallothionein. A circular dichroism and magnetic circular dichroism study of cadmium and zinc binding at temperature extremes

MJ Stillman and AJ Zelazowski
Department of Chemistry, University of Western Ontario, London, Canada.

Rabbit liver Zn metallothionein-(MT) will bind cadmium readily between - 26 degrees C and 70 degrees C. The binding reaction was monitored by recording the circular dichroism and magnetic circular dichroism spectra, in the region of the RS(-)----Cd2+ charge transfer transition at 250 nm, at intervals as aliquots of cadmium were added. For all temperatures, these data can be analyzed in terms of a distributed mechanism for cadmium binding when Zn-MT is used, and a domain-specific mechanism when apo-MT is used. The CD spectrum measured at -26 degrees C for Cd,Zn-MT, which was made by adding excess cadmium directly to Zn7- MT at -26 degrees C, is not the same as the CD spectrum of Cd-MT prepared at room temperature from the same Zn7-MT. Measurements of the stoichiometry of the cadmium and zinc bound to MT in the presence of excess cadmium at different temperatures indicates that below 5 degrees C at least one zinc atom remains bound to the protein. The mixed metal metallothionein, Cd/Zn-MT, that always forms below 5 degrees C, is characterized by a single maximum near 250 nm in the CD spectrum, rather than the derivative-shaped CD envelope that is diagnostic of the (Cd4-S11)alpha cluster, which indicates that the zinc occupies a site in the alpha domain. Rearrangement of the bound metals to the domain- specific distribution takes place if Cd,Zn-MT, prepared at subzero temperatures, is warmed above 30 degrees C.
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