Cadmium binding to metallothioneins. Domain specificity in reactions of alpha and beta fragments, apometallothionein, and zinc metallothionein with Cd2+.

Abstract

The cadmium-binding properties of rabbit liver Zn7metallothionein (MT) 2 and apo-MT, rat liver apo-a MT and &,-a MT, and calf liver apo-@ MT, have been studied using circular dichroism (CD) and magnetic circular dichroism (MCD) spectroscopies.Both sets of spectra recorded during the titration of Zn7-MT 2 with Cd2+ exhibit a complicated pattern that is quite unexpected.Such behavior is not found at all in sets of spectra recorded during titrations of the apo-species (apo-MT, apo-a MT, and apo-#I MT), and is observed to a much lesser extent in the titration of Zn-a MT.Comparison between the band centers of the Cd-a MT and Cd-@ MT indicates that the CD spectrum of Cd,-MT is dominated by intensity from transitions that originate on Cd-S chromophores in the a domain, with little direct contribution from the @ domain.Analysis of the spectra recorded during titrations of ZnT-MT 2 with Cd2+ suggests: (i) that CdZ+ replaces Znz+ in Zn7-MT isomorphously; (ii) that cadmium binds in a nonspecific, "distributed" manner across both domains; (iii) that cluster formation in the a domain only occurs after 4 mol eq of cadmium have been added and is indicated by the presence of a cluster-sensitive, CD spectral feature; (iv) that the characteristic derivative CD spectrum of native Cd4,Zn3-MT is only obtained from "synthetic" Cd4,Zna-MT following a treatment cycle that allows the redistribution of cadmium into the a domain; warming the synthetic %ative," Cd,,Zn,-MT, to 65 OC results in cadmium being preferentially bound in the CY domain; and (v) Zn,-MT will bind Cd2+ quite normally at up to 65 OC but with greater specificity for the a domain compared with titrations carried out at 25 O C .These results suggest that the initial presence of zinc in both domains is an important factor in the lack of any domain specificity during cadmium binding to Zn-MT which contrasts the domain specific manner observed for cadmium binding to apo-MT.Metallothionein (MT)' is a small, metal-binding protein that is rich in cysteine (1).The protein is commonly found in