Abstract
The cadmium-binding properties of rabbit liver Zn7-metallothionein (MT) 2 and apo-MT, rat liver apo-alpha MT and Zn4-alpha MT, and calf liver apo-beta 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-alpha MT, and apo-beta MT), and is observed to a much lesser extent in the titration of Zn-alpha MT. Comparison between the band centers of the Cd-alpha MT and Cd-beta MT indicates that the CD spectrum of Cd7-MT is dominated by intensity from transitions that originate on Cd-S chromophores in the alpha domain, with little direct contribution from the beta domain. Analysis of the spectra recorded during titrations of Zn7-MT 2 with Cd2+ suggests: (i) that Cd2+ replaces Zn2+ in Zn7-MT isomorphously; (ii) that cadmium binds in a nonspecific, "distributed" manner across both domains; (iii) that cluster formation in the alpha 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,Zn3-MT following a treatment cycle that allows the redistribution of cadmium into the alpha domain; warming the synthetic "native," Cd4,Zn3-MT, to 65 degrees C results in cadmium being preferentially bound in the alpha domain; and (v) Zn7-MT will bind Cd2+ quite normally at up to 65 degrees C but with greater specificity for the alpha domain compared with titrations carried out at 25 degrees 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.
Highlights
From the Department of Chemistp, University of Western Ontario, London, Ontario N6A 5B7, The cadmium-binding properties of rabbit liverZn7metallothionein (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
Analysis of the spectra recorded during titrations ofZnT-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 formationin the a domain only occurs after 4 mol eq of cadmium have been added and isindicated by the presence of a cluster-sensitive,CD spectral feature; (iv) that the characteristic derivative CD spectrum of native Cd4,Zn3-MTis only obtained from “synthetic” Cd4,Zna-MTfollowing a treatment cycle that allows the redistributionof cadmiuminto thea domain; warming the synthetic%ative,” Cd,Zn,MT, to 65 OC results incadmium being preferentially bound in theCY 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 carriedout at 25 O C
These results suggest that the initialpresence of zinc in both domains is an important factor in the lackof any domain specificity during cadmium binding to ZnMT which contrasts the domain specific manner observed forcadmium binding to apo-MT
Summary
From the Department of Chemistp, University of Western Ontario, London, Ontario N6A 5B7, The cadmium-binding properties of rabbit liverZn7metallothionein (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 titratioonf Zn7-MT2 with Cd2+exhibit a complicated pattern that is quite unexpected. The preference for metals such as copper and silver appears to be for the /3 domain, while cadmium and zinc bind to thea domain While these results have greatly advanced our understandingof the capabilities of metallothioneins as metal chelators, the experiments entailconsiderable chemistry, and itis not known how mobile the metals are under the conditions used in the isolation of the fragments.
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