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Electrospray ionization mass spectrometry of zinc, cadmium, and copper metallothioneins: Evidence for metal-binding cooperativity

Published online by Cambridge University Press:  01 February 2000

PETER M. GEHRIG
Affiliation:
Biochemisches Institut der Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland
CHUNHUI YOU
Affiliation:
Biochemisches Institut der Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland
REINHARD DALLINGER
Affiliation:
Institut für Zoologie und Limnologie (Abteilung Oekophysiologie) der Universität Innsbruck, Technikerstrasse 25, A-6020 Innsbruck, Austria
CHRISTINE GRUBER
Affiliation:
Institut für Zoologie und Limnologie (Abteilung Oekophysiologie) der Universität Innsbruck, Technikerstrasse 25, A-6020 Innsbruck, Austria
MARIUS BROUWER
Affiliation:
Institute of Marine Sciences, University of Southern Mississippi, Ocean Springs, Mississippi 39564
JEREMIAS H.R. KÄGI
Affiliation:
Biochemisches Institut der Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland
PETER E. HUNZIKER
Affiliation:
Biochemisches Institut der Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland
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Abstract

Electrospray ionization (ESI) mass spectra of both well-characterized and novel metallothioneins (MTs) from various species were recorded to explore their metal-ion-binding modes and stoichiometries. The ESI mass spectra of the zinc- and cadmium-binding MTs showed a single main peak corresponding to metal-to-protein ratios of 4, 6, or 7. These findings combined with data obtained by other methods suggest that these MTs bind zinc or cadmium in a single predominant form and are consistent with the presence of three- and four-metal clusters. An unstable copper-specific MT isoform from Roman snails (Helix pomatia) could be isolated intact and was shown to preferentially bind 12 copper ions. To obtain additional information on the formation and relative stability of metal-thiolate clusters in MTs, a mass spectrometric titration study was conducted. One to seven molar equivalents of zinc or of cadmium were added to metal-free human MT-2 at neutral pH, and the resulting complexes were measured by ESI mass spectrometry. These experiments revealed that the formation of the four-metal cluster and of the thermodynamically less stable three-metal cluster is sequential and largely cooperative for both zinc and cadmium. Minor intermediate forms between metal-free MT, Me4MT, and fully reconstituted Me7MT were also observed. The addition of increasing amounts of cadmium to metal-free blue crab MT-I resulted in prominent peaks whose masses were consistent with apoMT, Cd3MT, and Cd6MT, reflecting the known structure of this MT with two Me3Cys9 centers. In a similar reconstitution experiment performed with Caenorhabditits elegans MT-II, a series of signals corresponding to apoMT and Cd3MT to Cd6MT species were observed.

Type
Research Article
Copyright
© 2000 The Protein Society

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