A method involving the reversible chemical modification of an active site, zinc-binding cysteine residue (Cys221) for the specific removal of one of the two zinc ions in the metallo-p-lactamase IMP-1 was explored. Covalent modification of Cys221 by 5,5'-ditllio-bis(2-nitrobenzoic acid) was greatly enhanced by the presence of dipicolinic acid, and subsequent removal of the modifying group was easily achieved by reduction of the disulfide bond. However, mass spectrometric analyses and an assessment of IMP-1's catalytic competence are consistent with the maintenance of the enzyme's binuclear status. The consequences arising from chemical modification of Cys221 are thus distinct from those reported for Cys -> Ala/Ser Mutants of IMP-1 and other metallo-beta-lactamases, which are mononuclear. (C) 2009 Elsevier Inc. All rights reserved.