Glucosamine-6-phosphate isomerase deaminase from Escherichia coli, a typical allosteric enzyme, becomes less cooperative and 50% inhibited when treated with zinc. This metal cation behaving as a tight-bound and slow partial inhibitor. Modification of a pair of vicinal reactive thiols with some sulfhydryl reagents mimics this effect. On the other hand, sulfhydryl reactivity disappears in the presence of saturating concentrations of Zn2+, which does not modify the kinetics of S-methylated enzyme, a finding that indicates that vicinal thiols are an essential part of the zinc-binding site. Allosteric activation of the deaminase causes trapping of the metal, which cannot be released by dialysis against a buffer containing EDTA. Cadmium and nickel(II) cations also produce a similar effect.