The ATP–metallothionein complex

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Abstract
We have previously shown that glutathione (GSH) and glutathione disulfide interact with metallothionein (MT) and modulate its capacity to donate and transfer zinc. In this paper, we show that ATP also forms a 1:1 complex with MT (Kd = 176 ± 33 μM, pH 7.4) that enhances the transfer of zinc to zinc-depleted sorbitol dehydrogenase, increases the rate of thiol–disulfide interchange with Ellman’s reagent [5,5′-dithiobis (Z-nitrobenzoic acid)], and changes the apparent shape of the protein. GTP produces almost identical effects. The corresponding di- or monophosphates and pyrimidine nucleotides, however, neither bind as strongly as ATP nor enhance zinc transfer. Carbamoylation of MT lysines abolishes ATP binding, indicating that these highly conserved residues are part of the binding site. GSH decreases, whereas glutathione disulfide increases, ATP binding. The interaction of MT with two critical cellular ligands, i.e., GSH and ATP, and ensuing effects on zinc transfer and reactivity suggest that MT is not merely a cellular zinc buffer but, rather, actively participates in zinc distribution. Apparently, when isolated, MT lacks two important effectors that affect its redox behavior and function. The magnitude of the binding constant and the cellular concentration of ATP indicate that in the cell MT could be essentially saturated with ATP at low concentrations of GSH. Both the redox and energy states of the cell seem to control zinc distribution from MT, but their relative contributions require further studies.