As3+ bound to the two-domain, recombinant human metallothionein (isoform 1a) is stable at pH 7 and translocates via protein-protein interactions to other metallothionein proteins. The data show As3+ transfer from the two-domain beta-alpha-hMT to binding sites in the isolated apo-beta-hMT and apo-alpha-hMT. Under conditions of equilibrium, apo- and partially-metallated species coexist indicating that noncooperative demetallation of the As-6-beta alpha-hMT occurrs. As3+ transfer under conditions (pH 7) where the free As3+ ion is not stable, provides evidence that Cd2+ and Zn2+ transfer may also take place through protein-protein interactions and that partially metallated Cd-MT and Zn-MT would be stable. (C) 2010 Elsevier Inc. All rights reserved.