Bismuth compounds are currently used to treat gastric ailments and to prevent the toxic side effects of cancer treatments. The affinity of bismuth for binding to sulfur compounds has been reported and one such target biomolecule is the cysteine-rich metalloprotein metallothionein. Renal mammalian metallothionein has been shown to be induced by Bi salts, with the Bi3+ binding to the renal MT. However, the exact metal-to-metallothionein stoichiometric ratios for the 2-domain beta alpha mammalian protein and the individual beta and alpha domain fragments remain unknown. We now report that the maximum metal-to-MT stoichiometries for the individual domain fragments and the entire 2-domain protein are Bi-3-S-9-beta hMT, Bi-4-S-11-alpha hMT, and Bi-7-S-20-beta alpha hMT, respectively. Electrospray mass spectrometry data also unambiguously show the existence of partially metalated Bi-containing MT species during the titration of apo-MT with Bi3+, which demonstrates that Bi-metalation to MT occurs in a noncooperative manner. (C) 2010 Elsevier Inc. All rights reserved.