Arsenic transfer between metallothionein proteins at physiological pH

Area of Science:

• Biochemistry
• Metalloprotein Chemistry
• Toxicology

Background:

• Metallothioneins (MTs) are cysteine-rich proteins known for metal binding and detoxification.
• Understanding metal transfer mechanisms is crucial for MTs' biological roles and toxicological implications.

Purpose of the Study:

• To investigate the stability and transfer of arsenic (As³+) bound to recombinant human metallothionein (hMT) at physiological pH.
• To explore the potential protein-protein interaction mechanisms for metal translocation involving hMT.

Main Methods:

• Studied As³+ binding and stability with two-domain, recombinant human metallothionein (isoform 1a) at pH 7.
• Analyzed As³+ transfer from metallated hMT to apo-hMT domains (β-hMT and α-hMT).
• Examined equilibrium conditions to determine demetallation patterns of As(6)-βα-hMT.

Main Results:

• As³+ remains stable when bound to hMT at pH 7.
• Evidence of As³+ transfer from the two-domain hMT to isolated apo-hMT domains via protein-protein interactions.
• Noncooperative demetallation of As(6)-βα-hMT observed, with coexistence of apo- and partially-metallated species at equilibrium.
• As³+ transfer at pH 7 suggests similar mechanisms for Cd²+ and Zn²+ transfer via protein-protein interactions.

Conclusions:

• Protein-protein interactions are a viable mechanism for As³+ transfer between hMT molecules, even when free As³+ is unstable.
• Partially metallated cadmium-MT and zinc-MT species are likely stable under physiological conditions.
• Findings provide insights into metal homeostasis and detoxification pathways involving metallothioneins.