Quantification of metallothioneins by a silver-saturation method

Area of Science:

• Biochemistry
• Toxicology
• Analytical Chemistry

Background:

• Metallothioneins (MTs) are crucial proteins involved in metal detoxification and homeostasis.
• Accurate quantification of MTs in biological samples is essential for understanding metal toxicity and biological responses.

Purpose of the Study:

• To investigate the binding of silver ions (Ag+) to metallothionein (MT).
• To develop and validate a novel Ag-saturation assay (Ag-hem) for measuring MT concentrations in various tissues.
• To compare the efficacy of the Ag-hem assay with the existing cadmium-saturation (Cd-hem) assay.

Main Methods:

• Titration of purified Zn-MT and Cd-MT with Ag+ followed by hemolysate-heat treatment to determine Ag+ saturation levels.
• Metal displacement assays using 110mAg-labeled Ag-MT at pH 8.5 in glycine buffer to establish metal potency rank order.
• Comparative analysis of Ag-hem and Cd-hem assays using purified MT and tissue samples from various experimental conditions (Cd-injected rats, Hg-injected rats, Cu-injected rats).

Main Results:

• MT saturation occurred at 17-18 g-atoms Ag+/mol protein.
• The rank order of metal potency to displace Ag+ from Ag-MT was Ag+ > Cu2+ > Cd2+ > Hg2+ > Zn2+.
• The Ag-hem assay showed excellent agreement with the Cd-hem assay for most samples.
• The Cd-hem assay underestimated MT concentrations in samples with high copper content (Cu-MT), unlike the Ag-hem assay.

Conclusions:

• The Ag-hem assay is a reliable method for quantifying metallothioneins.
• The Ag-hem assay is particularly advantageous over the Cd-hem assay when significant amounts of copper-metallothionein are present.
• The Ag-hem assay provides a more accurate assessment of MT concentrations in diverse biological contexts, especially those involving copper exposure.