Spectrophotometric detection of labile zinc(II) released from metallothionein: A simple method to evaluate heavy metal toxicity

A new and simple method based on the mechanism of detoxification of metallothionein was developed by using a water-soluble porphyrin and Zn(II)-bound metallothionein for evaluating heavy metal toxicity. Labile Zn(II) ions were released when toxic metal ions such as Cu(II), Pb(II), Bi(III), Cd(II), Hg(II), Co(II), Ag(I), and Ni(II) bound to Zn(II)-bound metallothionein. The water-soluble porphyrin, 5,10,15,20-tetraphenyl-21 H,23 H-porphinetetrasulfonic acid, a chromogenic reagent that is highly sensitive to Zn(II), formed a complex with the labile Zn(II) ions. The absorption change at 423 nm resulting from the formation of the Zn(II)–porphyrin complex was used to evaluate the toxicity of sample solutions containing different metal ions. The absorption change was well correlated with the toxicity, which was evaluated by a bioluminescence inhibition assay using the bioluminescent bacteria Vibrio fischeri. This observation indicated that the absorption change determined by our method was a good indicator of heavy metal toxicity. The proposed method was more sensitive than conventional bioassays and could be used to detect metal toxicity at submicromolar concentrations of toxic metal ions.