From Coastal Wiki
Definition of metallothionein:
Protein involved in cell detoxification mechanisms for various heavy metals.
This is the common definition for metallothionein, other definitions can be discussed in the article
Metallothioneins occur in almost all organisms. They can regulate heavy metal concentrations by binding them. Organisms can produce metalollothioneins in response to the presence of heavy metals. However, when the amount of heavy metals exceeds what the metallothioneins can handle, the toxic effects of the contaminants will be manifested.