The cell system that detoxificates organic xenobiotics comprises three stages. Each of them involves a number of enzymes that, depending on the character of the degraded organic compound, lead to its transformations. The enzymes of the first stage – bioactivation – are generally polysubstrate mono- oxygenases, flavine-containing monooxygenases (FMO), and the enzymes showing hydrolytic and oxido- reductive activity. Induction of the enzymes influenced by xenobiotic has been confirmed by the SAGE studies (Serial Analysis of Gene Expression).

Metallothioneins are low molecular weight, cysteine-rich, heavy-metal binding proteins. They are present in some Prokaryota, fungi, plants and animals. Attempts to isolate native plant proteins failed but many metallothionein genes were identified in mono- and dicotyledons and also in some gymno- sperms and algae. They are organised in small gene families, like in animals genomes and their expression is upregulated by different endo- and exogenous factors, including metal ions.