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A xenobiotic is a chemical which is found in an organism but which is not normally produced or expected to be present in it. It can also cover substances which are present in much higher concentrations than are usual.

For example, drugs such as antibiotics are human xenobiotics because the human body does not produce them itself nor would they be expected to be present as part of a normal diet. However, the term is usually used in the context of pollutants such as dioxins and polychlorinated biphenyls and their effect on the biota.

The body gets rid of xenobiotics by xenobiotic metabolism. This consists of the deactivation and the secretion of xenobiotics, and happens mostly at the liver. Secretion routes are urine, faeces, breath and sweat. Hepatic enzymes are responsible for the metabolism of xenobiotics, by first activating them (oxidation, reduction, hydrolysis and/or hydration of the xenobiotic) and then conjugating the active secondary metabolite with glucuronic or sulphuric acid, or glutathione, followed by excretion in bile or urine. An example of a group of enzymes involved in xenobiotic metabolism is the hepatic microsomal cytochrome P450s.

Enzymes that metabolize xenobiotics are very important for the pharmaceutical industry, as they are responsible for the breakdown of drugs.

Xenobiotic substances are becoming an increasingly large problem in Sewage Treatment systems, since they are relatively new substances and are very difficult to categorize. Antibiotics, for example, were derived from plants originally, and so mimic naturally occurring substances. This, along with the natural monopoly nature of municipal Waste Water Treatment Plants makes it nearly impossible to remove this new pollutant load.