Xenobiotics: An Essential Precursor for Living System

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. Specifically, drugs such as antibiotics are xenobiotics in humans because the human body does not produce them itself, nor are they part of a normal diet. Natural compounds can also become xenobiotics if they are taken up by another organism, such as the uptake of natural human hormones by fish found downstream of sewage treatment plant outfalls, or the chemical defenses produced by some organisms as protection against predators. Xenobiotic metabolism is the set of metabolic pathways that modify the chemical structure of xenobiotics. In general, drugs are metabolized more slowly in fetal, neonatal and elderly humans and animals than in adults. The body removes xenobiotics by xenobiotic metabolism. This consists of the deactivation and the excretion of xenobiotics and happens mostly in the liver. Excretion routes are urine, feces, 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. Environmental xenobiotics are xenobiotic substances with a biological activity that are found as pollutants in the natural environment.