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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 Specifically medication|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
    However the term xenobiotics is very often used in the context of pollutants such as dioxins and polychlorinated biphenyls and their effect on the biota because xenobiotics are understood as substances foreign to an entire biological system ie artificial substances which did not exist in nature before their synthesis by humans The term xenobiotic is derived from the Greek words ξένος (xenos) = foreigner stranger and βίος (bios vios) = life plus the Greek suffix for adjectives -τικός -ή -ό (tic)

    Xenobiotic metabolism

    The body removes xenobiotics by xenobiotic metabolism This consists of the deactivation and the secretion of xenobiotics and happens mostly in the liver Secretion 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 An example of a group of enzymes involved in xenobiotic metabolism is hepatic microsomal cytochrome P450 These enzymes that metabolize xenobiotics are very important for the pharmaceutical industry because they are responsible for the breakdown of medications
    Organisms can also evolve to tolerate xenobiotics An example is the co-evolution of the production of tetrodotoxin in the rough-skinned newt and the evolution of tetrodotoxin resistance in its predator the common garter snake In this predator-prey pair an evolutionary arms race has produced high levels of toxin in the newt and correspondingly high levels of resistance in the snake This evolutionary response is based on the snake evolving modified forms of the ion channels that the toxin acts upon so becoming resistant to its effects

    Xenobiotics in the environment

    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
    Some xenobiotics are resistant to degradation For example they may be synthetic organochlorides such as plastics and pesticides or naturally occurring organic chemicals such as polyaromatic hydrocarbons (PAHs) and some fractions of crude oil and coal However it is believed that microorganisms are capable of degrading all the different complex and resistant xenobiotics found on the earth

    Inter-species organ transplantation

    The term xenobiotic is also used to refer to organ transplant from one speciesto another For example some researchers hope that hearts and other organs could be transplanted from pigs to humans Many people die every year whose lives could have been saved if a critical organ had been available for transplant Kidneys arecurrently the most commonly transplanted organ Xenobiotic organs would need to be developed in such a way that they would not be rejected by the immune system

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