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Not to be confused with xenobiology.

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.

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, i.e. 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

Main article: Xenobiotic metabolism

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. 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.^[1] 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.^[2]

Xenobiotics in the environment

Main article: Environmental xenobiotic

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 fungi 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 almost all the different complex and resistant xenobiotics found on the earth.^[3] Many xenobiotics produce a variety of biological effects, which is used when they are characterized using bioassay.

Inter-species organ transplantation

Main article: Xenotransplantation

The term xenobiotic is also used to refer to organs transplanted from one species to 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 are currently 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.

References

^ Brodie ED, Ridenhour BJ, Brodie ED (2002). "The evolutionary response of predators to dangerous prey: hotspots and coldspots in the geographic mosaic of coevolution between garter snakes and newts". Evolution 56 (10): 2067–82. PMID 12449493.
^ Geffeney S, Brodie ED, Ruben PC, Brodie ED (2002). "Mechanisms of adaptation in a predator-prey arms race: TTX-resistant sodium channels". Science 297 (5585): 1336–9. doi:10.1126/science.1074310. PMID 12193784.
^ Alexander M. (1999) Biodegradation and Bioremediation, Elsevier Science.

UpToDate Contents

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1. 抗てんかん剤と骨疾患 antiepileptic drugs and bone disease
2. 薬理ゲノム学の概要 overview of pharmacogenomics

English Journal

Improved xenobiotic-degrading activity of Rhodococcus opacus strain 1cp after dormancy.

Solyanikova IP, Mulyukin AL, Suzina NE, El-Registan GI, Golovleva LA.Sourcea G.K. Skryabin Institute of Biochemistry and Physiology of Microorganisms, Russian Academy of Sciences , Moscow , Russia.
Journal of environmental science and health. Part. B, Pesticides, food contaminants, and agricultural wastes.J Environ Sci Health B.2011 Oct;46(7):638-47.
The goals of the present work were as follows: to obtain the dormant forms of R. opacus 1cp; to study the phenotypic variability during their germination; to compare phenotypic variants during the growth on selective and elective media; and to reveal changes in the ability of the strain to destruct
PMID 21749252

Comparative effects of butyl benzyl phthalate (BBP) and di(2-ethylhexyl) phthalate (DEHP) on the aquatic larvae of Chironomus riparius based on gene expression assays related to the endocrine system, the stress response and ribosomes.

Planello R, Herrero O, Martinez-Guitarte JL, Morcillo G.SourceGrupo de Biologia y Toxicologia Ambiental, Facultad de Ciencias, Universidad Nacional de Educacion a Distancia, UNED, Senda del Rey 9, 28040 Madrid, Spain.
Aquatic toxicology (Amsterdam, Netherlands).Aquat Toxicol.2011 Sep;105(1-2):62-70. Epub 2011 May 20.
In this work, the effects of butyl benzyl phthalate (BBP) and di(2-ethylhexyl) phthalate (DEHP), two of the most extensively used phthalates, were studied in Chironomus riparius under acute short-term treatments, to compare their relative toxicities and identify genes sensitive to exposure. The ecot
PMID 21684242

Japanese Journal

胆汁酸代謝とビタミンD受容体

槇島誠
ビタミン 85(2), 47-56, 2011-02-25
… VDR activation by 1,25(OH)_2D_3 or LCA induces the xenobiotic metabolism of bile acids. …
NAID 110008460669

Systems biology analysis of xenobiotic responses in insects(Resistance Management: Insect Disease Vectors & Agricultural Pests,1) Pest Management, Crop Protection and Vector Control)