Editing Polychlorinated biphenyl (section)

===Overview===
PCBs undergo xenobiotic biotransformation, a mechanism used to make [[lipophilic]] toxins more polar and more easily excreted from the body.<ref name="pubs.acs.org">{{cite journal | vauthors = Richardson KL, Schlenk D | title = Biotransformation of 2,2',5,5'-tetrachlorobiphenyl (PCB 52) and 3,3',4,4'-tetrachlorobiphenyl (PCB 77) by liver microsomes from four species of sea turtles | journal = Chemical Research in Toxicology | volume = 24 | issue = 5 | pages = 718–725 | date = May 2011 | pmid = 21480586 | doi = 10.1021/tx1004562 }}</ref> The biotransformation is dependent on the number of chlorine atoms present, along with their position on the rings. Phase I reactions occur by adding an oxygen to either of the benzene rings by [[Cytochrome P450]].<ref>{{cite journal | vauthors = Forgue ST, Preston BD, Hargraves WA, Reich IL, Allen JR | title = Direct evidence that an arene oxide is a metabolic intermediate of 2,2',5,5'-tetrachlorobiphenyl | journal = Biochemical and Biophysical Research Communications | volume = 91 | issue = 2 | pages = 475–483 | date = November 1979 | pmid = 42397 | doi = 10.1016/0006-291x(79)91546-8 }}</ref> The type of P450 present also determines where the oxygen will be added; phenobarbital (PB)-induced P450s catalyze oxygenation to the meta-para positions of PCBs while 3-methylcholanthrene (3MC)-induced P450s add oxygens to the ''ortho''–''meta'' positions.<ref>{{cite journal | vauthors = Parke DV | year = 1985 | title = The role of cytochrome P-450 in the metabolism of pollutants | journal = Environmental Research | volume = 17 | issue = 2–4| pages = 97–100 | doi=10.1016/0141-1136(85)90049-2| bibcode = 1985MarER..17...97P }}</ref> PCBs containing ''ortho''–''meta'' and ''meta''–''para'' protons can be metabolized by either enzyme, making them the most likely to leave the organism. However, some metabolites of PCBs containing ''ortho''–''meta'' protons have increased [[steric hindrance]] from the oxygen, causing increased stability and an increased chance of accumulation.<ref>{{cite journal | vauthors = McFarland VA, Clarke JU | title = Environmental occurrence, abundance, and potential toxicity of polychlorinated biphenyl congeners: considerations for a congener-specific analysis | journal = Environmental Health Perspectives | volume = 81 | pages = 225–239 | date = May 1989 | pmid = 2503374 | pmc = 1567542 | doi = 10.1289/ehp.8981225 | bibcode = 1989EnvHP..81..225M }}</ref>