Editing Polychlorinated biphenyl (section)

==Biochemical metabolism==
===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>

===Species dependent===
Metabolism is also dependent on the species of organism; different organisms have slightly different P450 enzymes that metabolize certain PCBs better than others. Looking at the PCB metabolism in the liver of four sea turtle species (green, [[olive ridley]], loggerhead and hawksbill), green and hawksbill sea turtles have noticeably higher [[hydroxylation]] rates of PCB 52 than olive ridley or loggerhead sea turtles. This is because the green and hawksbill sea turtles have higher P450 2-like protein expression. This protein adds three hydroxyl groups to PCB 52, making it more polar and water-soluble. P450 3-like protein expression that is thought to be linked to PCB 77 metabolism, something that was not measured in this study.<ref name="pubs.acs.org"/>

===Temperature dependent===
Temperature plays a key role in the ecology, physiology and metabolism of aquatic species. The rate of PCB metabolism was temperature dependent in [[yellow perch]] (''Perca flavescens''). In fall and winter, only 11 out of 72 introduced PCB congeners were excreted and had [[halflife|halflives]] of more than 1,000 days. During spring and summer when the average daily water temperature was above 20&nbsp;°C, persistent PCBs had halflives of 67 days. The main excretion processes were fecal egestion, growth dilution and loss across respiratory surfaces. The excretion rate of PCBs matched with the perch's natural [[bioenergetics]], where most of their consumption, respiration and growth rates occur during the late spring and summer. Since the perch is performing more functions in the warmer months, it naturally has a faster metabolism and has less PCB accumulation. However, multiple cold-water periods mixed with toxic PCBs with coplanar chlorine molecules can be detrimental to perch health.<ref>{{cite journal | vauthors = Paterson G, Drouillard KG, Haffner GD | title = PCB elimination by yellow perch (Perca flavescens) during an annual temperature cycle | journal = Environmental Science & Technology | volume = 41 | issue = 3 | pages = 824–829 | date = February 2007 | pmid = 17328189 | doi = 10.1021/es060266r | bibcode = 2007EnST...41..824P }}</ref>

===Sex dependent===
[[Enantiomer]]s of chiral compounds have similar chemical and physical properties, but can be metabolized by the body differently. This was looked at in [[bowhead whale]]s (''Balaena mysticetus'') for two main reasons: they are large animals with slow metabolisms (meaning PCBs will accumulate in fatty tissue) and few studies have measured chiral PCBs in cetaceans. They found that the average PCB concentrations in the blubber were approximately four times higher than the liver; however, this result is most likely age- and sex-dependent. As reproductively active females transferred PCBs and other poisonous substances to the fetus, the PCB concentrations in the blubber were significantly lower than males of the same body length (less than 13 meters).<ref>{{cite journal | vauthors = Hoekstra PF, Wong CS, O'Hara TM, Solomon KR, Mabury SA, Muir DC |display-authors=3| title = Enantiomer-specific accumulation of PCB atropisomers in the bowhead whale (Balaena mysticetus) | journal = Environmental Science & Technology | volume = 36 | issue = 7 | pages = 1419–1425 | date = April 2002 | pmid = 11999046 | doi = 10.1021/es015763g | bibcode = 2002EnST...36.1419H }}</ref>