|Molar mass||252.31 g·mol−1|
|Density||1.24 g/cm3 (25 °C)|
|Melting point||179 °C (354 °F; 452 K)|
|Boiling point||495 °C (923 °F; 768 K)|
|0.2 to 6.2 ug/L|
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
|what is: / ?)(|
Benzo[a]pyrene is a polycyclic aromatic hydrocarbon found in coal tar with the formula C20H12. Its metabolites are mutagenic and highly carcinogenic, and it is listed as a Group 1 carcinogen by the IARC. The compound is one of the benzopyrenes, formed by a benzene ring fused to pyrene, and is the result of incomplete combustion at temperatures between 300 °C (572 °F) and 600 °C (1,112 °F).
In the 18th century, young British chimney sweeps who climbed into chimneys suffered from chimney sweeps' carcinoma, a scrotal cancer peculiar to their profession, and this was connected to the effects of soot in 1775, in the first work of occupational cancer epidemiology and also the first connection of any chemical mixture to cancer formation. Frequent skin cancers were noted among fuel industry workers in the 19th century. In 1933, benzo[a]pyrene was determined to be the compound responsible for these cases, and its carcinogenicity was demonstrated when skin tumors occurred in laboratory animals repeatedly painted with coal tar. Benzo[a]pyrene has since been identified as a prime carcinogen in cigarette smoke. When the body attempts to metabolise benzo[a]pyrene, the resulting diol epoxide reacts and binds to DNA, resulting in mutations and eventually cancer.
Sources of benzo[a]pyrene
The main source of atmospheric benzo[a]pyrene is residential wood burning. It is also found in coal tar, in automobile exhaust fumes (especially from diesel engines), in all smoke resulting from the combustion of organic material (including cigarette smoke), and in charbroiled food. Cooked meat products, regular consumption of which has been epidemiologically associated with increased levels of colon cancer (although this in itself does not prove carcinogenicity), have been shown to contain up to 4 ng/g of benzo[a]pyrene, and up to 5.5 ng/g in fried chicken and 62.6 ng/g in overcooked charcoal barbecued beef.
In February 2014, NASA announced a greatly upgraded database for tracking polycyclic aromatic hydrocarbons (PAHs), including benzo[a]pyrene, in the universe. According to scientists, more than 20% of the carbon in the universe may be associated with PAHs, possible starting materials for the formation of life. PAHs seem to have been forming "only a couple of billion years after the Big Bang", are widespread throughout the universe, and are associated with new stars and exoplanets.
A vast number of studies over the previous three decades have documented links between benzo[a]pyrene and cancers. It has been more difficult to link cancers to specific benzo[a]pyrene sources, especially in humans, and difficult to quantify risks posed by various methods of exposure (inhalation or ingestion). Researchers at Kansas State University recently discovered a link between vitamin A deficiency and emphysema in smokers. Benzo[a]pyrene was found to be behind the link, since it induces vitamin A deficiency in rats.
In 1996, a study was published that provided the clear molecular evidence conclusively linking components in tobacco smoke to lung cancer. Benzo[a]pyrene, found in tobacco smoke (including cigarette smoke), was shown to cause genetic damage in lung cells that was identical to the damage observed in the DNA of most malignant lung tumours.
A 2001 National Cancer Institute study found levels of benzo[a]pyrene to be significantly higher in foods that were cooked well-done on the barbecue, particularly steaks, chicken with skin, and hamburgers. Japanese scientists showed that cooked beef contains mutagens, chemicals that are capable of altering the chemical structure of DNA. However, the foods themselves are not necessarily carcinogenic, even if they contain trace amounts of carcinogens, because the gastrointestinal tract protects itself against carcinomas by shedding its outer layer continuously. Furthermore, detoxification enzymes, such as cytochromes P450 have increased activities in the gut due to the normal requirement for protection from food-borne toxins. Thus, in most cases, small amounts of benzo[a]pyrene are metabolized by gut enzymes prior to being passed into the blood. The lungs are not protected in either of these manners.
A recent study has found that cytochrome P450 1A1 (CYP1A1) and cytochrome P450 1B1 (CYP1B1) are both protective and, confusingly, necessary for benzo[a]pyrene toxicity. Experiments with strains of mice engineered to remove (knockout) CYP1A1 and CYP1B1 reveal that CYP1A1 primarily acts to protect mammals from low doses of benzo[a]pyrene, and that removing this protection causes the biological accumulation of large concentrations of benzo[a]pyrene. Unless CYP1B1 is also knocked out, benzo[a]pyrene toxicity results from the bioactivation of benzo[a]pyrene to the ultimate toxic compound, benzo[a]pyrene -7,8-dihydrodiol-9,10-epoxide (see below).
Interaction with DNA
Properly speaking, benzo[a]pyrene is a procarcinogen, meaning that the mechanism of carcinogenesis of benzo[a]pyrene depends on enzymatic metabolism of benzo[a]pyrene to the ultimate mutagen, benzo[a]pyrene diol epoxide, pictured above at right. This molecule intercalates in DNA, covalently bonding to the nucleophilic guanine nucleobases at the N2 position. X-ray crystallographic and nuclear magnetic resonance structure studies show that this binding distorts the DNA by perturbing the double-helical DNA structure. This disrupts the normal process of copying DNA and induces mutations, which explains the occurrence of cancer after exposure. This mechanism of action is similar to that of aflatoxin which binds to the N7 position of guanine.
There are indications that benzo[a]pyrene diol epoxide specifically targets the protective p53 gene. This gene is a transcription factor that regulates the cell cycle and hence functions as a tumor suppressor. By inducing G (guanine) to T (thymidine) transversions in transversion hotspots within p53, there is a probability that benzo[a]pyrene diol epoxide inactivates the tumor suppression ability in certain cells, leading to cancer.
Benzo[a]pyrene-7,8-dihydrodiol-9,10-epoxide is the carcinogenic product of three enzymatic reactions:
- Benzo[a]pyrene is first oxidized by cytochrome P450 1A1 to form a variety of products, including (+)benzo[a]pyrene-7,8-epoxide.
- This product is metabolized by epoxide hydrolase, opening up the epoxide ring to yield (-)benzo[a]pyrene-7,8-dihydrodiol.
- The ultimate carcinogen is formed after another reaction with cytochrome P4501A1 to yield the (+)benzo[a]pyrene-7,8-dihydrodiol-9,10-epoxide. It is this diol epoxide that covalently binds to DNA.
Benzo[a]pyrene induces cytochrome P4501A1 (CYP1A1) by binding to the AHR (aryl hydrocarbon receptor) in the cytosol. Upon binding the transformed receptor translocates to the nucleus where it dimerises with ARNT (aryl hydrocarbon receptor nuclear translocator) and then binds xenobiotic response elements (XREs) in DNA located upstream of certain genes. This process increases transcription of certain genes, notably CYP1A1, followed by increased CYP1A1 protein production. This process is similar to induction of CYP1A1 by certain polychlorinated biphenyls and dioxins. Seemingly, CYP1A1 activity in the intestinal mucosa prevents major amounts of ingested benzo(a)pyrene to enter portal blood and systemic circulation. Intestinal, but not hepatic, expression of CYP1A1 depends on TOLL-like receptor 2 (TLR2), which is a eucaryotic receptor for bacterial surface structures such as lipoteichoic acid.
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- Created from PDB 1JDG
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