2-Amino-1-methyl-6-phenylimidazo(4,5-b)pyridine
Encyclopedia
2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is one of the most abundant heterocyclic amines
(HCAs) in cooked meat. It is classified as a IARC Group 2B carcinogen, possibly carcinogenic to humans, and is considered "reasonably anticipated to be a human carcinogen" by the U.S. Department of Health and Human Services National Toxicology Program. PhIP is formed at high temperatures from the reaction between creatine
or creatinine
(found in muscle meats), amino acids, and sugar. PhIP formation increases with the temperature and duration of cooking and also depends on the method of cooking, as well as the variety of meat being cooked. There is inadequate evidence in humans that PhIP is carcinogenic, however, there is sufficient evidence in experimental animals, as well as in vitro
models, for the carcinogenicity of PhIP.
in cooked ground meat and chicken and up to 500 ppb in well-done flame-grilled chicken. Individual exposures can also differ due to various anti-carcinogens in the diet. Fiber or chlorophyllin, which directly bind to PhIP and prevent absorption, flavonoids, known inhibitors of metabolic activation, and substances that prevent formation of the carcinogen during cooking further complicate the exposure assessment to PhIP. Thus, the dose (exposure) plays a major role in individual risk related to PhIP and merely understanding the degree of doneness of the meat products may not be adequate.
). PhIP can be further metabolized into a more potent metabolite through O-acetylation by hepatic or colonic N-acetyltransferase
1 (NAT1) and N-acetyltransferase 2
(NAT2), or by sulfotransfereases. However, PhIP may also undergo a detoxification pathyway through Phase II
conjugation reaction via UDP-glucuronosyltransferase
s (UGTs) to form N-glucuronide conjugates. PhIP's nitrenium ion intermediate is a powerful electrophile
that has propensity to form C-8 guanine
adducts with the DNA
.
In addition, some of these metabolic enzymes are inducible and have polymorphic variation. CYP1A2 displays a 40-fold variation in expression among humans and can be induced by smoking, diet, and chronic hepatitis. The expression of UGTs also displays inducability, however NATs do not. Individuals can be classified as either rapid or slow N-oxidizers and O-acetylators by assessing CYP1A2 and NAT2 activities. Individuals with the rapid phenotype
of either CYP1A2 or NAT2 metabolize PhIP more effectively and are therefore at greater risk of PhIP’s carcinogenic metabolite and could be at a higher risk of cancer.
and in vitro
studies have demonstrated that PhIP is a potent mutagen
and can induce tumors of multiple sites in animal models. PhIP was positive in bacterial (Ames
) test and induced chromosomal abnormalities in human and Chinese hamster cells in vitro. PhIP has also formed DNA adducts in vivo in both rats and monkeys. PhIP has been tested for carcinogenicity in both mice and rats by oral administration. Increases in lymphomas were seen in mice and increases in adenocarcinomas of the small and large intestine in males and mammary adenocarcinomas in female were seen in rats. Also, an increasing number of epidemiological studies have evaluated the association of well-done meat intake and HCA exposure with cancer risk in humans. In general, these studies have reported that high intake of well-done and/or high exposure to PhIP may be associated with cancer of the colorectum, breast, prostate, pancreas, lung, stomach, and esophagus.
PhIP has been shown to induce DNA adducts and mutations. These adducts have been found in a wide variety sensitive of tissues and organs such as the colon. However, adducts also formed in sites that did not commonly form tumors, such as the kidneys. In humans receiving a dose of PhIP equivalent to that found in very well-done chicken, DNA and protein adducts were formed in the colon and blood, however the adducts were unstable and declined over a 24 hour period.
Heterocyclic amines
Heterocyclic amines, also sometime referred to as HCAs, are chemical compounds containing at least one heterocyclic ring, which by definition has atoms of at least two different elements, plus the compound has at least one amine group. While some heterocycles are amines themselves, the amines...
(HCAs) in cooked meat. It is classified as a IARC Group 2B carcinogen, possibly carcinogenic to humans, and is considered "reasonably anticipated to be a human carcinogen" by the U.S. Department of Health and Human Services National Toxicology Program. PhIP is formed at high temperatures from the reaction between creatine
Creatine
Creatine is a nitrogenous organic acid that occurs naturally in vertebrates and helps to supply energy to all cells in the body, primarily muscle. This is achieved by increasing the formation of Adenosine triphosphate...
or creatinine
Creatinine
Creatinine is a break-down product of creatine phosphate in muscle, and is usually produced at a fairly constant rate by the body...
(found in muscle meats), amino acids, and sugar. PhIP formation increases with the temperature and duration of cooking and also depends on the method of cooking, as well as the variety of meat being cooked. There is inadequate evidence in humans that PhIP is carcinogenic, however, there is sufficient evidence in experimental animals, as well as in vitro
In vitro
In vitro refers to studies in experimental biology that are conducted using components of an organism that have been isolated from their usual biological context in order to permit a more detailed or more convenient analysis than can be done with whole organisms. Colloquially, these experiments...
models, for the carcinogenicity of PhIP.
Sources of PhIP
PhIP has been found in cooked beef, pork, chicken, and fish products at concentrations up to 70 ng/g. Estimates of PhIP intake, from the HCA database, suggest that the mean daily intake of PhIP is between 43 and 110 ng/day. However, exposure to PhIP depends on the eating habits of the individual and can vary up to 5000-fold. This exposure is related to the type of meat, doneness, cooking method, and quantity consumed. PhIP can be quantitatively measured as low as 0.1 ppbPPB
PPB can stand for:* Party political broadcast, a type of political programming in the United Kingdom* parts-per-billion, a unit of concentration* Portland Police Bureau, a police agency for the city of Portland...
in cooked ground meat and chicken and up to 500 ppb in well-done flame-grilled chicken. Individual exposures can also differ due to various anti-carcinogens in the diet. Fiber or chlorophyllin, which directly bind to PhIP and prevent absorption, flavonoids, known inhibitors of metabolic activation, and substances that prevent formation of the carcinogen during cooking further complicate the exposure assessment to PhIP. Thus, the dose (exposure) plays a major role in individual risk related to PhIP and merely understanding the degree of doneness of the meat products may not be adequate.
PhIP and cooking methods
Early studies looking at the amount of PhIP in cooked meats have shown that a high level of exposure is possible. However, more recent studies estimate exposure to PhIP and other HCAs through intake questionnaires linked to an HCA database. This database was created by cooking different types of meats with a variety of different cooking methods to a range of doneness levels (rare, medium, well-done, and very well-done). The measured values of PhIP varied with the cut of beef, cooking method, and doneness level. In general, PhIP was the predominant HCA form. To reduce the formation of PhIP and other HCAs in meat, reduce the temperature and the length of cooking time or pre-heat the meat in the microwave oven, which removes creatine, a building block in the formation of PhIP.| Meat Type | Cooking Variation | PhIP ng/g ± SD |
|---|---|---|
| Beef (1.5 cm thick) | Fried - medium rare (51°C) | 0.29 ± 0.14 |
| Fried - well-done (63°C) | 0.73 ± 0.02 | |
| Fried - very well-done (74°C) | 7.33 ± 0.11 | |
| Lamb Chop | Fried - medium (75°C) | 0 |
| Fried - well-done (85°C) | 2.4 | |
| Pork (2 cm thick) | Fried - medium (63°C) | 0.37 ± 0.06 |
| Fried - well-done (83°C) | 7.82 ± 1.13 | |
| Mince Beef Patty (2 cm thick) | Fried - medium (51°C) | 0 |
| Fried - well-done (58°C) | 3.96 ± 0.13 | |
| Chicken (2.5 cm, no skin) | Fried - lightly browned (63°C) | 0.2 ± 0.005 |
| Fried - well-done (79°C) | 17.54 ± 0.17 | |
| Sausage | Fried - lightly browned (42°C) | 0 |
| Fried - well browned (70°C) | 0.61 ± 0.06 | |
| Bacon, middle | Fried - lightly cooked | 0.11 ± 0.002 |
| Fried - well cooked | 1.93 ± 0.37 |
Metabolism
Metabolic activation is required for PhIP to function as a mutagen, therefore the cancer risk posed by PhIP depends on the extent at which PhIP is metabolized. After absorption, PhIP is converted to a genotoxic metabolite in the liver by Phase I enzyme N-oxidation by Cytochrome P-450 1A2 (CYP1A2CYP1A2
Cytochrome P450 1A2 , a member of the cytochrome P450 mixed-function oxidase system, is involved in the metabolism of xenobiotics in the body...
). PhIP can be further metabolized into a more potent metabolite through O-acetylation by hepatic or colonic N-acetyltransferase
N-Acetyltransferase
N-acetyltransferase is an enzyme that catalyzes the transfer of acetyl groups from acetyl-CoA to arylamines. They have wide specificity for aromatic amines, particularly serotonin, and can also catalyze acetyl transfer between arylamines without CoA. EC 2.3.1.5....
1 (NAT1) and N-acetyltransferase 2
N-acetyltransferase 2
N-acetyltransferase 2 , also known as NAT2, is an enzyme which in humans is encoded by the NAT2 gene.- Function :...
(NAT2), or by sulfotransfereases. However, PhIP may also undergo a detoxification pathyway through Phase II
Phase II
Phase II may refer to:* Phase II clinical trial* Drug metabolism Phase II* Star Trek: Phase II, a planned Star Trek television series that eventually evolved into Star Trek: The Motion Picture...
conjugation reaction via UDP-glucuronosyltransferase
Glucuronosyltransferase
Uridine 5'-diphospho-glucuronosyltransferase is a glycosyltransferase that catalyzes addition of the glycosyl group from a UTP-sugar to a small hydrophobic molecule.This is glucuronidation reaction.Alternative names:...
s (UGTs) to form N-glucuronide conjugates. PhIP's nitrenium ion intermediate is a powerful electrophile
Electrophile
In general electrophiles are positively charged species that are attracted to an electron rich centre. In chemistry, an electrophile is a reagent attracted to electrons that participates in a chemical reaction by accepting an electron pair in order to bond to a nucleophile...
that has propensity to form C-8 guanine
Guanine
Guanine is one of the four main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine . In DNA, guanine is paired with cytosine. With the formula C5H5N5O, guanine is a derivative of purine, consisting of a fused pyrimidine-imidazole ring system with...
adducts with the DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
.
In addition, some of these metabolic enzymes are inducible and have polymorphic variation. CYP1A2 displays a 40-fold variation in expression among humans and can be induced by smoking, diet, and chronic hepatitis. The expression of UGTs also displays inducability, however NATs do not. Individuals can be classified as either rapid or slow N-oxidizers and O-acetylators by assessing CYP1A2 and NAT2 activities. Individuals with the rapid phenotype
Phenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...
of either CYP1A2 or NAT2 metabolize PhIP more effectively and are therefore at greater risk of PhIP’s carcinogenic metabolite and could be at a higher risk of cancer.
Associated cancers
Numerous in vivoIn vivo
In vivo is experimentation using a whole, living organism as opposed to a partial or dead organism, or an in vitro controlled environment. Animal testing and clinical trials are two forms of in vivo research...
and in vitro
In vitro
In vitro refers to studies in experimental biology that are conducted using components of an organism that have been isolated from their usual biological context in order to permit a more detailed or more convenient analysis than can be done with whole organisms. Colloquially, these experiments...
studies have demonstrated that PhIP is a potent mutagen
Mutagen
In genetics, a mutagen is a physical or chemical agent that changes the genetic material, usually DNA, of an organism and thus increases the frequency of mutations above the natural background level. As many mutations cause cancer, mutagens are therefore also likely to be carcinogens...
and can induce tumors of multiple sites in animal models. PhIP was positive in bacterial (Ames
Ames
Ames was an American chain of discount stores based in Rocky Hill, Connecticut, USA. The company was founded in 1958 with a store in Southbridge, Massachusetts, and at its peak operated 700 stores in 20 states, including the Northeast, Upper South, Midwest and the District of Columbia, making it...
) test and induced chromosomal abnormalities in human and Chinese hamster cells in vitro. PhIP has also formed DNA adducts in vivo in both rats and monkeys. PhIP has been tested for carcinogenicity in both mice and rats by oral administration. Increases in lymphomas were seen in mice and increases in adenocarcinomas of the small and large intestine in males and mammary adenocarcinomas in female were seen in rats. Also, an increasing number of epidemiological studies have evaluated the association of well-done meat intake and HCA exposure with cancer risk in humans. In general, these studies have reported that high intake of well-done and/or high exposure to PhIP may be associated with cancer of the colorectum, breast, prostate, pancreas, lung, stomach, and esophagus.
PhIP has been shown to induce DNA adducts and mutations. These adducts have been found in a wide variety sensitive of tissues and organs such as the colon. However, adducts also formed in sites that did not commonly form tumors, such as the kidneys. In humans receiving a dose of PhIP equivalent to that found in very well-done chicken, DNA and protein adducts were formed in the colon and blood, however the adducts were unstable and declined over a 24 hour period.