Heterocyclic amine

Niacin, essential to many types of life, is an example of a heterocyclic amine.

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, as well as at least one amine (nitrogen-containing) group. Typically it is a nitrogen atom of an amine group that also makes the ring heterocyclic (e.g., pyridine), though compounds exist in which this is not the case (e.g., the drug Zileuton). The biological functions of heterocyclic amines vary, including vitamins and carcinogens. Carcinogenic heterocyclic amines are created by high temperature cooking of meat, for example.

Five-membered heterocyclic amines

The compound pyrrolidine is composed of molecules that contain a saturated ring of five atoms. This cyclic structure is composed of one atom of nitrogen and four carbon. Nicotine is a molecule containing a pyrrolidine ring attached to a ring of pyridine (other heterocyclic amine). Nicotine belongs to a group of compounds known as alkaloids, which are naturally occurring organic compounds with nitrogen in them. Pyrrole is another compound made up of molecules with a five-membered heterocyclic ring. These molecules are unsaturated and contain a nitrogen atom in the ring. Four pyrrole rings are joined in a ring structure called a porphyrin. The rings of porphyrin are components of hemoglobin, myoglobin, vitamin B12, chlorophyll, and cytochromes. In the centers of heme in hemoglobin, myoglobin, and cytochromes, iron is an ion; in the first two, iron ion is bound to oxygen.

Six-membered heterocyclic amines

The structure of pyridine is similar to that of benzene except that a nitrogen atom replaces one carbon atom. Pyridine is used as a flavoring agent. The pyridine ring is part of two B vitamins: niacin and pyridoxine. Niacin, also called nicotinic acid, is found in most organisms. Via metabolism, it becomes nicotinamide adenine dinucleotide NAD, a coenzyme which is involved in oxidation and reduction in metabolic cells. A deficiency of niacin leads to a disease called pellagra. Pyridoxine or vitamin B6, it becomes a major compound in the metabolism of amino acids.

Pyrimidine is a heterocyclic amine that contains two nitrogen atoms in an unsaturated six-membered ring. An example of a molecule that contains pyrimidine is thiamine, which is also known as vitamin B1. Thiamine decifiency produces beriberi. Pyrimidine is a component of the nucleobases cytosine, uracil, and thymine. The other two nucleobases, adenine and guanine, are also heterocyclic amines called purines; they are composed of a fused pyrimidine and imidazole.

Heterocyclic amines and cancer

High-temperature cooking (particularly charring) of meat forms some cancer-causing heterocyclic amines.

Some heterocyclic amines (HCAs) found in cooked and especially burned meat are known carcinogens. Research has shown that heterocyclic amine formation in meat occurs at high cooking temperatures. For example, heterocyclic amines are the carcinogenic chemicals formed from the cooking of muscle meats such as beef, pork, fowl, and fish. HCAs form when amino acids and creatine (a chemical found in muscles) react at high cooking temperatures. Researchers have identified 17 different HCAs resulting from the cooking of muscle meats that may pose human cancer risk.^[1][2] The United States National Cancer Institute's Division of Cancer Epidemiology and Genetics found a link between individuals with stomach cancer and the consumption of cooked meat, and other studies for colorectal, pancreatic, and breast cancer is associated with high intakes of well-done, fried, or barbecued meats. People who eat medium-well or well-done beef were more than three times as likely to suffer stomach cancer as those who ate rare or medium-rare beef. Other sources of protein (milk, eggs, tofu, and organ meats such as liver) have very little or no HCA content naturally or when cooked.

Heterocyclic amines and neurological disorders

Harmane, a β-carboline alkaloid found in meats, has been shown to have strong neurotoxic characteristics, and in particular, is "highly tremorogenic" (tremor inducing).^[3] These chemicals are formed during the cooking process of meat, particularly the longer they are cooked, and the more they are exposed to high temperatures during cooking.^[4][5] Harmane has been found in roughly 50% higher concentrations in patients with essential tremor than in controls;^[6] essential tremor is a condition 20 times more prevalent than Parkinson's disease.^[7] There is no direct correlation between blood-levels and levels of daily meat consumption, suggesting a difference in metabolism of this chemical plays a greater role.^[3]

Heterocyclic amines and cooking

Research has shown that an olive oil, lemon juice, and garlic marinade cut HCA levels in chicken by as much as 90%. Six hours of marinating in beer or red wine cut levels of two types of HCA in beef steak by up to 90% compared with unmarinated steak. However, researchers say that "It may seem appetising to marinate steak in beer or wine, but this will have a minimal impact on the effect of the meat on your cancer risk and the best way to reduce your risk of cancer from eating red and processed meat is to eat less of it overall".^[8]

See also

• Heterocyclic amine formation in meat
• Polycyclic aromatic hydrocarbons

References

External links

• Sugimura T, Nagao M, Wakabayashi K (May 1996). "Carcinogenicity of food mutagens". Environmental Health Perspectives. 104 (Suppl 3): 429–33. doi:10.2307/3432798. PMC 1469643. PMID 8781358. 
• Louis ED, Zheng W, Jiang W, Bogen KT, Keating GA (June 2007). "Quantification of the neurotoxic beta-carboline harmane in barbecued/grilled meat samples and correlation with level of doneness". Journal of Toxicology and Environmental Health. Part A. 70 (12): 1014–9. doi:10.1080/15287390601172015. PMID 17497412.