A purine is a heterocyclic aromatic organic compound.
It consists of a pyrimidine ring fused to an imidazole ring.
Purines, which include substituted purines and their tautomers, are the most widely occurring
nitrogen-containing heterocycle in nature.
Purines and pyrimidines make up the two groups of nitrogenous bases, including the two groups
of nucleotide bases.
Two of the four deoxyribonucleotides and two of the four ribonucleotides, the respective
building-blocks of DNA and RNA, are purines.
Notable purines There are many naturally occurring purines.
Two of the five bases in nucleic acids, adenine and guanine, are purines.
In DNA, these bases form hydrogen bonds with their complementary pyrimidines thymine and
cytosine, respectively.
This is called complementary base pairing.
In RNA, the complement of adenine is uracil instead of thymine.
Other notable purines are hypoxanthine, xanthine, theobromine, caffeine, uric acid and isoguanine.
Functions Aside from the crucial roles of purines in
DNA and RNA, purines are also significant components in a number of other important
biomolecules, such as ATP, GTP, cyclic AMP, NADH, and coenzyme A. Purine itself, has not
been found in nature, but it can be produced by organic synthesis.
They may also function directly as neurotransmitters, acting upon purinergic receptors.
Adenosine activates adenosine receptors.
History The word purine was coined by the German chemist
Emil Fischer in 1884.
He synthesized it for the first time in 1899.
The starting material for the reaction sequence was uric acid, which had been isolated from
kidney stones by Scheele in 1776.
Uric acid was reacted with PCl5 to give 2,6,8-trichloropurine, which was converted with HI and PH4I to give
2,6-diiodopurine.
The product was reduced to purine using zinc-dust.
Purines are also much larger than pyrimidines.
Metabolism
Many organisms have metabolic pathways to synthesize and break down purines.
Purines are biologically synthesized as nucleosides.
Accumulation of modified purine nucleotides is defective to various cellular processes,
especially those involving DNA and RNA.
To be viable, organisms possess a number ofpurine phosphohydrolases, which hydrolyze these purine
derivatives removing them from the active NTP and dNTP pools.
Deamination of purine bases can result in accumulation of such nucleotides as ITP, dITP,
XTP and dXTP.
Defects in enzymes that control purine production and breakdown can severely alter a cell’s
DNA sequences, which may explain why people who carry certain genetic variants of purine
metabolic enzymes have a higher risk for some types of cancer.
Purine Sources Purines are found in high concentration in
meat and meat products, especially internal organs such as liver and kidney.
In general, plant-based diets are low in purines.
Examples of high-purine sources include: sweetbreads, anchovies, sardines, liver, beef kidneys,
brains, meat extracts, herring, mackerel, scallops, game meats, beer and gravy.
A moderate amount of purine is also contained in beef, pork, poultry, other fish and seafood,
asparagus, cauliflower, spinach, mushrooms, green peas, lentils, dried peas, beans, oatmeal,
wheat bran, wheat germ, and hawthorn.
Higher levels of meat and seafood consumption are associated with an increased risk of gout,
whereas a higher level of consumption of dairy products is associated with a decreased risk.
Moderate intake of purine-rich vegetables or protein is not associated with an increased
risk of gout.
Laboratory synthesis In addition to in vivo synthesis of purines
in purine metabolism, purine can also be created artificially.
Purine is obtained in good yield when formamide is heated in an open vessel at 170 °C for
28 hours.
This remarkable reaction and others like it have been discussed in the context of the
origin of life.
Oro, Orgel and co-workers have shown that four molecules of HCN tetramerize to form
diaminomaleodinitrile, which can be converted into almost all natural-occurring purines.
The Traube purine synthesis is a classic reaction between an amine-substituted pyrimidine and
formic acid.
See also Pyrimidine
Simple aromatic rings Gout, a disorder of purine metabolism
References
External links Purine Content in Food