Purine is a heterocyclicaromaticorganic compound that consists of a two rings in their structure. It is water-soluble. Purine also gives its name to the wider class of molecules, purines, which include substituted purines and their tautomers. They are the most widely occurring nitrogen-containing heterocycles in nature. 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 red meat, beef, pork, poultry, fish and seafood, asparagus, cauliflower, spinach, mushrooms, green peas, lentils, dried peas, beans, oatmeal, wheat bran, wheat germ, and haws. 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. In order to form DNA and RNA, both purines and pyrimidines are needed by the cell in approximately equal quantities. Both purine and pyrimidine are self-inhibiting and activating. When purines are formed, they inhibit the enzymes required for more purine formation. This self-inhibition occurs as they also activate the enzymes needed for pyrimidine formation. Pyrimidine simultaneously self-inhibits and activates purine in similar manner. Because of this, there is nearly an equal amount of both substances in the cell at all times.
Properties
Purine is both a very weak acid and an even weaker base. If dissolved in pure water, the pH will be halfway between these two pKa values.
The word purine was coined by the German chemistEmil Fischer in 1884. He synthesized it for the first time in 1898. The starting material for the reaction sequence was uric acid, which had been isolated from kidney stones by Carl Wilhelm 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.
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 of purine 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.
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. Similar results have been found with the risk of hyperuricemia.
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 naturally occurring purines. For example, five molecules of HCN condense in an exothermic reaction to make adenine, especially in the presence of ammonia. The Traube purine synthesis is a classic reaction between an amine-substituted pyrimidine and formic acid.
