A purine is a type of chemical that is found in DNA and RNA. It is also found in energy-containing molecules such as ATP, and in some signaling molecules. Purine metabolism refers to the chemical reactions purines undergo in the body. Defects in purine metabolism can lead to disorders.
Digestion and Absorption
Some amount of purines are found in most foods because they help to form DNA, which is found in most organisms. However, certain foods, such as meat, contain more. Purines are ingested in the diet into the gastrointestinal tract. There, the large molecules in the food, such as DNA, are broken down into their component parts. DNA is composed of a sugar, phosphate, pyrimidine and purine backbone. These chemicals are absorbed across the wall of the intestine and into the bloodstream.
Anabolism
Anabolism refers to building up molecules from smaller chemical components. In regards to purines, this can mean building purines and using purines to build larger chemical structures. The purines adenine and gaunine are synthesized from the purine inosine monophosphate, or IMP. This is synthesized from many reagents, including the amino acids glycine, glutamine and aspartate as well as carbon atoms from formyl groups and bicarbonate. The purines are added to a sugar called ribose and phosphate. The purine, ribose and phosphate together are then added to DNA, RNA or energy-containing compounds.
Catabolism
Catabolism is the breaking down of chemicals. First, the sugar and phosphate are removed from the purine. Then, through several reactions, they are turned into the chemical xanthine, then uric acid, which is excreted by the kidneys. During catabolism, enzymes called salvage enzymes can convert some of the chemicals back into purines, if needed.
Disorders
In gout, uric acid builds up in the blood and deposits in the joints, causing a very painful arthritis. This occurs through two general mechanisms. The first is the overproduction of uric acid. For example, if a large mass of rapidly dividing tumor cells containing a large amount of DNA receives a dose of chemotherapy or radiation, the cells will break apart and release the DNA. It will be catabolized to uric acid. The second mechanism is damage to the kidneys so the uric acid is not excreted. Other causes may be genetic -- for example, changes to the enzymes breaking down purines. Other problems to the purine metabolizing enzymes can cause severe disease. A deficiency of the salvage enzyme HGPRT causes Lesch-Nyhan syndrome, which is characterized by developmental delay and self-mutilation.
References
- "Robbins and Cotran Pathologic Basis of Disease"; Vinay Kumar, et al.; 8th Ed 2009
- "Molecular Biology of the Cell"; Bruce Alberts et al.; 4th Ed 2002
- "Harrison's Principles of Internal Medicine"; Anthony S Fauci, et al.; 17th Ed 2008
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