What is hypoxanthine?
Hypoxanthine is a purine derivative produced by the degradation of the purine bases adenine and guanine. Besides xanthine, it is an intermediate in the synthesis of uric acid. Under the influence of xanthine oxidase hypoxanthine is usually degraded first in xanthine and then in uric acid.
Like all purine derivatives, it consists of two heterocyclic rings containing six or five atoms. The rings contain a total of nine atoms. These are five carbon atoms and four nitrogen atoms. Two carbon atoms belong to both rings. At the carbon atom of position 6, a hydroxyl group is bonded. By stabilizing effects, the molecule may exist in several tautomeric forms that are in equilibrium with each other. Hypoxanthine consists of solid transparent crystals that melt at 250 degrees. It does not dissolve in cold water or alcohol. However, it is easily soluble in hot water, acids or alkalis.
Function, effect & tasks
Hypoxanthine, as mentioned, is an intermediate in the degradation of purine bases. The enzyme xanthine oxidase oxidizes it to xanthine. Together with xanthine, it is then broken down into uric acid by xanthine oxidase. The difference between hypoxanthine and xanthine is that xanthine additionally has one hydroxyl group attached to position 2.
Furthermore, hypoxanthine can be degraded to uric acid as well as fed back into the purine metabolism via the salvage pathway. With xanthine, on the other hand, only degradation to uric acid takes place. Hypoxanthine forms with ribose the nucleoside inosine. Inosine is very rarely incorporated into the anticodon of tRNA. It is used in the production of degenerate primers that initiate a polymerase chain reaction. It is a neutral base that can mate with all nucleobases. However, the combination with cytosine is energetically most favorable.
Another important compound derived from hypoxanthine is inosine monophosphate. This compound is a phosphoric acid ester of inosine. Inosine monophosphate (IMP) is a key intermediate for the synthesis of guanosine monophosphate (GMP) and adenosine monophosphate (AMP), both of which can be used for nucleic acid synthesis. The synthesis of IMP takes place from hypoxanthine directly via the salvage pathway. The two enzymes AICAR-formyltransferase / IMP-cyclase and hypoxanthine-guanine-phosphoribosyltransferase are responsible for this. Thus, hypoxanthine is at the interface between degradation of purine bases to uric acid and buildup of nucleic acids. Inosine monophosphate is also used as a flavor enhancer.
Education, occurrence, properties & optimal values
Hypoxanthine forms as an intermediate in the purine metabolism and is at the threshold between degradation and rebuilding of the purine bases. When oxidized to xanthine by the enzyme xanthine oxidase, the reverse reaction to the nucleic bases adenine and guanine is no longer possible.
Hypoxanthine is formed from the purine base adenine, while the degradation of guanine leads to xanthine. However, the reactions of different nucleosides and nucleotides are interconnected through a complicated network. Thus, adenosine nucleotides lead directly to hypoxanthine, with the key substance being AMP. However, GMP can also be converted to AMP via IMP and adenylosuccinate. Among other things, AMP leads to the formation of adenosine and inosine to hypoxanthine. In addition to guanine and adenine, hypoxanthine can then restore nucleotides as nucleic acid components via the salvage pathway.
Diseases & Disorders
There may be multiple disorders associated with hypoxanthine. When purine decomposition arise equivalent hypoxanthine and xanthine. Hypoxanthine is transformed into xanthine by xanthine oxidase. The same enzyme then degrades xanthine to uric acid.
However, when xanthine oxidase is absent, xanthine and hypoxanthine accumulate in the blood. The uric acid values are very low. However, the concentration of xanthine increases mainly because hypoxanthine has the opportunity to be recycled via the salvage pathway. It develops the clinical picture of Xanthinurie. The excretion of xanthine in urine can increase by 1500 percent. But the levels of hypoxanthine do not increase that much. The high levels of xanthine can damage the kidneys. With low hydration, kidney stones or stones may form in the urinary tract. The excretion of urinary crystals is possible.
In very severe cases it can lead to a fatal kidney failure. However, since xanthine and hypoxanthine have some water solubility, the best therapy is to drink a lot. Purine-rich foods such as fish, clams, legumes or beer should be avoided. However, there are also more severe forms of xanthineuria. Thus, besides severe kidney diseases, delayed mental development, autism or even tooth development disorders can occur. Since hypoxanthine can also be recycled via the salvage pathway as opposed to xanthine, the disorders within this process lead to increased uric acid formation, because only the pathway of purine bases works.
The resulting hypoxanthine can be oxidized only in xanthine, which in turn is converted into uric acid. Often there is a hereditary defect of the enzyme hypoxanthine-guanine phosphoribosyltransferase. The uric acid concentration in the blood increases sharply, which can lead to precipitation of uric acid crystals in the joints. The result is gout attacks. In severe cases, Lesch-Nyham syndrome develops.