Xanthine

Xanthine ( or ; archaically xanthic acid) (3,7-dihydro-purine-2,6-dione), is a purine base found in most human body tissues and fluids and in other organisms. A number of stimulants are derived from xanthine, including caffeine and theobromine.

Xanthine is a product on the pathway of purine degradation.
 * It is created from guanine by guanine deaminase.
 * It is created from hypoxanthine by xanthine oxidoreductase.
 * It is also created from xanthosine by purine nucleoside phosphorylase(PNP).

Xanthine is subsequently converted to uric acid by the action of the xanthine oxidase enzyme.

Studies reported in 2008, based on 12C/13C isotopic ratios of organic compounds found in the Murchison meteorite, suggested that xanthine and related chemicals, including the RNA component uracil, were formed extraterrestrially. In August 2011, a report, based on NASA studies with meteorites found on Earth, was published suggesting xanthine and related organic molecules, including the DNA and RNA components adenine and guanine, were made in outer space.

Pathology
People with the rare genetic disorder xanthinuria lack sufficient xanthine oxidase and cannot convert xanthine to uric acid.

Clinical significance of xanthine derivatives
Derivatives of xanthine, known collectively as xanthines, are a group of alkaloids commonly used for their effects as mild stimulants and as bronchodilators, notably in treating the symptoms of asthma. In contrast to other, more potent stimulants, their main effect consists mainly of opposing the actions of the sleepiness-inducing adenosine, making them somewhat less effective as stimulants than sympathomimetic amines. Due to widespread effects, the therapeutic range of xanthines is narrow, making them merely a second-line asthma treatment. The therapeutic level is 10-20 micrograms/mL blood; signs of toxicity include tremor, nausea, nervousness, and tachycardia/arrhythmia.

Methylxanthines affect not only the airways but stimulate heart rate, force of contraction, cardiac arrhythmias at high concentrations. In the CNS they increase alertness, stimulate the respiratory centre, and are used for treatment of infantile apnea. In high doses they can lead to convulsions that are resistant to anticonvulsants. Methylxanthines induce acid and pepsin secretions in the GI tract. Methylxanthines are metabolized by Cytochrome P450 in the liver. Methylated xanthines include caffeine, aminophylline, IBMX, paraxanthine, pentoxifylline, theobromine, and theophylline. These drugs act as both But different analogues show varying potency at the numerous subtypes, and a wide range of synthetic xanthines (some nonmethylated) have been developed searching for compounds with greater selectivity for phosphodiesterase enzyme or adenosine receptor subtypes. Xanthines are also found very rarely as constituents of nucleic acids.
 * 1) competitive nonselective phosphodiesterase inhibitors which raise intracellular cAMP, activate PKA, inhibit TNF-alpha and leukotriene synthesis, and reduce inflammation and innate immunity and
 * 2) nonselective adenosine receptor antagonists which inhibit sleepiness-inducing adenosine.