Xanthine

Overview
Xanthine (3,7-dihydro-purine-2,6-dione), is a purine base found in most body tissues and fluids and in other organisms.

Reactions
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.

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

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. On the contrary, they really only eliminate the actions of adenosine to some extent, adenosine causing sleepiness, thus they are by far less in effectiveness as stimulants than sympathomimetic amines. Their effects, however, are widespread and their therapeutic range is narrow, so they are not the drug of choice in asthma treatment. Therapeutic level is 10-20 micrograms/mL blood. Signs of toxicity include tremor, nausea, nervousness, and tachycardia/arrhythmia.

Methylated xanthine derivatives include caffeine, paraxanthine, theophylline, and theobromine (found mainly in chocolate). These drugs inhibit phosphodiesterase and antagonise adenosine. Xanthines are also found very rarely as constituents of nucleic acids.