Xanthine: A substance found in caffeine, theobromine, and theophylline and encountered in tea, coffee, and the colas. Chemically, xanthine is a purine.There is a genetic disease of xanthine metabolism, xanthinuria, due to deficiency of an enzyme, xanthine dehydrogenase, needed to process xanthine in the body.reatment is by avoiding foods and drinks containing xanthine derivatives, such as coffee, tea, and colas. The name xanthine was taken from the Greek xanqos meaning yellow + quinine since it forms yellow salts and its solution forms a blue fluorescence as does quinine.