Catabolism of caffeine in plants and microorganisms.

Caffeine has been found in tissues of several plants. Because of its stimulating effect on the central nervous system, a great number of reports have been published on its content in beverages and foodstuffs. However, a much more restricted number of reports have dealt specifically with caffeine metabolism in plants. This review presents, in chronological manner, the contribution of these reports to the vast knowledge accumulated on caffeine catabolism in plants and microorganisms over the last 40 years. In plants, the accumulated data indicate the operation of a main catabolic pathway: caffeine --> theophylline --> 3-methylxantine --> xanthine --> uric acid --> allantoin --> allantoic acid --> glyoxylic acid + urea --> NH3 + CO2. Some studies have shown that, depending on the plant species, other minor routes may operate with the formation of theobromine and 7-methylxantine, which are salvaged for caffeine formation since they also appear in the biosynthetic pathway. A specific group of coffee known as liberio-excelsioides has the ability to convert caffeine to the corresponding methyluric acid, which is methylated to other uric acid derivatives. In bacteria caffeine is either degraded to theobromine or paraxanthine. Both dimethylxanthines are demethylated to 7-methylxantine which in turn is demethylated to xanthine and then enters the catabolic pathway of purines. In bacteria, theobromine, paraxanthine and 7-methylxantine may also be oxidized to their corresponding methyluric acids.