Abstract
In a study of purine alkaloid catabolism pathways in coffee, C-14-labelled theobromine, caffeine, theophylline and xanthine were incubated with leaves of C. arabica. Incorporation of label into (CO2)-C-14 was determined and methanol-soluble metabolites were analysed by HPLC-RC. The data obtained demonstrate catabolism of caffeine -> theophylline -> 3-methylxanthine -> xanthine. Xanthine is degraded further by the conventional purine catabolism pathway to CO2 and NH3 via uric acid, allantoin and allantoic acid. The conversion of caffeine to theophylline is the rate limiting step in purine alkaloid catabolism and provides a ready explanation the high concentrations of endogenous caffeine that are found in C. arabica leaves. Although theobromine is converted primarily to caffeine, a small portion of the theobromine pool appears to be degraded to xanthine by a caffeine-independent pathway. In addition to being broken down to CO2, via the purine catabolism pathway, xanthine is metabolised to 7-methylxanthine. Metabolism of [2-C-14]xanthine by C. arabica leaves in the presence of 5 mM allopurinol results in very large increases in incorporation of radioactivity into 7-methylxanthine as degradation of the substrate via the purine catabolism pathway is blocked. The identity of 7-methylxanthine in these studies was confirmed by GC-MS.