CATABOLISM OF CAFFEINE AND RELATED PURINE ALKALOIDS IN LEAVES OF COFFEA-ARABICA L

In a study of purine alkaloid catabolism pathways in coffee, C-14-labe lled theobromine, caffeine, theophylline and xanthine were incubated w ith leaves of Coffea arabica. Incorporation of label into (CO2)-C-14 w as determined and methanol-soluble metabolites were analysed by high-p erformance liquid chromatography-radiocounting. The data obtained demo nstrate catabolism of caffeine --> theophylline --> 3-methylxanthine - -> xanthine. Xanthine is degraded further by the conventional purine c atabolism pathway to CO2 and NH3 via uric acid, allantoin and allantoi c acid. The conversion of caffeine to theophylline is the rate-limitin g step in purine alkaloid catabolism and provides a ready explanation for the high concentration of endogenous caffeine found in C. arabica leaves. Although theobromine is converted primarily to caffeine, a sma ll 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 i ncorporation of radioactivity into 7-methylxanthine as degradation of the substrate via the purine catabolism pathway is blocked. The identi ty of 7-methylxanthine in these studies was confirmed by gas chromatog raphy-mass spectrometry analysis.