Metabolism of caffeine and related purine alkaloids in leaves of tea (Camellia sinensis L)

Purine alkaloid catabolism pathways in young, mature and aged leaves of tea (Camellia sinensis L.) were investigated by incubating leaf sections with C-14-labelled theobromine, caffeine, theophylline and xanthine. Incorporation of label into CO2 was determined and methanol-soluble metabolites were analysed by high-performance liquid chromatography-radiocounting and thin layer chromatography. The data obtained demonstrate that theobromine is the immediate precursor of caffeine, which accumulates in tea leaves because its conversion to theophylline is the rate limiting step in the purine alkaloid catabolism pathway. The main fate of [8-C-14]theophylline incubated with mature and aged leaves, and to a lesser extent young leaves, is conversion to 3-methylxanthine and onto xanthine which is degraded to (CO2)-C-14 via the purine catabolism pathway. However, with young leaves, sizable amounts of [8-C-14]theophylline were salvaged for the synthesis of caffeine via a 3-methylxanthine --> theobromine --> caffeine pathway. Trace amounts of [2-C-14]xanthine were also salvaged for caffeine biosynthesis in young leaves, by conversion to 3-methylxanthine, and this was enhanced in the presence of 5 mM allopurinol which inhibits purine catabolism, Feeds of [2-C-14]xanthine to young leaves also indicated that 3-methylxanthine, as well as being salvaged for theobromine and caffeine production, is also converted, via N-1-methylation, to theophylline.