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A study was undertaken to identify the pathway(s) leading to the synthesis of caffeine and theobromine in leaves of Coffea arabica. The relative contribution of purine nucleosidcs and bases to the biosynthesis of these alkaloids was assessed by measuring the incorporation of radiolabeled inosine, adenosine, adenine, hypoxanthine, and xanthine into caffeine and theobromine.The results of this investigation suggest that caffeine and theobromine are end products of two distinctively different pathways. The incorporation of radiolabeled formate, adenosine, and xanthine was significantly greater into caffeine than into theobromine. Furthermore, exogenously supplied theobromine did not dilute the incorporation of [14C]formate, [14C]inosine, or [14C]xanthine into caffeine. In contrast, radiolabeled adenine was incorporated into theobromine but not into caffeine, and exogenously supplied adenine diluted the incorporation of [14 C]adenosine into theobromine, but not into caffeine.Taken together, these results provide strong evidence that theobromine is not a precursor of caffeine biosynthesis in leaves of C. arabica.Supported by the Citrus Research Center and Agricultural Experimental Station of the University of California, Riverside,