Biosynthesis of Caffeine from Purine Nucleotides in Tea Plant

The radioactivity of (14)C-labeled adenosine, inosine, and guanosine fed to excised tea shoots was incorporated into caffeine at a high rate (about 50% within 24 hr), after the rapid labeling of free nucleotides, while the radioactivity of xanthosine was directly incorporated into 7-methylxanthosine, and then into 7-methylxanthine, theobromine, and caffeine. Cell-free extracts from tea leaves also phosphorylated four purine nucleosides into the respective nucleotides. The phosphorylation rate was adenosine»inosine≧guanosine» xanthosine. A small amount of enzyme activity involved in the reaction from guanosine to xanthosine was detected, but the reaction from inosine to xanthosine was not observed. Based on these results, it is considered that the pathway leading to the formation of xanthosine from adenine nucleotides in caffeine biosynthesis is via AMP→IMP→XMP→xanthosine in tea plants, while there is a possibility that guanine nucleotides are converted to xanthosine via guanosine.