CAFFEINE BIOSYNTHESIS IN YOUNG LEAVES OF CAMELLIA-SINENSIS - IN-VITROSTUDIES ON N-METHYLTRANSFERASE ACTIVITY INVOLVED IN THE CONVERSION OFXANTHOSINE TO CAFFEINE

The aim of this study was to investigate the S-adenosylmethionine depe ndent N-methyltransferase(s) (NMT) associated with the three methylati on steps in the caffeine biosynthesis pathway in tea (Camellia sinensi s L.). NMT activity in cell-free preparations from young leaves was pu rified by anion-exchange and gel-filtration column chromatography. In both systems, a single zone of NMT activity, with broad substrate spec ificity was detected. The N-3 position of dimethylxanthine and monomet hylxanthines was methylated more readily than N-1 while comparatively little substitution occurred at the N-7 locus. When xanthosine was use d as a substrate only the N-7 position was methylated These results in dicate that a single NMT may participate in the conversion of xanthosi ne to caffeine. The apparent M(r) of the NMT, estimated by gel filtrat ion chromatography, was 61 000. The substrate specificity of the NMT i s compatible with the operation of a xanthosine --> 7-methylxanthosine --> 7-methylxanthine --> theobromine --> caffeine pathway as the main biosynthetic route to caffeine in young tea leaves. The data also ind icate that the conversion of 7-methylxanthine --> paraxanthine --> caf feine may function as one of a number of minor pathways that also cont ribute to the production of caffeine.