Profiles of Purine Metabolism in Leaves and Roots of Camellia sinensis Seedlings

To determine the metabolic profiles of purine nucleotides and related compounds in leaves and roots of tea (Camellia sinensis), we studied the in situ metabolic fate of 10 different C-14-labeled precursors in segments from tea seedlings. The activities of key enzymes in tea leaf extracts were also investigated. The rates of uptake of purine precursors were greater in leaf segments than in root segments. Adenine and adenosine were taken up more rapidly than other purine bases and nucleosides. Xanthosine was slowest. Some adenosine, guanosine and inosine was converted to nucleotides by adenosine kinase and inosine/guanosine kinase, but these compounds were easily hydrolyzed, and adenine, guanine and hypoxanthine were generated. These purine bases were salvaged by adenine phosphoribosyltransferase and hypoxanthine/guanine phosphoribosyltransferase. Salvage activity of adenine and adenosine was high, and they were converted exclusively to nucleotides. Inosine and hypoxanthine were salvaged to a lesser extent. In situ C-14-tracer experiments revealed that xanthosine and xanthine were not salvaged, although xanthine phosphoribosyltransferase activity was found in tea extracts. Only some deoxyadenosine and deoxyguanosine was salvaged and utilized for DNA synthesis. However, most of these deoxynucleosides were hydrolyzed to adenine and guanine and then utilized for RNA synthesis. Purine alkaloid biosynthesis in leaves is much greater than in roots. In situ experiments indicate that adenosine, adenine, guanosine, guanine and inosine are better precursors than xanthosine, which is a direct precursor of a major pathway of caffeine biosynthesis. Based on these results, possible routes of purine metabolism are discussed.