METABOLISM OF PURINE NUCLEOSIDES AND BASES IN SUSPENSION-CULTURED ARABIDOPSIS THALIANA CELLS
We study the metabolism of purine nucleosides and purine bases in suspension-cultured cells of the model plant Arabidopsis thaliana. [8-14C]Adenosine, [8-14C]guanosine, [8-14C]inosine, [8-14C]xanthosine, [8-14C]adenine, [8-14C]guanine, [8-14C]hypoxanthine and [8-14C]xanthine were administered to cells in the cell division phase, and the uptake and metabolic fate of these compounds were monitored for 4h. The rates of uptake of most purines were within the range 60-70 nmol/gFW. Xanthine and xanthosine were taken up more slowly. The rate of uptake was ordered as hypoxanthine > adenine > inosine > guanosine > guanine > adenosine > xanthosine> xanthine. A large amount of radioactivity from [8-14C]adenosine, [8-14C]guanosine, [8-14C]adenine and [8-14C]guanine, and a limited amount from [8-14C]inosine and [8-14C]hypoxanthine, was incorporated into nucleotides and RNA. The so-called purine salvage pathways of adenosine, guanosine, adenine, guanine, inosine and hypoxanthine are therefore functional in A. thaliana. These tracer experiments also reveal that significant amounts of these compounds were converted to xanthine, and enter the catabolic pathway via allantoin. Neither xanthosine nor xanthine is used in the synthesis of nucleotides and RNA. These compounds are entirely catabolized via allantoin and allantoic acid. Deamination of adenine and guanine rings takes place at the stage of AMP deaminase and guanosine deaminase, respectively. The pattern of purine metabolism in A. thaliana is similar to that in other plants. Adenine salvage activity estimated from the metabolism of [8-14C]adenine, the cellular concentration of ATP, and expression of the APT1 gene encoding adenine phosphoribosyltransferase all increased markedly at the lag phase of cell proliferation. These observations imply that the salvage pathway is important during the early stages of cell culture in A. thaliana.
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