The biosynthesis of 9-[5′-deoxy-5′-(methylthio)-beta-D-xylofuranosyl]adenine (xylosyl-MTA), a naturally occurring analogue of 5′-deoxy-5′-methylthioadenosine (MTA) recently characterized, was studied in the nudibranch mollusc Doris verrucosa. Experiments performed in vivo with putative labelled precursors such as [8-14C]adenine, [Me-14C]methionine and [Me-14C]MTA indicate that xylosyl-MTA originates from MTA. Experiments with MTA double-labelled at critical positions are consistent with a 3′-isomerization of the nucleoside through the formation of a 3′-oxo intermediate. In addition, experiments with the newly synthesized [3′-3H]xylosyl-MTA are indicative for a very low turnover rate of this molecule, which therefore accumulates in the mollusc.

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