Biosynthesis of (+)-syringaresinol inLiriodendron tulipifera I: feeding experiments withl-[U-14C]phenylalanine and [8-14C]sinapyl alcohol

To clarify the biosynthesis of syringyl lignans and lignan formation by stereoselective coupling of monolignols, formation of (+)-syringaresinol and (+)-pinoresinol inLiriodendron tulipifera were investigated by means of feeding experiments. Following individual administration ofl-[U-14C]phenylalanine and [8-14C]sinapyl alcohol to excised shoots ofL. tulipifera and their subsequent metabolism for 3h, free [14C] lignans and [14C] lignan glucosides were extracted from both of the stems and leaves with methanol and divided into an ether fraction and an aqueous one, respectively. The glucosides were hydrolyzed by a combination of cellulase andΒ-glucosidase to liberate [14C]lignans as aglycones.l-[U-14C]Phenylalanine was incorporated into free (+)-[14C]syringaresinol and its glucosides; the (+)-[14C]syringaresinols in the stems and leaves had 52% enantiomeric excess (% e.e.) and 42% e.e., respectively; and the (+)-[14C]syringaresinol aglycones from the glucosides in the stems and leaves had 20% e.e. and 22% e.e., respectively. Furthermore, [8-14C]sinapyl alcohol was incorporated into (+)-[14C]syringaresinol and its glucosides in the stems. These results suggest that the (+)-enantiomer of syringaresinol was enantioselectively formed from two molecules of sinapyl alcohol inL. tulipifera followed by transformation into the (+)-syringaresinol glucosides, accompanying the formation of racemic syringaresinol by nonselective coupling and the subsequent transformation of the racemate into their glucosides.l-[U-14C]Phenylalanine was incorporated also into free (+)-[14C]pinoresinol and its glucosides with 12%–42% e.e.