Biosynthesis of (+)-syringaresinol in Liriodendron tulipifera I: feeding experiments with L-[U-C-14]phenylalanine and [8-C-14]sinapyl alcohol

To clarify the biosynthesis of syringyl lignans and lignan formation by ste reoselective coupling of monolignols, formation of (+)-syringaresinol and ( +)-pinoresinol in Liriodendron tulipifera were investigated by means of fee ding experiments. Following individual administration of L-[U-C-14]phenylal anine and [8-C-14]sinapyl alcohol to excised shoots of L. tulipifera and th eir subsequent metabolism for 3 h, free [C-14]lignans and [C-14]lignan gluc osides were extracted from both of the stems and leaves with methanol and d ivided into an ether fraction and an aqueous one, respectively. The glucosi des were hydrolyzed by a combination of cellulase and beta -glucosidase to liberate [C-14]lignans as aglycones. r-[U-C-14]Phenylalanine was incorporat ed into free (+)-[C-14]syringaresinol and its glucosides; the (+)-[C-14]syr ingaresinols in the stems and leaves had 52% enantiomeric excess (% e.e.) a nd 42% e.e., respectively: and the (+)-[C-14]syringaresinol aglycones from the glucosides in the stems and leaves had 20% e.e. and 22% e.e., respectiv ely. Furthermore, [8-C-14]sinapyl alcohol was incorporated into (+)-[C-14]s yringaresinol and its glucosides in the stems. These results suggest that t he (+)-enantiomer of syringaresinol was enantioselectively formed from two molecules of sinapyl alcohol in L. tulipifera followed by transformation in to the (+)-syringaresinol glucosides, accompanying the formation of racemic syringaresinol by nonselective coupling and the subsequent transformation of the racemate into their glucosides. L-[U-C-14]Phenylalanine was incorpor ated also into free (+)-[C-14]pinoresinol and its glucosides with 12%-42% e .e.