Pinoresinol/Lariciresinol Reductases and Secoisolariciresinol Dehydrogenases Involved in the Specific Production of (+)-Enantiomer of Matairesinol in <i>Daphne genkwa</i>

Lignans are phenylpropanoid dimers in which the monomers are linked by the central carbon atoms of their propyl side chains. In addition to the biosynthesis of biologically active lignans, the unique stereochemical mechanisms of biosynthetic reactions have been a long-standing interest. The enantiomeric composition of pinoresinol and lariciresinol-the two lignans farthest upstream in the lignan biosynthetic pathway-varies greatly among the plant species; these compounds are not enantiomerically pure and are mixtures of both enantiomers. The enantiomeric composition of the next intermediate, secoisolariciresinol, also varies by plant species, with some being enantiomerically pure. Conversely, the next compound, matairesinol, is always enantiomerically pure. In most cases, it is levorotatory, while the opposite dextrorotatory enantiomer, (+)-matairesinol, has been obtained from Thymelaeaceae plants, including Daphne genkwa. The conversion of pinoresinol to secoisolariciresinol via lariciresinol is catalyzed by pinoresinol/lariciresinol reductases (PLRs). Secoisolariciresinol is then oxidized by secoisolariciresinol dehydrogenases (SIRDs) to afford matairesinol. Many PLRs from various plant species preferentially reduce (+)-pinoresinol to (-)-secoisolariciresinol, while others preferentially reduce (-)-pinoresinol to (+)-secoisolariciresinol. However, the specific formation and accumulation of (+)-matairesinol in Thymelaeaceae plants remain unknown. This study demonstrated that DgPLRs selectively reduce (-)-lariciresinol but not (+)-lariciresinol, resulting in the formation of (+)-secoisolariciresinol. Furthermore, DgSIRDs were found to selectively oxidize (+)-secoisolariciresinol over (-)-secoisolariciresinol, yielding (+)-matairesinol. Thus, the selectivity of DgPLRs for lariciresinol enantiomers rather than pinoresinol enantiomers and the selectivity of DgSIRDs for secoisolariciresinol enantiomers are responsible for the accumulation of nearly enantiomerically pure (+)-secoisolariciresinol and enantiomerically pure (+)-matairesinol in D. genkwa.