Enzymology and Stereochemistry in Lignan Biosynthesis

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The biosynthesis of lignans is not well-known. The value of specific rotation of a particular lignan may vary with the plant species. The biosynthetic difference between lignans and lignins is unclear. Absolute configuration of furofuran lignans [e.g. (-) -pinoresinol] in Zanthoxylum ailanthoides is (8S,8'S). There is no biosynthetic study on the (8S,8'S) lignans. Interestingly, the configuration of (-) -secoisolariciresinol in the plant is opposite, (8R,8'R), although it must be derived from unnatural (+) - (8R,8'R) -pinoresinol. To investigate biosynthesis and stereochemistry in Zanthoxylum lignans, feeding experiments and crude enzyme reactions were performed using shoots of Z.ailanthoides and Z.schinifolium. following results were obtained.(1) Lignans and [8-^<14>C] coniferyl alcohol (CA) were prepared and used as authentic compounds and substrates. (2) Analytical conditions by HPLC were established. (3) Administration of [8-^<14>C] CA to excised shoots of the plants for 3 h result … More ed in conversion into pinoresinol, not into epipinoresinol, sesamin or asarinin in this experimental condition. Incorporation into lariciresinol and secoisolariciresinol is under study. (4) Incubation of (<plus-minus>) -pinoresinol with cell-free extracts of the plants in the presence of NADPH showed formation of (+) -lariciresinol. Incubation of [8-^<14>C] CA with the same extracts in the presence of NADPH and H_2O_2 showed formation of (+) -larisiresinol and (+) and (-) -pinoresinol with the small depletion of (+) -form. The amount of the depletion is similar to that of the (+) -lariciresinol. It was concluded that CA was transformed to (<plus-minus>) -pinoresinol, (+) -form of which was reduced to (+) -lariciresinol. This configuration is the same as that of (-) -secoisolariciresinol. But (-) -secoisolariciresinol formation have not found yet. Residual pinoresinol might have the natural (-) -sign. (5) Incubation of [8-^<14>C] CA with the insoluble residue from the same stems in the absence of cofactors gave (<plus-minus>) -pinoresinol. This formation might be catalyzed by a cell wall bound laccase. When NAD and malate were supplied, the ratio of (+) and (-) forms was 46.5 : 53.5. Further study is necessary to know whether direct formation of (-) -pinoresinol occurred by the insoluble residue. Less