Secoisolariciresinol dehydrogenase purification, cloning, and functional expression - Implications for human health protection

Matairesinol is a central precursor in planta in the biosynthesis of numero us lignans, including that of the important antiviral and anticancer agent, podophyllotoxin, In this study, the similar to 32-kDa NAD-dependent secois olariciresinol dehydrogenase, which catalyzes the enantiospecific conversio n of (-)-secoisolariciresinol into (-)-matairesinol in Forsythia intermedia , was purified >6,000-fold to apparent homogeneity, The 831-base pair cDNA clone encoding this 277-amino acid protein was next obtained from a library constructed from F intermedia stem tissue, whose fully functional recombin ant protein, produced by expression of this cDNA in Escherichia coil, catal yzed the same enantiospecific conversion via the corresponding lactol inter mediate. A homologous secoisolariciresinol dehydrogenase gene was also isol ated from a Podophyllum peltatum rhizome cDNA library, whose 834-base pair cDNA clone encoded a 278-amino acid protein with a calculated molecular mas s of similar to 32 kDa, Expression of this protein in E, coil produced a fu lly functional recombinant protein that also catalyzed the enantiospecific conversion of (-)-secoisolariciresinol into (-)-matairesinol via the interm ediary lactol, Various kinetic parameters were defined and established conv ersion of the intermediary lactol as being rate-limiting. With this overall enzymatic conversion now unambiguously defined, the entire biochemical pat hway to the lignans, secoisolariciresinol and matairesinol, has been elucid ated. Last, both secoisolariciresinol and matairesinol are metabolized in t he gut of mammals, following digestion of high fiber dietary grains, seeds, and berries, into the so-called "mammalian" lignans, enterodiol and entero lactone, respectively; these in turn confer significant protection against the onset of breast and prostate cancers.