Matairesinol is a central precursor in planta in the biosynthesis of numerous
lignans, including that of the important
antiviral and
anticancer agent,
podophyllotoxin. In this study, the approximately 32-kDa
NAD-dependent
secoisolariciresinol dehydrogenase, which catalyzes the enantiospecific conversion 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
recombinant protein, produced by expression of this
cDNA in Escherichia coli, catalyzed the same enantiospecific conversion via the corresponding lactol intermediate. A homologous
secoisolariciresinol dehydrogenase gene was also isolated from a Podophyllum peltatum rhizome cDNA library, whose 834-base pair
cDNA clone encoded a 278-amino
acid protein with a calculated molecular mass of approximately 32 kDa. Expression of this
protein in E. coli produced a fully functional
recombinant protein that also catalyzed the enantiospecific conversion of (-)-
secoisolariciresinol into (-)-
matairesinol via the intermediary lactol. Various kinetic parameters were defined and established conversion of the intermediary lactol as being rate-limiting. With this overall enzymatic conversion now unambiguously defined, the entire biochemical pathway to the
lignans,
secoisolariciresinol and
matairesinol, has been elucidated. Last, both
secoisolariciresinol and
matairesinol are metabolized in the gut of mammals, following digestion of high
fiber dietary grains, seeds, and berries, into the so-called "mammalian"
lignans,
enterodiol and
enterolactone, respectively; these in turn confer significant protection against the onset of breast and
prostate cancers.