Abstract
In vivo labeling experiments of Forsythia intermedia plant tissue with [8-14C]- and [9,9-2H2,OC2H3]coniferyl alcohols revealed that the lignans, (-)-secoisolariciresinol and (-)-matairesinol, were derived from two coniferyl alcohol molecules; no evidence for the formation of the corresponding (+)-enantiomers was found. Administration of (+-)-[Ar-3H]secoisolariciresinols to excised shoots of F. intermedia resulted in a significant conversion into (-)-matairesinol; again, the (+)-antipode was not detected. Experiments using cell-free extracts of F. intermedia confirmed and extended these findings. In the presence of NAD(P)H and H2O2, the cell-free extracts catalyzed the formation of (-)-secoisolariciresinol, with either [8-14C]- or [9,9-2H2,OC2H3]coniferyl alcohols as substrates. The (+)-enantiomer was not formed. Finally, when either (-)-[Ar-3H] or (+-)-[Ar-2H]secoisolariciresinols were used as substrates, in the presence of NAD(P), only (-)- and not (+)-matairesinol formation occurred. The other antipode, (+)-secoisolariciresinol, did not serve as a substrate for the formation of either (+)- or (-)-matairesinol. Thus, in F. intermedia, the formation of the lignan, (-)-secoisolariciresinol, occurs under strict stereochemical control, in a reaction or reactions requiring NAD(P)H and H2O2 as cofactors. This stereoselectivity is retained in the subsequent conversion into (-)-matairesinol, since (+)-secoisolariciresinol is not a substrate. These are the first two enzymes to be discovered in lignan formation.
Highlights
In vivo labeling experiments of Forsythia intermedia plant tissue with [8-_4C]- and [9,9-"H.,OC"H:dconiferyl alcohols revealed that the lignans, (-)-secoisolariciresinol and (-)-matairesinol, two coniferyl alcohol molecules; were derived from no evidence for the formation of the corresponding
Administration of (-+)-[Ar-:_H]secoisolariciresi nols to excised shoots of F. intermedia resulted in a significant conversion into (-)-matairesinol; again, the
With a method to rapidly determine chirality, we examined F. intermedia plant extracts to establish the optical purity of the secoisolariciresinol
Summary
The first goal of our research was to identify the key enzymatic reaction affording entry into the specialized biosynthetic pathway to the Forsythia lignans. Antipodes due to the addition of unlabeled HPLC analysis; the large preponderance carrier for chiral of the (-)-form reflects the amount of naturally occurring 2b already present in F. intermedia tissue.). Chiralcel OD (Daicel) elution details: 1% AcOH in hexanes:EtOH (85:15); flow rate: 1 ml rain _ These experiments did not, prove that coniferyl alcohol 8 had been incorporated intactly into either lignan; enzymatic conversion of this alcohol to the acid or aldehyde could have occurred prior to coupling. If intact incorporation of coniferyl alcohol 8 occurred, the (-)-secoisolariciresinol lb and (-)-matairesinol 2b formed de novo would contain 10 and 8 deuterium atoms, respectively This could be proven by mass spectrometry. D, deuterated (-)-matairesinol obtained following incubation of (_+)-[Ar-2H]secoisolariciresinols with cell-free extracts of F. intermedia in the presence of NADP. Absolute incorporation of radioactivity into (-) -matairesino"l 2b (-)-Matairesinol 2b formation
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