BIOSYNTHESIS OF LIGNANS IN FORSYTHIA-INTERMEDIA

Feeding experiments with young shoots of Forsythia intermedia have shown that phenylalanine and ferulic acid are good precursors of the dibenzylbutyrolactone lignan arctigenin and the furofuran lignans phillygenin and epipinoresinol. Although matairesinol was incorporated into arctigenin, and epipinoresinol into phillygenin, in accord with late methylation steps in the biosynthetic pathway, some incorporation of label from one lignan class into the other was observed, particularly from epipinoresinol into arctigenin. This may indicate a high degree of catabolic activity followed by non-specific incorporation. [2′-14C,1′-3H]Coniferyl alcohol was incorporated into all three lignans with a significant increase in the 1H: 14C isotopic ratio. This supports the hypothesis that the lignans arise by oxidative coupling of two coniferyl alcohol units, rather than from two ferulic acid or coniferaldehyde units. The observed increase in isotopic ratio probably results from a primary isotope effect in the reversible oxidation-reduction reactions interrelating cinnamic acids and cinnamyl alcohols. A similar increase in isotopic ratio was measured during biosynthesis of the aryltetralin lactone lignan podophyllotoxin in Podophyllum hexandrum. © 1982.