Valine and phenylalanine as precursors in the biosynthesis of alkamides in Acmella radicans

Acmella radicans (Asteraceae) produces at least seven alkamides, most with either an isobutyl- or phenylethyl group as the amine moiety. These moieties suggest that the amino acids valine and phenylalanine are the biosynthetic precursors of these alkamides. On the basis of labeled feeding experiments using either L-2H8]valine or L-2H8]phenylalanine we present evidence for the involvement of these two amino acids in the biosynthesis of (2E,6Z,8E)-N-isobutyl-2,6,8-decatrienamide (affinin) (1), (2Z,4E)-N-(2-phenylethyl)-2,4-octadienamide (2), (2E)-N-(2-phenylethyl)-nona-2-en-6,8-diynamide (3), and 3-phenyl-N-(2-phenylethyl)-2-propenamide (4). Alkamides were isolated from young A. radicans plants and analyzed by gas chromatography-mass spectrometry (GC-MS). Additionally, in cell free in vitro experiments based on isobutyl and phenylethylamide biosynthesis, using a colorimetric assay and GC-MS, valine and phenylalanine decarboxylase activities were assayed in the soluble extract of A. radicans leaves.