BIOSYNTHESIS OF MUSTARD OIL GLUCOSIDES: V. FORMATION OF GLUCONASTURTIIN FROML-γ-PHENYLBUTYRINE-C14-N15IN WATERCRESS

Abstract

C14-Labelled compounds were fed to watercress (Nasturtium officinale R. Br.) and their efficiency as precursors of the aglycone portion of gluconasturtiin compared. Phenylalanine-2- and -3-C14and sodium acetate-2-C14were efficient precursors of the aglycone but neither compound was as efficient a precursor as γ-phenylbutyrine (2-amino-4-phenylbutyric acid). Approximately 40% of the C14from γ-phenylbutyrine-2- or -3-C14was incorporated into the aglycone side chain. Results of studies with doubly labelled L-γ-phenylbutyrine-C14-N15show that the amino nitrogen is incorporated directly into the thioglucoside. Evidence that indicates that D-γ-phenylbutyrine is converted to its L-isomer by the plant is also presented. The results of an isotope competition experiment provide some evidence for the existence of a chain-lengthening pathway (analogous to the formation of leucine from valine) for the biosynthesis of γ-phenylbutyrine from phenylalanine and acetate in watercress.