Biosynthesis of mustard oil glucosides: 3-benzylmalic acid, a precursor of 2-amino-4-phenylbutyric acid and of gluconasturtiin

Abstract

2-Amino-4-phenylbutyric acid was isolated from Nasturtium officinale R. Br. (watercress) and identified by gas–liquid and paper chromatography.Various 14C-labelled compounds, including 3-benzylmalic-1,2-14C acid, were fed to N. officinale and their efficiencies as precursors of the aglycone moiety of gluconasturtiin and of 2-amino-4-phenylbutyric acid were determined. The efficiency of conversion of 14C from 3-benzylmalic-1,2-14C acid into gluconasturtiin aglycone was greater than that from DL-phenylalanine-2-14C or -3-14C, acetate-2-14C, shikimic-G-14C acid, and D-glucose-G-14C, but was less than that from DL-2-ammo-4-phenylbutyric-2-14C acid. Degradation of the aglycone obtained from plants fed 3-benzylmalic-1,2-14C acid revealed the tracer had been incorporated specifically into the isothiocyanate carbon and it was concluded that the acid was converted to gluconasturtiin with the loss of C-1. As was reported previously, the carboxyl carbons of acetate and phenylalanine were not incorporated into gluconasturtiin aglycone.The 14C from DL-phenylalanine-2-14C, acetate-1-14C, acetate-2-14C, and from 3-benzylmalic-1,2-14C acid was incorporated into 2-amino-4-phenylbutyric acid with high efficiency. The results support the view that 2-amino-4-phenylbutyric acid is formed from phenylalanine and acetate via 3-benzylmalic acid.