Use of 13C in biosynthetic studies. Incorporation of isotopically labeled acetate and aspartate into fusaric acid

Abstract

Acetate-1-13C, acetate-2-13C, and aspartate-4-13C were used as substrates to elucidate the biosynthetic pathway for fusaric acid, the phytotoxic metabolite of Fusarium oxysporum Schlecht. It was established that C-2, C-3, C-4, and C-7 were derived from acetate via aspartate or a related four-carbon dicarboxylic acid, whereas C-5, C-6, C-8, C-9, C-10, and C-11 were derived more directly from acetate. The 13C-labeling patterns were determined by measuring the relative intensities of the 13C—H satellites by proton magnetic resonance spectroscopy.Further information was derived from experiments with D- and L-isomers of aspartate-1-14C and L-aspartate-1-14C:15N. Radioactivity from both enantiomers of aspartate-1-14C was incorporated into fusaric acid, but more efficiently from the L-isomer. Furthermore, 15N was incorporated more efficiently into fusaric acid and the aspartic acid of mycelial protein than was 14C. These results suggest that aspartic acid is metabolized to fusaric acid via oxalacetate, and that L-aspartate serves as a donor of nitrogen, in an amino-transferase reaction, to a separate oxalacetate pool of primarily endogenous origin.