INCORPORATION OF LABEL FROM C-13-LABELED, H-2-LABELED AND N-15-LABELED METHIONINE MOLECULES DURING THE BIOSYNTHESIS OF 2'-DEOXYMUGINEIC ACID IN ROOTS OF WHEAT

The biosynthetic pathway of 2'-deoxymugineic acid, a key phytosiderophore, was investigated by feeding C-13-, H-2-, and N-15-labeled methionine, the first precursor, to the roots of hydroponically cultured wheat (Triticom aestivum L. cv Minori). The incorporation of label from each methionine species was observed during their conversion to 2'-deoxymugineic acid, using H-2-, N-15-, and C-13-nuclear magnetic resonance (NMR). L-[1-C-13]Methionine (99% C-13) was efficiently incorporated, resulting in C-13 enrichment of the three carboxyl groups of 2'-deoxymugineic acid. Use of D,L-[N-15]methionine (95% N-15) resulted in N-15 enrichment of 2'-deoxymugineic acid at the azetidine ring nitrogen and the secondary amino nitrogen. When D,L-[2,3,3-H-2(3)-S-methyl-H-2(3)]methionine (98.2% H-2) was fed to the roots, H-2-NMR results indicated that only six deuterium atoms were incorporated, and that the deuterium atom from the C-2 position of each methionine was almost completely lost. [2,2,3,3-H-2(4)]1-Aminocyclopropane-1-carboxylic acid (98% H-2) was not incorporated into 2'-deoxymugineic acid. These data and our previous findings demonstrated that only the deuterium atom from the C-2 position of L-methionine was lost, and that other atoms were completely incorporated when three molecules of methionine were converted to 2'-deoxymugineic acid. These observations are consistent with the conversion of L-methionine to azetidine-2-carboxylic acid, suggesting that L-methionine is first converted to azetidine-2-carboxylic acid during biosynthesis leading to 2'-deoxymugineic acid. Based on these results, a hypothetical pathway from L-methionine to 2'-deoxymugineic acid was postulated.