The methionine chain elongation pathway in the biosynthesis of glucosinolates in Eruca sativa (Brassicaceae)

Glucosinolates are nitrogen- and sulfur-containing plant natural products t hat have become increasingly important in human affairs as flavor precursor s, cancer-prevention agents, and crop protectants. While many glucosinolate s are biosynthesized front common amino acids, the major glucosinolates in economically important species of the Brassicaceae, such as Brassica napus (oilseed rape), are thought to be formed from chain-elongated derivatives o f methionine or phenylalanine. We investigated the chain elongation pathway for methionine that is involved in glucosinolate biosynthesis in Eruca sat iva. Isotopically labeled methionine and acetate were administered to cut l eaves and the major product, 4-methylthiobutylglucosinolate (isolated as it s desulfated derivative), was analyzed by MS and NMR. Administration of [U- C-13]methionine showed that the entire carbon skeleton of this amino acid, with the exception of the COOH carbon, is incorporated as a unit into 4MTB. Administration of [C-13]- and [C-14]acetate gave a labeling pattern consis tent with the operation of a three-step chain elongation cycle which beans with the condensation of acetyl-CoA with a 2-oxo acid derived from methioni ne and ends with an oxidative decarboxylation forming a new 2-oxo acid with one additional methylene group. Administration of [N-15]methionine provide d evidence for the transfer of an amino group to the chain-elongated 2-oxo acid, forming an extended amino acid which serves as a substrate for the re maining steps of glucosinolate biosynthesis. The retention of a high level of N-15 in the products suggests that the amino transfer reactions and the chain elongation cycle are localized in the same subcellular compartment, ( C) 2000 Academic Press.