The Methionine Chain Elongation Pathway in the Biosynthesis of Glucosinolates in Eruca sativa (Brassicaceae)

Glucosinolates are nitrogen- and sulfur-containing plant natural products that have become increasingly important in human affairs as flavor precursors, cancer-prevention agents, and crop protectants. While many glucosinolates are biosynthesized from 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 of methionine or phenylalanine. We investigated the chain elongation pathway for methionine that is involved in glucosinolate biosynthesis in Eruca sativa. Isotopically labeled methionine and acetate were administered to cut leaves and the major product, 4-methylthiobutylglucosinolate (isolated as its desulfated derivative), was analyzed by MS and NMR. Administration of [U-13C]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 [13C]- and [14C]acetate gave a labeling pattern consistent with the operation of a three-step chain elongation cycle which begins with the condensation of acetyl-CoA with a 2-oxo acid derived from methionine and ends with an oxidative decarboxylation forming a new 2-oxo acid with one additional methylene group. Administration of [15N]methionine provided 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 remaining steps of glucosinolate biosynthesis. The retention of a high level of 15N in the products suggests that the amino transfer reactions and the chain elongation cycle are localized in the same subcellular compartment.