title: Quantitative deuterium isotopic profiling at natural abundance indicates mechanistic differences for delta 12-epoxidase and delta 12-desaturase in Vernonia galamensis.
abstract: Quantitative (2)H NMR spectroscopy can determine the natural abundance ((2)H/(1)H) ratio at each site of a molecule. In natural products, variation in these values is related to the reaction mechanisms in the pertinent biosynthetic pathway. For the first time, this novel approach has been exploited to probe for mechanistic differences in the introduction of different functionalities into a long-chain fatty acid. Vernolic acid, a major component of the seed oil of Vernonia galamensis, contains both an epoxide and a desaturation. The site-specific isotopic distribution ((2)H/(1)H)(i) has been determined for both vernolic acid and linoleic acid isolated from the same V. galamensis oil. It is found that the ((2)H/(1)H) ratio of vernolic acid shows a pattern along the entire length of the chain, consistent with linoleic acid being its immediate precursor. Notably, the C13 relates to the C13 of linoleic acid but not to the C13 of oleic acid. Furthermore, the C12 and C13 positions in vernolic acid are less depleted, consistent with a change in hybridization state from sp(2) to sp(3). However, the C11 position shows a marked relative enrichment in the vernolic acid, implying that it plays a role in the epoxidase but not the desaturase mechanism. Thus, although it can be concluded that the catalytic mechanisms for the epoxidase and desaturase activities are similar, marked differences in the residual ((2)H/(1)H) patterns indicate that the reaction mechanisms are not identical.
authors.affiliation: Groupe de Fractionnement Isotopique de Métabolismes, Laboratoire d'Analyse Isotopique et Electrochimique de Métabolismes, CNRS UMR6006, Université de Nantes, BP 99208, F-44322 Nantes, France. isabelle.billault@univ-nantes.fr
authors.forename: Isabelle
authors.initials: I
authors.lastname: Billault
authors.forename: Jia-Rong
authors.initials: JR
authors.lastname: Duan
authors.forename: Sébastien
authors.initials: S
authors.lastname: Guiet
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Robins
mesh_terms: D041961:Deuterium Exchange Measurement / Q000379:methods; D044943:Fatty Acid Desaturases / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D009097:Multienzyme Complexes / Q000737:chemistry*; D010936:Plant Extracts / Q000032:analysis; D010938:Plant Oils / Q000032:analysis; D010940:Plant Proteins / Q000737:chemistry*; D012639:Seeds / Q000737:chemistry; D013251:Steroid Isomerases / Q000737:chemistry*; D035741:Vernonia
chemical_list: D009097:Multienzyme Complexes; D010936:Plant Extracts; D010938:Plant Oils; D010940:Plant Proteins; D044943:Fatty Acid Desaturases; C040448:delta-12 fatty acid desaturase; D013251:Steroid Isomerases; C022660:steroid delta-isomerase