title: Dihydroceramide delta(4) desaturase initiates substrate oxidation at C-4.
abstract: The intermolecular primary deuterium isotope effects on the individual C-H bond cleavage steps involved in dihydroceramide Delta(4) desaturation have been determined for the first time by incubating rat liver microsomes with 1:1 mixtures of nonlabeled substrate and the appropriate regiospecifically dideuterated analogue. Analysis of the enzymatic products via gas chromatography coupled to mass spectrometry showed that the introduction of the (E) double bond between C-4 and C-5 occurs in two discrete steps: cleavage of the C4-H bond was found to be very sensitive to isotopic substitution (k(H)/k(D) = 8.0 +/- 0.8), while a negligible isotope effect (k(H)/k(D) = 1.02 +/- 0.07) was observed for the C5-H bond-breaking step. According to a mechanistic model that we have previously proposed, these results suggest that initial oxidation for this desaturation reaction occurs at C-4. This finding correlates nicely with the observation that 4-hydroxylated products are produced from a similar substrate by a closely related oxidative enzyme in yeast.
authors.affiliation: Department of Chemistry, Ottawa-Carleton Chemistry Institute, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario K1S 5B6, Canada.
authors.forename: C K
authors.initials: CK
authors.lastname: Savile
authors.forename: G
authors.initials: G
authors.lastname: Fabriàs
authors.forename: P H
authors.initials: PH
authors.lastname: Buist
mesh_terms: D000818:Animals; D002518:Ceramides / Q000737:chemistry / Q000378:metabolism*; D002850:Chromatography, Gel; D066298:In Vitro Techniques; D009682:Magnetic Resonance Spectroscopy; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology / Q000378:metabolism; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000378:metabolism*; D051381:Rats; D013056:Spectrophotometry, Ultraviolet
chemical_list: D002518:Ceramides; D010088:Oxidoreductases; C108138:dihydroceramide desaturase