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Natural abundance deuterium 2D NMR spectroscopy in weakly ordering, polypeptide chiral liquid crystals is a powerful technique that enables determination of enantiotopic isotopic ratios (H/H) at the methylene groups of long-chain fatty acids. This technique has been used to study the bioconversion of linoleic acid to vernoleic acid with the objective of establishing the in-vivo site-specific fractionation of H associated with this process. The fractionation pattern was investigated in Euphorbia lagascae and Vernonia galamensis, plants that use different enzyme systems to perform the Δ-epoxidation: a cytochrome P450 monooxygenase in the former and a di-iron dioxygenase in the latter. The specific interest in this study was to ascertain whether different (H/H) isotopic ratios in substrate and product might reflect distinct features of the nature of the reaction centre. However, both the linoleate (substrate) samples and both vernoleate (product) samples isolated from the seed oils of the two plants had remarkably similar H isotope profiles, with selection against H in the positions around the Δ-epoxidation site. This is interpreted as indicating that, despite differences in the form in which the activated Fe is presented and in the architecture of the active site, the (H/H) isotopic pattern is determined by features common to the reaction. It is suggested that the effects acting as the overall determinants of the final (H/H) distribution in the product are the encumbrance of the active site pocket and constraints to conformational readjustment during the linoleate to vernoleate transformation. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Metabolism of isobutene (2-methylpropene) in rats (Sprague Dawley) and mice (B6C3F) follows kinetics according to Michaelis-Menten. The maximal metabolic elimination rates are 340 μmol/kg/h for rats and 560 μmol/kg/h for mice. The atmospheric concentration at which V/2 is reached is 1200 ppm for rats and 1800 ppm for mice. At steady state, below atmospheric concentrations of about 500 ppm the rate of metabolism of isobutene is direct proportional to its concentration. 1,1-Dimethyloxirane is formed as a primary reactive intermediate during metabolism of isobutene in rats and can be detected in the exhaled air of the animals. Under conditions of saturation of isobutene metabolism the concentration of 1,1-dimethyloxirane in the atmosphere of a closed exposure system is only about 1/15 of that observed for ethene oxide and about 1/100 of that observed for 1,2-epoxy-3-butene as intermediates in the metabolism of ethene or 1,3-butadiene. [ABSTRACT FROM AUTHOR]

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