The biosynthesis of cyclopropane fatty acids. II. Mechanistic studies using methionine labelled with one, two, and three deuterium atoms in the methyl group

Abstract

The mechanism of formation of monodeuterated cyclopropane fatty acids on administration of L-methione-methyl-d3 to L. plantarum has been investigated. It has been found that the extent of exchange is not affected by the presence of oxygen in the medium, that neither formate-d0 nor formaldehyde-d2 is significantly incorporated into the fatty acid, and that methionine reisolated from the intracellular pool does not contain any exchanged species. These experiments militate against any mechanism involving exchange prior to methyl group transfer. Parallel feeding experiments using methionine labelled in the methyl group with one, two, or three deuterium atoms were carried out and in this way that extent of exchange was quantitated. It was found that at least one-third of the cyclopropane fatty acid from methionine methyl-d3 experiments is derived from an exchanged methyl group. The intramolecular and intermolecular primary deuterium isotope effects for biological cyclopropane ring formation were measured and found to be 3.2 ± 0.5, and 1.01 ± 0.04, respectively. These results are consistent with a mechanism in which a methyl group is transferred in a slow step which is followed by a fast, partially reversible, protonation-deprotonation step.