Desaturation of trans-octadecenoyl-acyl carrier protein by stearoyl-acyl carrier protein Δ9 desaturase

Positional isomers of mono-unsaturated 18:1-ACP have been used as substrates for stearoyl-acyl carrier protein Delta(9) desaturase to test whether a C-H bond abstraction from either the C-9 or C-10 position could lead to rearranged products diagnostic for the production of an allylic radical intermediate. The reconstituted enzyme complex was able to desaturate trans-Delta(11)-18:1-ACP and trans-Delta(7)-18:1-ACP, but not trans-Delta(9)-18:1-ACP, or any of the corresponding cis-isomers. Enzymatic desaturation of trans-Delta(11)-18:1-ACP gave a single product, cis-Delta(9),trans-Delta(11)-18:2-ACP, as characterized by gas chromatography-electron ionization mass spectrometry of the molecular ions, the fragmentation products of pyrrolidide and 4,4-dimethyloxazoline derivatives, and by comparison of chromatographic retention times with authentic standards. Reaction of trans-Delta(7)-18:1-ACP gave two enzymic products, trans-Delta(7),cis-Delta(9)-18:2 ( similar to 80%) and trans-Delta(7),cis-Delta(10)-18:2 ( similar to 20%). The major product was likely formed in a reaction identical to that of 18:0-ACP desaturation, while the minor product was likely formed by alternative placement of the C-10 and C-11 positions of the substrate analog in a cis configuration relative to the diiron oxidant. Since none of the products observed are indicative of rearrangements originating with an allylic radical, a discussion of the origins and possible implications of these results is presented. (C) 2000 Elsevier Science Inc. All rights reserved.