Desaturation of trans-octadecenoyl-acyl carrier protein by stearoyl-acyl carrier protein delta9 desaturase.

Positional isomers of mono-unsaturated 18:1-ACP have been used as substrates for stearoyl-acyl carrier protein delta9 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-delta11-18:1-ACP and trans-delta7-18:1-ACP, but not trans-delta9-18:1-ACP, or any of the corresponding cis-isomers. Enzymatic desaturation of trans-delta11-18:1-ACP gave a single product, cis-delta9,trans-delta11-18:2-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-delta7-18:1-ACP gave two enzymic products, trans-delta7,cis-delta9-18:2 (approximately 80%) and trans-delta7,cis-delta11-18:2 (approximately 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.