Manganese lipoxygenase oxidizes bis-allylic hydroperoxides and octadecenoic acids by different mechanisms

Manganese lipoxygenase (MnLOX) oxidizes (11R)-hydroperoxylinolenic acid (11R-HpOTrE) to a peroxyl radical. Our aim was to compare the enzymatic oxidation of 11R-HpOTrE and octadecenoic acids with LOO-H and allylic C-H bond dissociation enthalpies of similar to 88 and similar to 87 kcal/mol. Mn(III)LOX oxidized (11Z)-, (12Z)-, and (13Z)-18:1 to hydroperoxides with R configuration, but this occurred at insignificant rates (<1%) compared to 11R-HpOTrE. We next examined whether transitional metals could mimic this oxidation. Ce4+ and Mn3+ transformed 11R-HpOTrE to hydroperoxides at C-9 and C-13 via oxidation to a peroxyl radical at C-11, whereas Fe3+ was a poor catalyst. Our results suggest that MnLOX oxidizes bis-allylic hydroperoxides to peroxyl radicals in analogy with Ce4+ and Mn3+. The enzymatic oxidation likely occurs by proton-coupled electron transfer of the electron from the hydroperoxide anion to Mn(III) and H+ to the catalytic base, Mn(III) OH-. Hydroperoxides abolish the kinetic lag times of MnLOX and FeLOX by oxidation of their metal centers, but 11R-HpOTrE was isomerized by MnLOX to (13R)-hydroperoxy-(9Z,11E,15Z)-octadecatrienoic acid (13R-HpOTrE) with a kinetic lag time. This lag time could be explained by two competing transformations, dehydration of 11R-HpOTrE to 11-ketolinolenic acid and oxidation of 11R-HpOTrE to peroxyl radical; the reaction rate then increases as 13R-HpOTrE oxidizes MnLOX with subsequent formation of two epoxyalcohols. We conclude that oxidation of octadecenoic acids and bis-allylic hydroperoxides occurs by different mechanisms, which likely reflect the nature of the hydrogen bonds, steric factors, and the redox potential of the Mn(III) center. (C) 2010 Elsevier B.V. All rights reserved.