Oxidative cleavage of polyunsaturated fatty acid chains via dioxygenation/pseudoperoxidation by 15-lipoxygenase

Aberrant progression of one-electron reduction of fatty acid hydroperoxides in cells is thought to be responsible for cell death, such as ferroptosis caused by reactive carbonyl compounds and hydrocarbon radicals generated through the cleavage of C-C bonds of fatty acid alkoxyl radicals belonging to oxygen-centered radicals. In the present study, we investigated the mechanism underlying the oxidative cleavage of polyunsaturated fatty acid chains by 15-lipoxygenase under anaerobic conditions using soybean 15-lipoxygenase as a model enzyme. The apparent reaction rate constant of lipoxygenase(Fe²⁺) with 13-hydroperoxyoctadecadienoic acid (13-HpODE) was approximately 7.3 times higher than that with 9-HpODE. The production of 13-oxo-tridecadienoic acid (13-OTA) in this reaction was remarkably inhibited in the presence of a five-membered nitroxyl radical, 3-carbamoyl-2,2,5,5-tetramethyl-3-pyrroline-N-oxyl (CmΔP), which is a specific spin-trapping agent for carbon-centered radicals. The trapped adduct appeared to be a CmΔP adduct with a linoleate epoxyallyl radical, which was derived from the linoleate alkoxyl radical through intramolecular rearrangement at the reaction site on the enzyme. Furthermore, 13-OTA production in the α-linolenate hydroperoxide/lipoxygenase(Fe²⁺) system was six times higher than that in the linoleate hydroperoxide/lipoxygenase(Fe²⁺) system. Based on these facts, we hypothesized that under anaerobic conditions, the fatty acid epoxyallyl radical at the reaction site on the enzyme spontaneously degrades into reactive carbonyl compounds and hydrocarbon radicals through the cleavage of C-C bonds. In conclusion, radical scavengers against carbon-centered radicals should inhibit the lipoxygenase-inducing cell death through preventing the cleavage of C-C bond of fatty acid epoxyallyl radicals.