Monolayer autoxidation of arachidonic acid to epoxyeicosatrienoic acids as a model of their potential formation in cell membranes

Abstract

In light of the importance of epoxyeicosatrienoic acids (EETs) in mammalian pathophysiology, a nonenzymatic route that might form these monoepoxides in cells is of significant interest. In the late 1970s, a simple system of arranging linoleic acid molecules on a monolayer on silica was devised and shown to yield monoepoxides as the main autoxidation products. Here, we investigated this system with arachidonic acid and characterized the primary products. By the early stages of autoxidation (∼10% conversion of arachidonic acid), the major products detected by LC-MS and HPLC-UV were the 14,15-, 11,12-, and 8,9-EETs, with the 5,6-EET mainly represented as the 5-δ-lactone-6-hydroxyeicosatrienoate as established by 1H-NMR. The EETs were mainly the cis epoxides as expected, with minor trans configuration EETs among the products. 1H-NMR analysis in four deuterated solvents helped clarify the epoxide configurations. EET formation in monolayers involves intermolecular reaction with a fatty acid peroxyl radical, producing the EET and leaving an incipient and more reactive alkoxyl radical, which in turn gives rise to epoxy-hydro(pero)xides and other polar products. The monolayer alignment of fatty acid molecules resembles the arrangements of fatty acids in cell membranes and, under conditions of lipid peroxidation, this intermolecular mechanism might contribute to EET formation in biological membranes.