Macrocyclic oxygen transfer in the conversion of fatty acid hydroperoxide to a single species of triol in physiological saline

Abstract

We analyzed the autoxidation and non-enzymic reactions of the 17S-hydroperoxide of docosahexaenoic acid 1, a common lipoxygenase product in mammalian cells, which, with its six double bonds, can undergo reactions not accessible to all polyunsaturated fatty acids. Incubation in phosphate-buffered saline for one or more days at 37 °C revealed a dominant product identified by comparison to synthetic standards as diastereomers of the γ-lactone 2 of 4,5,17-trihydroxy-docosapentaenoic acid. Over several days in PBS at 37 °C, the γ-lactone hydrolyzed to the more polar 4,5,17-trihydroxy derivative 3. The same γ-lactone is formed by acid hydrolysis of synthetic 4,5-cis-epoxy-17-hydroxy-22:5ω3 4; however, this epoxide is stable in PBS at pH 7.4, indicating that the γ-lactone is a primary product rather than a secondary product formed from hydrolysis of the 4,5-epoxide. Mass spectrometric analysis of γ-lactone and trihydroxy derivatives from incubation with [17-¹⁸O₂H]-hydroperoxide demonstrated intramolecular oxygen transfer with the retention of both hydroperoxy oxygens on the 5- and 17-carbons. Direct involvement of the 17-hydroperoxide group in oxygen transfer with the participation of the C1 carboxyl in a mechanism with high “effective molarity” at the 4,5-double bond can account for the findings. Other fatty acid hydroperoxides with a similar spatial relationship between the peroxide and double bonds could also undergo this intramolecular oxygen transfer, a novel pathway in lipid peroxidation.