Modification of lysine amino groups by the lipid peroxidation product 4,5(E)‐Epoxy‐2(E)‐hepteal

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Abstract
The reaction between 4,5(E)‐epoxy‐2(E)‐heptenal (EH) andl‐lysine was studied to characterize some of the compounds that may be produced when proteins react with peroxidizing lipids. A mixture of EH and lysine was incubated overnight at room temperature and then fractionated by high‐performance liquid chromatography (HPLC). Fractions were freeze‐dried and characterized by1H and13C nuclear magnetic resonance (NMR) and mass spectrometry. Four major pyrrole derivatives were obtained, namely 1‐(5′‐amino‐1′‐carboxypentyl)‐pyrrole (3), 1‐(5′‐amino‐1′‐carboxypentyl)‐2‐(1″‐hydroxypropyl)pyrrole (diastereomers 5 and 8), 1‐(5′‐amino‐5′‐carboxypentyl)pyrrole (7), and 1‐(5′‐amino‐5′‐carboxypentyl)‐2‐(1″‐hydroxypropyl)pyrrole (9). In addition, several lysine complexes were detected. A polymer (1b) that was responsible for the color and the fluorescence produced in the reaction was isolated by gel filtration chromatography from a fraction obtained by HPLC. Formation of pairs of analogs (5 and 3, 9 and 7) with and without a substituent in position 2 of the pyrrole ring suggested that the compounds were produced by the same mechanism, with the formation of the 2‐unsubstituted pyrroles corresponding to the loss of the 2‐substituent as propanal; propanal was detected by headspace capillary gas chromatography. A reaction mechanism is proposed based on the NMR data obtained when the reaction was monitored in real time in an NMR tube. The results suggest that pyrrolic amino acids 7 and 9 may be present in proteins that have been damaged by peroxidizing lipids.