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

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.