Model studies on the modification of proteins by lipoxidation-derived 2-hydroxyaldehydes

2-hydroxyaldehydes have been previously identified as products of lipid peroxidation, and although they represent the simplest reducing sugars, their potential for modification of proteins under physiological conditions has not been investigated. Here, 2-hydroxyaldehydes were found to condense with amines in two ways, implicating potential pathways for modification of lysine residues. A fluorescent 4,5-dialkyl-3-hydroxypyridinium with ex/em 327/390 nm and a nonfluorescent 4-alkylimidazolium cross-linking product were isolated and characterized by (1)H NMR, (13)C NMR, high-resolution mass spectrometry, and, in the former case, through independent synthesis. Both reactions appear to proceed through Amadori rearrangement of the initial Schiff base. On the basis of the UV absorbance of the 3-hydroxypyridinium, the latter was estimated to represent modification of 1.5% of the lysines of RNase incubated with 0.5 mM 2-hydroxyheptanal for 10 days at 25 degrees C. The 4-alkylimidazolium is proposed to contribute to the protein cross-linking observed by gel electrophoresis in the incubation of RNase with higher concentrations of 2-hydroxyheptanal.