Heterogeneity of peptide adducts with carbonylated lipid peroxidation products.

Highly reactive lipid peroxidation-derived carbonyls (oxoLPP) modify protein nucleophiles via Michael addition or Schiff base formation. Once formed, Michael adducts can be further stabilized via cyclic hemiacetals with or without loss of water. Depending on the mechanism of their formation, peptide-oxoLPP can carry aldehyde or keto groups and thus be a part of the total protein carbonylation level. If a carbonyl function is lost during consecutive reactions, the oxoLPP-peptide adducts will not be detected using the common carbonyl labeling protocols. Because of the differences in adduct stabilities, it is possible to address the heterogeneity of peptide/protein-oxoLPP adducts by careful evaluation of tandem mass spectra of modified peptides. Here, we used hydrophilic interaction liquid chromatography-tandem mass spectrometry analysis of lysine, cysteine and histidine containing model peptides co-incubated with oxidized 1-palmitoyl-2-linoleoyl-sn-glycerophosphatidylcholine to characterize the collision-induced dissociation behavior of peptide-carbonyl adducts. Numerous modifications were detected based on the analysis of tandem mass spectra, including Schiff bases on lysine (two), Michael adducts on lysine (six), cysteine (eleven) and histidine (two), as well as 4-hydroxy-2-aldehydes derived dehydrated cyclic hemiacetals on cysteine (five) and histidine (one). Additionally, cysteine and histidine side chains were modified by lipid-bound aldehydes as Michael adducts and dehydrated hemiacetals. The tandem mass spectra revealed collision-induced dissociation characteristics specific for each class of oxoLPP-peptide adducts.