Stereochemical configuration of 4-hydroxy-2-nonenal-cysteine adducts and their stereoselective formation in a redox-regulated protein.

4-Hydroxy-2-nonenal (HNE), a major racemic product of lipid peroxidation, preferentially reacts with cysteine residues to form a stable HNE-cysteine Michael addition adduct possessing three chiral centers. Here, to gain more insight into sulfhydryl modification by HNE, we characterized the stereochemical configuration of the HNE-cysteine adducts and investigated their stereoselective formation in redox-regulated proteins. To characterize the HNE-cysteine adducts by NMR, the authentic (R)-HNE- and (S)-HNE-cysteine adducts were prepared by incubating N-acetylcysteine with each HNE enantiomer, both of which provided two peaks in reversed-phase high performance liquid chromatography (HPLC). The NMR analysis revealed that each peak was a mixture of anomeric isomers. In addition, mutarotation at the anomeric center was also observed in the analysis of the nuclear Overhauser effect. To analyze these adducts in proteins, we adapted a pyridylamination-based approach, using 2-aminopyridine in the presence of sodium cyanoborohydride, which enabled analyzing the individual (R)-HNE- and (S)-HNE-cysteine adducts by reversed-phase HPLC following acid hydrolysis. Using the pyridylamination method along with mass spectrometry, we characterized the stereoselective formation of the HNE-cysteine adducts in human thioredoxin and found that HNE preferentially modifies Cys(73) and, to the lesser extent, the active site Cys(32). More interestingly, the (R)-HNE- and (S)-HNE-cysteine adducts were almost equally formed at Cys(73), whereas Cys(32) exhibited a remarkable preference for the adduct formation with (R)-HNE. Finally, the utility of the method for the determination of the HNE-cysteine adducts was confirmed by an in vitro study using HeLa cells. The present results not only offer structural insight into sulfhydryl modification by lipid peroxidation products but also provide a platform for the chemical analysis of protein S-associated aldehydes in vitro and in vivo.