Albumin is the main nucleophilic target of human plasma: a protective role against pro-atherogenic electrophilic reactive carbonyl species?

The aim of this work was to study the metabolic fate of 4-hydroxy- trans-2-nonenal (HNE) in human plasma, which represents the main vascular site of reactive carbonyl species (RCS) formation and where the main pro-atherogenic target proteins are formed. When HNE was spiked in human plasma, it rapidly disappeared (within 40 s) and no phase I metabolites were detected, suggesting that the main fate of HNE is due to an adduction mechanism. HNE consumption was then monitored in two plasma fractions: low molecular weight plasma protein fractions (<10 kDa; LMWF) and high molecular weight plasma protein fractions (>10 kDa; HMWF). HNE was almost stable in LMWF, while in HMWF it was consumed by almost 70% within 5 min. Proteomics identified albumin (HSA) as the main protein target, as further confirmed by a significantly reduced HNE quenching of dealbuminated plasma. LC-ESI-MS/MS analysis identified Cys34 and Lys199 as the most reactive adduction sites of HSA, through the formation of a Michael and Schiff base adducts, respectively. The rate constant of HNE trapping by albumin was 50.61 +/- 1.89 M (-1) s (-1) and that of Cys34 (29.37 M (-1) s (-1)) was 1 order of magnitude higher with respect to that of GSH (3.81 +/- 0.17 M (-1) s (-1)), as explained by molecular modeling studies. In conclusion, we suggest that albumin, through nucleophilic residues, and in particular Cys34, can act as an endogenous detoxifying agent of circulating RCS.