Human serum albumin associated with reductive radical stress

Radiation-induced modifications of human serum albumin mainly occurring at S-containingresidues were investigated by Raman spectroscopy and mass spectrometry techniques. When HOradicals were scavenged by t-BuOH, the H atom and hydrated electron (eaq) attack led to thechemical transformation of Cys into Ala residues and the conversion of Met residues intoa-aminobutyric acid residues. Mapping experiments demonstrated that desulfurization selectivelyaffects Cys34, Met123, Met298, Cys514, Met548 and Cys567 since the first stages of reaction.Generation of thiol functionality was also detected at specific residues involved in disulfide bonds,namely Cys200, Cys392 and Cys514, together with partial oxidation at Met87, Met123, Met298,Met329 and Met548, forming sulfoxides. When HO radicals were not scavenged, lesser amountsof the previous modifications were observed, whereas some residues (Cys34, Cys461, Pro486,Phe488 and Phe502) resulted to be oxidatively modified. Based on the known cystine pairing innative albumin, no relationships were observed between desulfurization and disulfide reductionprocesses at Cys residues involved in disulfide bonds, thus suggesting either independentreactivities or, more probably, reactions at the same amino acids that then underwent quickdisulfide scrambling events toward more stable disulfide/thiol populations. When these reactionswere performed on protein species added to large unilamellar vesicles, desulfurization yieldedsulfur radicals able to induce a cis-trans isomerization of lipids at the onset of irradiation. Thisstudy provides a description of the human albumin modifications resulting from reductive radicalstress, thus suggesting the need for specific assays and future investigations to detect these eventsin proteins and lipids within challenged cells.