Radical-mediated damages in human serum albumin and models of biological membranes: some insights by Raman spectroscopy.

Damages induced by free radicals on human serum albumin (HSA), the most prominent protein in plasma, were investigated by Raman spectroscopy. The HSA chain is characterised by an abundance of charged and aromatic residues, 6 Met and 35 half-cystines which form 17 disulfide bridges. In general, HSA represents the major and predominant antioxidant in plasma, a body compartment known to be exposed continuous oxidative stress. HSA was undergone to oxidative and reductive radical stress. Gamma-irradiation was used to simulate the endogenous formation of reactive radical species such as hydrogen atoms (•H) , solvated electrons (eaq–), and hydroxyl radicals (•OH). Raman spectroscopy showed to be a useful tool in identifying conformational changes of the protein structure and specific damages occurring at sensitive amino acid sites, as previously showed for other proteins. In particular, the analysis of the S-S stretching region suggested a different ability among the radical species cause modifications in the 17 disulfide bridges of HSA. The concomitant action of eaq– and •H atoms was also able to cause the formation of cyclic disulfide bridges, showing how cystine pairs act as efficient interceptors of reducing species. This reactivity was placed in a biomimetic context, analogously to other proteins. Indeed, the radical-induced damages on Met and Cys/cystine occur with the parallel formation of thiyl radicals. CH3S• and S•– radicals are able to migrate from the aqueous to the membrane compartment and cause damage to unsaturated lipids by effecting the cis-trans geometrical isomerization of the lipid double bonds.