Characterisation of S-nitrosohaemoglobin by mass spectrometry

Recent studies have demonstrated the biological importance of the interaction of S-nitrosothiols, which can be considered as nitric oxide (NO) protein donors, especially haemoglobin, at the level of Cys residues. It was recently proposed that S-nitrosohaemoglobin is formed within red blood cells and serves as a regulatory function. In human haemoglobin the alpha-subunit contains one Cys residue and the beta-subunit contains two Cys residues, one of which (beta-Cys93) is highly reactive and conserved among species, although its function has remained unknown. Electrospray ionization mass spectrometry was used to monitor the results of exposure of haemolysates to S-nitrosocysteine under different conditions and thus addressed some aspects of NO-haemoglobin interaction. When an equimolar ratio of S-nitrosothiol was added to haemoglobin, only a single NO molecule was added. Peptide mapping by liquid chromatography-mass spectrometry located the nitrosyl group at the level of beta-Cys93 demonstrating that this was the preferred site of formation of S-nitrosohaemoglobin. The present data also suggest that electrospray mass spectrometry can allow quantification and characterisation of S-nitrosoproteins in blood.