Characterisation of S‐nitrosohaemoglobin by mass spectrometry

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Abstract
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 α‐subunit contains one Cys residue and the β‐subunit contains two Cys residues, one of which (β‐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 β‐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.