Size exclusion chromatography coupled to electrospray ionization mass spectrometry for analysis and quantitative characterization of arsenic interactions with peptides and proteins.

Arsenic-binding proteins are of toxicological importance since enzymatic activities can be blocked by arsenic interactions. In the present work, a novel methodology based on size exclusion chromatography coupled to electrospray ionization mass spectrometry (SEC-ESI-MS) was developed with special emphasis to preserve the intact proteins and their arsenic bindings. The eluent composition of 25 mMTris/HCl, pH 7.5, with the addition of 100-mM NaCl optimized for SEC with UV detection provided the highest SEC separation efficiency, but was not compatible with the ESI-MS because of the non-volatility of the buffer substance and of the salt additive. In order to find the best compromise between chromatographic separation and ionization of the arsenic-binding proteins, buffer type and concentration, pH value, portion of organic solvent in the SEC eluent as well as the flow rate were varied. In the optimized procedure five different arsenic-binding peptides and proteins (glutathione, oxytocin, aprotinin, alpha-lactalbumin, thioredoxin) covering a molar mass range of 0.3-14 kDa could be analyzed using 75% 10-mM ammonium formate, pH 5.0/25% acetonitrile (v : v) as eluent and a turbo ion spray source operated at 300 degrees C and 5.5 kV. A complete differentiation of all peptides and proteins involved in the arsenic-binding studies as well as of their arsenic-bound forms has become feasible by means of the extracted ion chromatograms (XIC) of the mass spectrometric detection. The new method offered the possibility to estimate equilibrium constants for the reaction of phenylarsine oxide with different thiol-containing biomolecules by means of the XIC peak areas of reactants and products. Limits of detection in the range of 2-10 microM were obtained by SEC-ESI-MS for the individual proteins.