APPLICATION OF THERMALLY ASSISTED ELECTROSPRAY-IONIZATION MASS-SPECTROMETRY FOR DETECTION OF NONCOVALENT COMPLEXES OF BOVINE SERUM-ALBUMIN WITH GROWTH-HORMONE RELEASING-FACTOR AND OTHER BIOLOGICALLY-ACTIVE PEPTIDES

Ion-spray ionization mass spectrometry with gentle conditions for solvent removal has been reported as a useful tool for detection of high-affinity noncovalent complexes of biological relevance formed in solution. Two main objectives of this study were (i) to find whether other types of electrospray ionization (ESI) sources, e.g. where the solvent is removed with the help of heat (thermally assisted electrospray), could be utilized for detection of noncovalent biological complexes of high and low affinity and (ii) to find whether ESI-MS can be used for detection of the association of bovine serum albumin (BSA) with biologically active peptides. Using a well-defined high-affinity association of FK506 with its binding protein (FKBP) as model system we proved that ESI-MS with thermally assisted interphase can be used for detection of the FK506-FKBP complexes in a similar way as was previously shown for electrospray mass spectrometry (Ganem et al., J. Am. Chem. Soc. 113, 6294 (1991)). In mixtures of BSA with a 9-10 molar excess of biologically active peptides, such as growth hormone releasing factor (GRF), glucagon, bradykinin or insulin in ammonium acetate at pH 7.5, complexes with a ratio of 1:1, 1:2 and in some cases 1:3 were detected. On the other hand, these complexes disappeared upon acidification, pointing to their noncovalent nature. To our knowledge, this is the first evidence that relatively weak and nonselective association of proteins (represented here by BSA) with peptide ligands have been detected by ESI-MS in addition to well documented examples of highly specific molecular recognition phenomena like, for example, those reported for receptor-ligand, enzyme-substrate, or enzyme-inhibitor systems. Our results also show that electrospray ion sources of different design are amenable for performing studies of binding.