Characterization of protein iv-glycosylation by reversed-phase microbore liquid chromatography / electrospray mass spectrometry, complementary mobile phases, and sequential exoglycosidase digestion

A mass spectrometry method has been developed for selective detection of glycopeptides at the low (< or = 25) picomole level during chromatography of glycoprotein digests and for differentiation of O-linked from N-linked oligosaccharides. The technique involves observation of diagnostic sugar oxonium-ion fragments, particularly the HexNAc+ fragment at m/z 204, from collisionally excited glycopeptides. Collision-induced fragmentation can be accomplished in either of two regions of a triple quadrupole mass spectrometer equipped with an atmospheric pressure, electrospray (ES) ionization source. If collisions before the first quadrupole are chosen, it is possible to enhance formation of carbohydrate-related fragment ions without distorting the distribution of peptide and glycopeptide signals by increasing the collisional excitation potential only during that portion of each scan in which the low mass carbohydrate-related ions are being detected. This procedure, requiring only a single quadrupole instrument, identifies putative glycopeptide-containing fractions in the chromatogram but suffers from a lack of specificity in the case of co-eluting peptides. Increased specificity is obtained by selectively detecting only those parent ions that fragment in Q2, the second collision region of the triple quadrupole, to produce an ion at m/z 204 (HexNAc+). Only (M + H)+ ions of glycopeptides are observed in these liquid chromatography-electrospray tandem mass spectrometry (LC-ESMS/MS) "parent-scan" spectra. N-linked carbohydrates are differentiated from O-linked by LC-ESMS/MS analysis of the digested glycoprotein prior to and after selective removal of N-linked carbohydrates by peptide N:glycosidase F. These methods, which constitute the first liquid chromatography-mass spectrometry (LC-MS)-based strategies for selective identification of glycopeptides in complex mixtures, facilitate location and preparative fractionation of glycopeptides for further structural characterization. In addition, these techniques may be used to assess the compositional heterogeneity at specific attachment sites, and to define the sequence context of the attachment site in proteins of known sequence. The strategy is demonstrated for bovine fetuin, a 42-kDa glycoprotein containing three N-linked, and at least three O-linked carbohydrates. Over 90% of the fetuin protein sequence was also corroborated by these LC-ESMS studies.

Mass spectrometric methods of glycopeptide-specific detection in liquid chromatography/electrospray mass spectrometry (LC/ESMS) of glycoprotein digests are explored using a variety of glycopeptide models and then applied to soluble complement receptor type I, a 240-kDa glycoprotein containing 25 potential sites of N-glycosylation. The most specific method, requiring a triple quadrupole, involves monitoring of sugar oxonium fragment ions during precursor-ion scan ESMS/MS. Signals derived from nonglycosylated peptides are virtually eliminated, resulting in a total-ion current chromatographic trace of only the glycopeptides present in the digest. The corresponding mass spectra yield molecular weight and glycopeptide microheterogeneity information. An alternative and complementary approach that we term collisional-excitation scanning also involves fragmentation of glycopeptides to sugar oxonium ion fragments but does not involve any mass-selection process, permitting the experiment to be performed on a single quadrupole instrument. The resulting total ion chromatogram is similar to the UV chromatogram (215 nm), but a selected-ion chromatogram for carbohydrate-specific ions such as the N-acetylhexosamine oxonium ion (m/z 204) produces a glycopeptide-specific trace. Although there can sometimes be peptide interferences in the spectra of the indicated glycopeptide-containing chromatographic peaks, this latter approach permits peptide mapping to be performed on the same data set that also indicates the location of glycopeptides in the chromatogram. Both methods are suitable for detection of glycopeptides with all common classes of oligosaccharides in either N- or O-linkage to the peptide.

Ten characterized sialylated oligosaccharides from bovine fetuin (B. Bendiak, M. Harris-Brandts, S. W. Michnick, J. P. Carver, and D. A. Cumming, Biochemistry, in press; and D. A. Cumming, C. G. Hellerqvist, M. Harris-Brandts, S. W. Michnick, J. P. Carver, and B. Bendiak, Biochemistry, in press) were chromatographed using high-performance anion-exchange chromatography with pulsed amperometric detection. At near neutral pH values, oligosaccharides were separated according to their number of formal negative charges from sialic acid; however, at alkaline pH, the neutral portion of the oligosaccharides enhanced resolution due to oxyanion formation. Specifically, trisialylated triantennary oligosaccharides containing a Gal-beta(1,3)GlcNAc sequence were more retained and could be completely separated from those having only Gal-beta(1,4)GlcNAc units. Oligosaccharides containing the same number of sialic acids were separated according to the combination of alpha(2,6)- and alpha(2,3)-linked sialic acids (alpha(2,6)-linked sialic acid reduced retention time). The relative molar electrochemical responses for di-, tri-, tetra-, and pentasialylated oligosaccharides were found to be similar (4.8 +/- 14% relative to glucose). Coelution studies were performed with each of the characterized oligosaccharides and the mixture of oligosaccharides which were released from fetuin with N-glycanase. The relative proportion of the major classes of sialylated oligosaccharides (bi-, tri-, tetra-, and penta-) varied significantly in bovine fetuin from different sources.