A Method for Mapping Glycosylation Sites in Proteins.

The analysis of protein glycosylation by mass spectrometry (MS) has been a challenging technical problem. Quantification by HPLC of N-linked glycans can be executed by the use of peptide-N-glycosidase F to release them from the protein, followed by attachment of a fluorescent label and subsequent fluorescence detection. Similar quantification of O-linked glycans is not possible, as a result of the lack of a universal deglycosylation enzyme. Site-specific analyses by MS, such as the use of proteases to digest the glycoprotein, are difficult to use for quantification of glycans, as a result of the presence of miscleavages. Here, we present a new application of a digestion method for native proteins using resin-bound, thermally stabilized proteases. The use of this enzymatic treatment eliminates miscleavages around the site of glycosylation, thereby allowing site-specific relative quantification of glycans on glycoproteins. A native, intact human mAb was digested using a thermally stable, resin-bound trypsin to produce glycopeptides from the Fc region using a single-step protocol. A 1 mg sample was treated with 60 µg trypsin for 3 h at 70°C. After digestion, acetonitrile was added, and the mixture was centrifuged to remove the resin before analysis. Liquid chromatography (LC)/MS with hydrophilic interaction chromatography was used to analyze the glycopeptides produced. All of the glycopeptides found resulted from a single peptide (EEQYNSTYR). The LC/MS analysis of the glycopeptides is compared with that of fluorescently labeled glycans. Quantitative analysis produced a correlation coefficient of 0.87 for the linear fit between the glycopeptide and released glycan methods.