Distinguishing Histidine Tautomers in Proteins Using Covalent Labeling-Mass Spectrometry

In this work, we use diethylpyrocarbonate (DEPC)-based covalent labeling together with LC-MS/MS analysis to distinguish the two sidechain tautomers of histidine residues in peptides and proteins. From labeling experiments on model peptides, we demonstrate that DEPC reacts equally with both tautomeric forms to produce chemically different products with distinct dissociation patterns and LC retention times, allowing the ratios of the two tautomers to be determined in peptides and proteins. Upon measuring the tautomer ratios of several histidine residues in myoglobin, we find good agreement with previous 2D NMR data on this protein. Because our DEPC labeling/MS approach is simpler, faster, and more precise than 2D NMR, our method will be a valuable way to determine how protein structure enforces histidine sidechain tautomerization. Because the tautomeric state of histidine residues is often important for protein structure and function, the ability of DEPC labeling/MS to distinguish histidine tautomers should equip researchers with a tool to understand the histidine residue structure and function more deeply in proteins.

Covalent Labeling/Mass Spectrometry of Monoclonal Antibodies with Diethylpyrocarbonate: Reaction Kinetics for Ensuring Protein Structural Integrity

Diethylpyrocarbonate (DEPC)-based covalent labeling together with mass spectrometry is a promising tool for the higher-order structural analysis of antibody therapeutics. Reliable information about antibody higher-order structure can be obtained, though, only when the protein's structural integrity is preserved during labeling. In this work, we have evaluated the applicability of DEPC reaction kinetics for ensuring the structural integrity of monoclonal antibodies (mAbs) during labeling. By monitoring the modification extent of selected proteolytic fragments as a function of DEPC concentration, we find that a common DEPC concentration can be used for different monoclonal antibodies in formulated samples without perturbing their higher-order structure. Under these labeling conditions, we find that the antibodies can accommodate up to four DEPC modifications without being structurally perturbed, indicating that multidomain proteins can withstand more than one label, which contrasts to previously studied single-domain proteins. This more extensive labeling provides a more sensitive measure of structure, making DEPC-based covalent labeling-mass spectrometry suitable for the higher-order structural analyses of mAbs.

Covalent labeling and mass spectrometry reveal subtle higher order structural changes for antibody therapeutics

Monoclonal antibodies are among the fastest growing therapeutics in the pharmaceutical industry. Detecting higher-order structure changes of antibodies upon storage or mishandling, however, is a challenging problem. In this study, we describe the use of diethylpyrocarbonate (DEPC)-based covalent labeling (CL) - mass spectrometry (MS) to detect conformational changes caused by heat stress, using rituximab as a model system. The structural resolution obtained from DEPC CL-MS is high enough to probe subtle conformation changes that are not detectable by common biophysical techniques. Results demonstrate that DEPC CL-MS can detect and identify sites of conformational changes at the temperatures below the antibody melting temperature (e.g., 55 ᴼC). The observed labeling changes at lower temperatures are validated by activity assays that indicate changes in the F_ab region. At higher temperatures (e.g., 65 ᴼC), conformational changes and aggregation sites are identified from changes in CL levels, and these results are confirmed by complementary biophysical and activity measurements. Given the sensitivity and simplicity of DEPC CL-MS, this method should be amenable to the structural investigations of other antibody therapeutics.