Several different low molecular weight haptens were conjugated to BSA to produce immunogens useful for antibody development. The extent of BSA modification due to covalent attachment of hapten was estimated by matrix-assisted laser desorption ionization mass spectrometry. The average number of hapten incorporated to immunogen was determined from the difference in the measured molecular weights of the conjugate from nonmodified BSA. The results from mass spectrometry were compared with results obtained from other more traditional methods of immunogen characterization (UV analysis, trinitrobenzenesulfonic acid titrations, and gel electrophoresis). In each case we were able to calculate the average number of hapten covalently bound to BSA for each synthetically prepared immunogen using matrix-assisted laser desorption ionization mass spectrometry. The other methods presented limitations in certain cases.
A mixture of 5- and 6-carboxyfluorescein was activated with 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide hydrochloride in the presence of either N-hydroxysuccinimide or pentafluorophenol to give the corresponding succinimidyl and pentafluorophenyl esters. The regioisomeric mixtures were separated to give the 5- and 6-succinimidyl and pentafluorophenyl active esters in > 98% purity.
Post-translational modifications can have a signification effect on antibody stability. A comprehensive approach is often required to best understand the underlying reasons the modification affects the antibody's potency or aggregation state. Monoclonal antibody 001 displayed significant variation in terms of potency, as defined by surface plasmon resonance testing (Biacore), from lot to lot independent of any observable aggregation or degradation, suggesting that a post-translational modification could be driving this variability. Analysis of different antibody lots using analytical hydrophobic interaction chromatography (HIC) uncovered multiple peaks of varying size. Electrospray ionization mass spectrometry (ESI-MS) indicated that the antibody contained a cysteinylation post-translational modification in complementarity-determining region (CDR) 3 of the antibody light chain. Fractionation of the antibody by HIC followed by ESI-MS and Biacore showed that the different peaks were antibody containing zero, one, or two cysteinylation modifications, and that the modification interferes with the ability of the modified antibody arm to bind antigen. Molecular modeling of the modified region shows that this oxidation of an unpaired cysteine in the antibody CDR would block a potential antigen binding pocket, suggesting an inhibition mechanism.
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