Capillary Electrophoretic Peptide Mapping to Probe the Immobilization/Digestion Conditions of Glutaraldehyde-crosslinked Chymotrypsin

The use of immobilized enzymes in proteomics studies has increased over the last decade not only due to their ease of use in fluidic systems but also because autoproteolysis is suppressed allowing use of high ratios of enzyme-to-substrate for rapid digestion. Chymotrypsin immobilization by crosslinking with excess aqueous glutaraldehyde (GA) has been used to produce a gel-like agglomerated pellet of high enzyme loading capable of digesting protein. The simple procedure involved drop-wise addition of 80 mu L GA (2.5% in water) to 80 mu L chymotrypsin in pH 6.4 phosphate buffer, reaction for 2 h at room temperature without stirring, washing with buffer and NaCl, then deactivation of any free aldehyde groups by reaction with 200 mM glycine for 3 h. The enzyme was ready for use after washing with buffer and water. The efficiency of the immobilized enzyme for protein digestion was followed by capillary electrophoretic (CE) peptide mapping with absorbance detection at 200 nm. A study comparing preparation methods showed that making a small immobilized enzyme pellet leads to better digestions (i.e., peptide maps) compared to using a five-fold larger pellet or a part of the latter. When three different buffers for the chymotrypsin crosslinking reaction were compared, sodium phosphate at pH 6.4 showed better batch-to-batch reproducibility and activity of the immobilized enzyme: overall peak area RSD was 27% versus 39% and 43% for the other pHs (with 11% of this variability due to the CE method) and up to 3 times more peptide peaks were detected. The immobilized chymotrypsin pellet could be used for two consecutive digestions of denatured BSA and was able to partially digest native BSA.