Integrated sample preparation methodology for proteomics: analysis of native proteins.

An innovative sample preparation strategy is reported for protein identification in complex mixtures based on integration of affinity chromatographic selection and accelerated trypsin digestion using a continuous flow immobilized enzyme reactor (cf-IMER). Affinity selected glycoproteins were released to a cf-IMER column which converted native proteins to peptides in 5 min at elevated temperature. Digestion with the cf-IMER was compared to the traditional 16 h solution-based trypsin digestion of reduced and alkylated proteins. With immobilized antibody selection of Lewis x (Le(x)) glycan bearing glycoproteins from plasma, 66 proteins were identified in total with the two methods while approximately 1/3 of the total proteins and peptides were only observed with the cf-IMER. This suggests that proteomics based on protein identification by reduction and alkylation with solution-based trypsin digestion alone may not be identifying large numbers of proteins or peptides present at detectable levels in samples. Furthermore, except for proteins containing a high content of disulfide bonds, the majority of proteins did not require reduction and alkylation steps for their identification. The validity of the proposed proteolysis was evaluated in several ways by analyses of a model protein and yeast lysates where the reproducibility of quantification was essentially the same with both cf-IMER and solution-based proteolysis.