Sample preparation of human serum for the analysis of tumor markers: Comparison of different approaches for albumin and γ-globulin depletion

LC–MS is a powerful method for the sensitive detection of proteins and peptides in biological fluids. However, the presence of highly abundant proteins often masks those of lower abundance and thus generally prevents their detection and identification in proteomic studies. In human serum the most abundant proteins are albumin and γ-globulins. We tested several approaches to specifically reduce the level of these proteins based on either specific antibodies, dye ligands (for albumin) and protein A or G (for γ-globulins). The resulting, depleted serum was analyzed by sodium dodecylsulfate–polyacrylamide gel electrophoresis and LC–MS for the residual presence of these abundant proteins as well as for other serum proteins that should remain after depletion. To test the applicability of this method to real-life samples, depleted serum of a cervical cancer patient was analyzed for the presence of a specific tumor marker protein SCCA1 (squamous cell carcinoma antigen 1; P29508), which is present at ng/ml concentrations. The results demonstrate that SCCA1 can be detected by LC–MS in patient serum following depletion of albumin and γ-globulins thus opening the possibility of screening patient sera for other, so far unknown, tumor markers.