Chromatofocusing fractionation and two‐dimensional difference gel electrophoresis for low abundance serum proteins

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Abstract
The technical challenge to analysis of the serum proteome is that the serum proteins are present at unequal concentrations. A few are so dominant, such as serum albumin and immunoglobulins, that they mask detection of other proteins. Because of these high abundance proteins, current technologies, while theoretically capable of analyzing protein amounts spanning four orders of magnitude, are only able to analyze proteins ranging over two orders of magnitude and cannot analyze the lower abundance proteins that may be the next biomarkers and drug targets. To facilitate the identification of low abundance proteins, we fractionated serum samples from patients with prostate cancer and patients with benign prostate hyperplasia using anion displacement liquid chromatofocusing chromatography, which separates proteins by a pH gradient and a positively charged column. Differential expression of proteins from fractions was then determined and identified by IEF gels and 2-D DIGE. Results demonstrate improved resolution of proteins within the chosen pH gradient when compared to the unfractionated samples. Several proteins that were differentially expressed in serum from patients with prostate cancer were identified in the fractionated serum. Three of these proteins, squamous cell carcinoma antigen 1 (SCCA1), calgranulin B, and haptoglobin-related protein, are present in the serum at levels below the classical protein level of mg/mL. SCCA1 is normally expressed in serum at ng/mL levels, and calgranulin B is an intracellular protein. Our results demonstrate that the use of anion displacement liquid chromatofocusing chromatography may reduce the complexity of the serum proteome by separating proteins into distinct pH ranges, and facilitate the identification of low abundance proteins.