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Submitted on April 2, 2007
Revised on October 24, 2007
Accepted on November 28, 2007
Biochemistry, Molecular Biology and Biophysics, University of Minnesota, Minneapolis, MN 55455
Corresponding Author: tgriffin{at}umn.edu
Whole human saliva possesses tremendous potential in clinical diagnostics, particularly for conditions within the oral cavity, such as oral cancer. Although many have studied the soluble fraction of whole saliva, few have taken advantage of the diagnostic potential of the cells present in saliva, and none have taken advantage of proteomic capabilities for their study. We report on a novel proteomic method with which we characterized for the first time cells contained in whole saliva from patients diagnosed with oral squamous cell carcinoma. Our method employs three dimensions of peptide fractionation, combining these steps: preparative IEF using Free Flow Electrophoresis; strong cation exchange step-gradient chromatography; and microcapillary reverse-phase liquid chromatography. We determined that the whole saliva samples contained enough cells, mostly exfoliated epithelial cells, providing adequate amounts of total protein for proteomic analysis. From a mixture of four oral cancer patient samples, the analysis resulted in a catalogue of over one thousand human proteins, each identified from at least two peptides, including numerous proteins with a role in oral squamous cell carcinoma signaling and tumorigenesis pathways. Additionally, proteins from over 30 different bacteria were identified, some of which putatively contribute to cancer development. The combination of preparative IEF followed by strong cation exchange chromatography effectively fractionated the complex peptide mixtures, despite the closely related physiochemical peptide properties of these separations (pI and solution phase charge, respectively). Furthermore, compared to our two-step method combining preparative IEF and reverse phase liquid chromatography, our three-step method identified significantly more cellular proteins while retaining higher confidence protein identification enabled by peptide pI information gained through IEF. Thus, for detecting salivary markers of oral cancer, and possibly other conditions of the oral cavity, the results confirm both the potential of analyzing the cells in whole saliva and doing so with our proteomic method.
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