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The combination of SDS‐PAGE and MS is one of the most powerful and perhaps most frequently used gel‐based proteomics approaches in protein identification. However, one drawback of this method is that separation takes place under denaturing and reducing (R) conditions and as a consequence, all proteins with identical apparent molecular mass (Mr) will run together. Therefore, low‐abundant proteins may not be easily identified. Another way of investigating proteins by proteomics is by analyzing subproteomes from a total proteome such as phosphoproteomics, glycoproteomics, or disulfide proteomics. Here, we took advantage of the property of secreted proteins to form disulfide bridges and investigated disulfide‐linked proteins, using SDS‐PAGE under nonreducing (NR) conditions. We separated sera from normal subjects and from patients with various diseases by SDS‐PAGE (NR) and (R) conditions …