In this paper, we present a method for the determination of low- and ultralow copy-number proteins in biomaterials based on a combination of concentrating the protein from the sample onto cyanogen bromide-activated Sepharose 4B (via nonspecific binding of free amino groups) and MRM. The detection limit and the dependence of the MRM peak areas on the concentration of protein in the sample were determined using the proteins CYP102 and BSA, as a model system, both in solution and after their addition to human plasma. Nonspecific protein enrichment of proteins from diluted sample volumes of 10-50 mL was found to increase the range of linear dependence of the chromatographic peak area on concentration by more than three orders of magnitude, allowing a lower LOD limit (LLOD) of as low as 10(-18) M. At this LLOD, at least two tryptic peptides of CYP102 and BSA could be detected with S/N of ≥7.0. The results were equally good for samples containing pure protein mixtures and proteins spiked into diluted depleted human blood plasma.
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