Abstract
INTRODUCTION: In the search for novel cancer biomarkers, various proteolytically derived peptides have been proposed to exhibit cancer or cancer-type specificity. As these peptides are presumably also generated after sample collection by tumor-specific proteases, extensive investigation of the involved proteolytic processes is crucial for further research. MATERIALS AND METHODS: Using two previously developed and fully validated liquid-chromatography coupled to tandem-mass spectrometry assays, absolute quantification of, in total, 13 proteolytically derived peptides in human serum was accomplished. The analytes included eight peptides derived from inter-α-trypsin inhibitor heavy chain-4 (ITIH(4)-30, ITIH(4)-29, ITIH(4)-28, ITIH(4)-27, ITIH(4)-26, ITIH(4)-25, ITIH(4)-22, and ITIH(4)-21), bradykinin, des-Arg(9)-bradykinin, Hyp(3)-bradykinin, and fragments from fibrinogen-α-chain (Fib-α ([605-629])) and complement component 4a (C4a ([1337-1350])). Samples were obtained from different healthy individuals and prepared with variable tube types, clotting times, and temperatures. Furthermore, stabilities in the serum fraction were assessed and compared to stabilities in serum from breast cancer patients. RESULTS AND DISCUSSION: The quantitative analyses showed either increasing or decreasing serum concentrations during blood coagulation, while comparable effects were observed in serum separated from the blood clot. Furthermore, comparisons of inter- and intra-individual variations suggested better reflection of an individual's protease activity after prolonged ex vivo incubation. This was illustrated for the putative breast cancer marker ITIH(4)-22, revealing better differentiation after incubation of serum at ambient temperature for 24 h. CONCLUSION: The presented study provides suggestions for more specific and optimized sample preparation, as well as extended knowledge necessary to further explore the opportunities of these proteolytic peptides as cancer biomarkers.