Profiling of endogenous serum phosphorylated peptides by titanium (IV) immobilized mesoporous silica particles enrichment and MALDI-TOFMS detection.

Phosphorylation is one of the most important post-translational modifications of proteins, which modulates a wide range of biological functions and activities of proteins. The phosphorylation of proteins is also associated with the pathway of cancer cells. We have previously enriched the low molecular weight proteome from human plasma based on the combination of size exclusion and adsorption mechanism by using highly ordered mesoporous silica particles. Herein, highly ordered mesoporous silica particles were modified with titanium phosphonate to selectively capture the phosphopeptides from complex peptide and protein mixtures. The limit of detection for phosphopeptides from beta-casein and standard phosphopeptide spiked in human serum was as low as 1.25 fmol based on MALDI-TOFMS detection. The modified mesoporous silica particles were further used to enrich phosphopeptides from serum of hepatocellular carcinoma patients and healthy individuals and then analyzed with MALDI-TOFMS. The combination of isobaric tagging for relative and absolute quantitation labeling with MALDI-TOFMS/MS was further applied to validate the serum phosphopeptide profiling result of MALDI-TOFMS. The profiling of the serum phosphopeptides between the cancer patients and healthy persons was distinguishingly different, which indicated the potential ability of this technique for cancer diagnosis and biomarker discovery. The approach developed here would be applicable to other biological samples and a wide variety of diseases.