I plan to use filters to get the fraction below 25kDa from serum proteins, then perform digestion and enrichment using TiO2 magnetic beads ( avoiding albumin depletion that can result in low reproducibility).
There are kits available for enrichment of phosphoproteins. I don't have personal experience with them. Maybe someone can comment on how well they work.
I am already using TiO2 magnetic beads (Thermo) to enrich phosphopeptides from serum. I wanted to know what is the best way to enrich lower abundance serum proteins (and deplete albumin at least) before phosphoprotein/phosphopeptide enrichment in order not to interfere with the phospo- enrichment process and have a high yield of phosphoprotein identification by LC-MS/MS