Casein, (a natural substrate for casein kinase II) is phosphorylated and
commercially available through many vendors. I have previously isolated a
phosphorylated peptide from a digest of casein for methods development for
MALDI-MS analysis and N-terminal sequencing of phospho-serine following removal
of the phosphate group.
Hope this helps.
Raymond Boynton
Analytical Protein Chemistry
N-terminal Sequencing Core Facility
Biogen, Inc.
Cambridge MA, 02142