Calcium stoichiometry determination for calcium binding proteins by electrospray ionization mass spectrometry

Electrospray ionization mass spectrometry was used for the determination of calcium-binding stoichiometry for calcium-binding proteins. Bovine calmodulin, bovine alpha-lactalbumin, and rabbit parvalbumin were found to bind specifically to 4, 1, and 2 Ca(2+) ions, respectively, in agreement with previously reported results obtained by using other physical methods. This mass spectrometry method could also be applied to proteins that bind more than one type of metal ion. The Zn(2+)- and Ca(2+)-binding stoichiometries for human stromelysin catalytic domain were determined to be 3 and 2, respectively.