ELECTROSPRAY-IONIZATION MASS-SPECTROMETRIC STUDY OF CALCIUM-BINDING TO CALMODULIN IN THE PRESENCE OF MAGNESIUM AND TERBIUM

Ca2+, Mg2+ and Tb3+ binding on calmodulin (CaM) was studied using elec trospray ionization mass spectrometry, The solvent used was ammonium a cetate (5 mM, pH 6.0), The calcium and magnesium stoichiometries were the same as previously described in the presence of 20% methanol, The study of competition between Ca2+ and Mg2+ showed that magnesium binds on auxiliary cationic sites, It was also found that six Tb3+, the mos t frequently used probe for the determination of calcium binding seque nce on CaM, bind to this protein in addition to four Ca2+ This result clearly confirms the existence of a maximum of ten cationic sites on C aM.