Calcium-Binding Proteins in Bovine Milk: Calcium-Binding Properties and Amino Acid Composition

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The Ca2+-binding properties and amino acid compositions of two calcium-binding proteins (mCaBP-3 and mCaBP-4) purified fromff bovine milk were studied. mCaBP-3 was identified as a Cat +-bound type and mCaBP-4 as a Ca2+-free type by means of ion-exchange chromatog-raphy on a DEAE-5ephadex A-25 column. In polyacrylamide gel disc electrophoresis, both mCaBP-3 and mCaBP-4 had the same mobility of Rf=0.73 and the addition of 5 mM CaCl2 to the electrode buffer decreased the mobility from R f=0.73 to R f=0.49. mCaBP-3 and mCaBP-4 con-sisted of 120 and 122 amino acid residues, respectively. The molecular weights were 13, 758 and 13, 967, respectively. The amino acid com-positions of the two milk CaBPs very closely resembled each other. Both milk CaBPs were rich in aspartic acid, glutamic acid, leucine and lysine, but did not contain trimethylated lysine and amino sugar. An interesting feature is that each milk CaBP contained eight cysteine sulfone and three tryptophan residues per molecule. From these results, it is suggested thal mCaBP-3 and mCaBP-4 are identical protein and that mCaBP-3 is formed from mCaBP-4 by means of a conformational change by binding of Cat + Thus, mCaBP-3 is a holoprotein and mCaBP-4 is an apoprotein. Furthermore, it is suggested that milk CaBP is different from calmodulin, troponin C and vitamin D-dependent calcium-binding protein.
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