Interactions of divalent metal ions with bovine, human, and goat alpha-lactalbumins.

Bovine, human, and goat alpha-lactalbumins prepared by the ordinary methods were found to contain 1.1-1.3 atoms of Ca per protein molecule. Removal of Ca2+ was shown to destabilize the tertiary structures in the three proteins. The three apoproteins were indicated to change in the conformation by heat from the native-like to the unfolded state. Degree of restoration of the native tertiary structure in 5 mM Tris-HCl and 0.1 mM EDTA at pH 7.2 and 25 degrees C by addition of Ca2+ was determined from change in CD ellipticity at 270 nm, and the apparent binding constant of Ca2+ was analyzed to be 2.5 X 10(8) (bovine), 3.0 X 10(8) (human), and 2.8 X 10(8) M-1 (goat). Also, value of the binding constant of Ca2+ to the native-like apoform was estimated from the apparent binding constant and equilibrium constant of the conformational change of the apoform. The binding properties of Mn2+, Mg2+, and Zn2+ to the bovine protein at neutral pH are also discussed.