Interactions of Divalent Metal Ions with Bovine, Human, and Goat α-Lactalbumins1

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Abstract
Bovine, human, and goat α-lactalbumins prepared by the ordinary methods were found to contain 1.1-1.3 atoms of Ca per protein molecule. Removal of Ca 2+ 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°C by addition of Ca 2+ was determined from change in CD ellipticity at 270 nm, and the apparent binding constant of Ca 2+ was analyzed to be 2.5×10 8 (bovine), 3.0×10 8 (human), and 2.8×10 8 M -1 (goat). Also, value of the binding constant of Ca 2+ 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 Mn 2+ Mg 2+ and Zn 2+ to the bovine protein at neutral pH are also discussed.