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Calcium Binding by -Lactalbumin in Human Milk and Bovine Milk

Department of Nutrition, University of California, Davis, CA 95616

The metal-binding property of -lactalbumin (-LA) in human milk was studied and compared to that of bovine milk -LA. Gel filtration on Sephadex G-75 at physiological pH and ionic strength separated -LA in human and bovine milk from most other proteins. The only metal ion associated with -LA under these conditions was Ca²⁺. Minor protein contaminants were removed by ion-exchange chromatography, and the ratio of Ca²⁺:-LA was determined in the isolated protein preparations. Concentrations of -LA in mature human milk were between 1.03 and 1.57 mg/ml; the Ca²⁺ concentration bound to -LA varied, yielding a molar ratio of Ca²⁺:-LA of approximately 1:1 (0.82–1.41 mol Ca²⁺/mol -LA) in mature milk. Gel filtration with excess Ca²⁺ in the running buffer showed that there is another weaker binding site for Ca²⁺, but this binding does not occur under physiological conditions. Less Ca²⁺ was bound to bovine -LA (0.6–0.9 mol Ca²⁺/mol -LA) than to human -LA. Calcium binding was abolished at pH 3.0 and resulted in a substantial increase in the hydrodynamic radius of -LA. Reconstitution of human -LA with Ca²⁺ and other divalent cations at native pH (6.8) and ionic strength showed a binding specific for Ca²⁺. Since only 1% of calcium from human milk and 0.15% from bovine milk is -LA bound, -LA is probably unimportant with respect to calcium nutrition of the infant. However, the metal binding of -LA may have a biological significance through its role in the lactose synthase complex.

KEY WORDS: • human milk • -lactalbumin • calcium

Manuscript received 22 February 1985. Revision accepted 7 June 1985.

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