The metal-binding property of α-lactalbumin (α-LA) in human milk was studied and compared to that of bovine milk α-LA. Gel filtration on Sephadex G-75 at physiological pH and ionic strength separated α-LA in human and bovine milk from most other proteins. The only metal ion associated with α-LA under these conditions was Ca2+. Minor protein contaminants were removed by ion-exchange chromatography, and the ratio of Ca2+:α-LA was determined in the isolated protein preparations. Concentrations of α-LA in mature human milk were between 1.03 and 1.57 mg/ml; the Ca2+ concentration bound to α-LA varied, yielding a molar ratio of Ca2+:α-LA of approximately 1:1 (0.82-1.41 mol Ca2+/mol α-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 α-LA (0.6-0.9 mol Ca2+/mol α-LA) than to human α-LA. Calcium binding was abolished at pH 3.0 and resulted in a substantial increase in the hydrodynamic radius of α-LA. Reconstruction of human α-LA with Ca2+ and other divalent cations at native pH and ionic strength showed a binding specific for Ca2+. Since only 1% of calcium from human milk and 0.15% from bovine milk is α-LA bound, α-LA is probably unimportant with respect to calcium nutrition of the infant. However, the metal binding of α-LA may have a biological significance through its role in the lactose synthase complex.
|Original language||English (US)|
|Number of pages||8|
|Journal||Journal of Nutrition|
|State||Published - 1985|
ASJC Scopus subject areas
- Food Science
- Medicine (miscellaneous)