Calcium binding by alpha-lactalbumin in human milk and bovine milk.
The metal-binding property of alpha-lactalbumin (alpha-LA) in human milk was studied and compared to that of bovine milk alpha-LA. Gel filtration on Sephadex G-75 at physiological pH and ionic strength separated alpha-LA in human and bovine milk from most other proteins. The only metal ion associated with alpha-LA under these conditions was Ca2+. Minor protein contaminants were removed by ion-exchange chromatography, and the ratio of Ca2+:alpha-LA was determined in the isolated protein preparations. Concentrations of alpha-LA in mature human milk were between 1.03 and 1.57 mg/ml; the Ca2+ concentration bound to alpha-LA varied, yielding a molar ratio of Ca2+:alpha-LA of approximately 1:1 (0.82-1.41 mol Ca2+/mol alpha-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 alpha-LA (0.6-0.9 mol Ca2+/mol alpha-LA) than to human alpha-LA. Calcium binding was abolished at pH 3.0 and resulted in a substantial increase in the hydrodynamic radius of alpha-LA. Reconstitution of human alpha-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 alpha-LA bound, alpha-LA is probably unimportant with respect to calcium nutrition of the infant. However, the metal binding of alpha-LA may have a biological significance through its role in the lactose synthase complex.
D000818:Animals; D002135:Calcium-Binding Proteins; D002417:Cattle; D002850:Chromatography, Gel; D002852:Chromatography, Ion Exchange; D004591:Electrophoresis, Polyacrylamide Gel; D005260:Female; D006801:Humans; D006863:Hydrogen-Ion Concentration; D005779:Immunodiffusion; D007768:Lactalbumin / Q000378:metabolism*; D007785:Lactose / Q000378:metabolism; D008892:Milk / Q000378:metabolism*; D008895:Milk, Human / Q000378:metabolism*; D011485:Protein Binding
D002135:Calcium-Binding Proteins; D007768:Lactalbumin; D007785:Lactose