Binding of zinc to bovine and human milk proteins

Zn binding by whole bovine and human casein and by purified bovine caseins and whey proteins was investigated by equilibrium dialysis. Bovine α_s1-casein had the greatest Zn-binding capacity (~ 11 atoms Zn/mol). Protein aggregation was observed as Zn concentration was increased and the protein precipitated at a free Zn concentration of 1.7mM. Zn binding increased with increasing pH in the range 5.4-7.0 and decreased with increasing ionic strength. Competition between Zn and Ca was observed for binding to _s1-casein indicating common binding sites for these two metals. Bovine β-casein bound up to 8 atoms Zn/mol and precipitated at a free Zn concentration of ~ 2.5 mM, while κ-casein bound 1–2 atoms Zn/mol. Whole bovine and human casein bound 5–8 atoms Zn/mol and precipitated at a free Zn concentration of ~ 2.0 mM. Scatchard plots for Zn binding to caseins showed upward convexity, possibly due to Zn-induced association of caseins. Apparent average association constants (K_app) for all caseins were similar (log K_app 3.0-3.2). Enzymic dephosphorylation of _s1-or whole bovine casein markedly reduced, but did not eliminate, Zn binding. Thus, phosphoserine residues appeared to be the primary Zn-binding sites in caseins. With the exception of bovine serum albumin, which bound over 8 atoms Zn/mol, the bovine whey proteins, β-lactoglobulin, α-lactalbumin and lactotransferrin, had little capacity for Zn binding.