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Binding of zinc to bovine and human milk proteins

Published online by Cambridge University Press:  01 June 2009

Harjinder Singh ,
Albert Flynn  and
Patrick F. Fox
Harjinder Singh
Affiliation:
Departments of Food Chemistry, University College, Cork, Irish Republic
Albert Flynn
Affiliation:
Nutrition, University College, Cork, Irish Republic
Patrick F. Fox
Affiliation:
Departments of Food Chemistry, University College, Cork, Irish Republic
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Summary

Zn binding by whole bovine and human casein and by purified bovine caseins and whey proteins was investigated by equilibrium dialysis. Bovine αs1 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 1·7 mM. 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 αs1-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 K-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 αs1- 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.

Type
Original Articles
Information
Journal of Dairy Research , Volume 56 , Issue 2 , May 1989 , pp. 235 - 248
Copyright
Copyright © Proprietors of Journal of Dairy Research 1989

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