Binding of zinc to casein

Area of Science:

• Biochemistry
• Food Science
• Nutritional Science

Background:

• Zinc is an essential trace element crucial for numerous biological functions.
• Cow's milk is a significant source of nutrients, including zinc, for infants and adults.
• Understanding zinc's interaction with milk proteins is vital for optimizing nutrient absorption.

Purpose of the Study:

• To investigate the binding characteristics of zinc with major cow's milk proteins.
• To determine the influence of pH and protein modification on zinc-milk protein interactions.
• To elucidate the mechanisms underlying zinc bioavailability from milk products.

Main Methods:

• pH-dependent zinc binding assays with casein and whey proteins.
• Analysis of zinc binding to dephosphorylated casein and casein phosphopeptides.
• Investigation of zinc release from casein using precipitation methods (acid, Ca2+, rennin).
• Competition studies with metal complexing agents (citrate, picolinic acid).

Main Results:

• Zinc exhibits avid, pH-dependent binding to casein, with maximal binding at slightly alkaline pH.
• No significant zinc binding occurs at acidic pH (pH 2) or to dephosphorylated casein at neutral pH.
• Bound zinc is released upon casein precipitation at pH 4.6, but not by Ca2+ or rennin.
• Zinc also binds to casein phosphopeptides, and this binding is competed by citrate and picolinic acid.

Conclusions:

• Casein is the primary milk protein responsible for zinc binding.
• The binding is pH-sensitive and involves phosphopeptide regions within casein.
• These interactions may contribute to the limited zinc bioavailability observed in cow's milk and some infant formulas.