Binding of zinc to casein.

An investigation of the binding of zinc to cow's milk proteins has shown that zinc binds avidly in a pH-dependent manner to casein, but has little affinity for the total whey protein fraction. At slightly alkaline pH 1 mg casein binds 8.4 micrograms zinc. No zinc binds to casein at pH 2 and to dephosphorylated casein at pH 7.4. Bound zinc is released only by casein precipitation at pH 4.6 but not by casein precipitation using Ca2+-ions or rennin. It could also be shown that zinc binds to phosphopeptides derived from tryptic or chymotryptic casein digestion and that metal complexing agents, such as citrate or picolinic acid compete for zinc binding with these phosphopeptides and casein. Binding of zinc to casein and its tryptic or chymotryptic phosphopeptides may explain in part the comparatively low zinc availability from cow's milk and some milk-based infant formulas.