Zinc and copper binding proteins in human milk.

The proteins binding zinc and copper in human milk have been fractionated and identified, and the distribution of these trace elements among the different binding compounds has been determined. Casein was separated by ultracentrifugation and was found to contain 14% (range 5 to 21%) of the total zinc content in the milk and 28% (range 7 to 48%) of the total copper. Another zinc- and copper-binding protein was isolated by gel filtration and ion-exchange chromatography and identified as serum albumin by gel electrophoresis. Serum albumin in breast milk binds 28% of total zinc and 39% of total copper. The remainder of Zn and Cu was found to be in a low molecular weight form (29% (range 24 to 36%) of Zn; 24% (range 15 to 47%) of Cu) or associated to the fat (29% (range 20 to 45%) of Zn; 9% (range 1 to 21% of Cu). It is hypothesized that association constants of the different binding compounds as well as their concentrations will determine the relative distribution of zinc and copper among them and may affect bioavailability of these elements for the infant.