Binding of Copper by Certain Milk Proteins as Measured by Equilibrium Dialysis

The binding of copper with micellar casein, fat globule-membrane protein (soluble fraction), a-casein (crude), Bcasein and fi-lactoglobulin was determined by equilibrium dialysis at pH 6.5. Solutions of these proteins at 200.0, 300.0, 243.6, 120.5 and 142.0 rag/liter were placed in washed bags and dialyzed against each of four copper solutions for 72 hours, an interval sufficient to attain equilibrium. Four copper solutions outside the dialysis bags were 16 10 -6, 32 10 6, 64 10 -6 or 80 >( 10 -6 M. Data were plotted in moles of bound copper per mole of protein against the logarithm of concentratration of unbound copper. Micellar casein bound the most copper. The fat-globule membrane protein had the second greatest affinity for copper ions. Much less copper was bound by a-casein, fi-lactoglobulin and B-casein.