Free zinc concentration in bovine milk measured by analytical affinity chromatography with immobilized metallothionein.

A new analytical affinity chromatography method was developed for measuring the free [Zn2+] concentration in bovine milk. The column was generated by immobilizing avidin and attaching biotinylated metallothionein (MT) on controlled-pore glass beads. Zinc bound to the MT column at physiological free [Zn2+] concentration and was dissociated again in an elution buffer of pH 2. The distributions of extrinsically added 65Zn and native zinc in different fractions of milk were virtually identical, validating the use of extrinsic labeling in studies of the free [Zn2+] concentration in milk. Extrinsically labeled whey fractions were mixed with standard solutions whose free [Zn2+] concentrations were calculated by computer model. 65Zn retained by the column provided an indication of free [Zn2+] concentration in the mixture, and by interpolation, in the original milk. The free [Zn2+] concentration measured by the affinity chromatography method in the milk of a group of six cows was 90.4 +/- 29.7 pM. This value is similar to estimates of free [Zn2+] concentrations in other biological fluids by entirely different methods. Measurement of free [Zn2+] may be helpful in understanding the physiology and biochemistry of zinc metabolism.