A NOVEL DIALYSIS PROCEDURE MEASURING FREE ZN2-MILK AND PLASMA( IN BOVINE)

A dialysis method was developed for measuring free Zn2+ concentration in plasma and milk for determination of the electrochemical gradient f or Zn2+ across the mammary gland. This method used the zinc content of casein after dialysis as the metal ion indicator because zinc in the calcium phosphate-citrate complex inside casein micelles is dependent on the free Zn2+ concentration of an associated aqueous phase. Zinc, c alcium and magnesium distribution in milk confirmed the high zinc bind ing by bovine casein. Zinc in the casein, dialyzed against standards o r unknowns, was measured by atomic absorption spectrophotometry. Avera ge free Zn2+ concentrations measured by the dialysis method in plasma and milk of five cows were 0.141 and 0.331 nmol/L, respectively. The e quilibrium potential of Zn2+ ions across the mammary epithelium, calcu lated from free Zn2+ concentrations in blood and milk measured by the dialysis method, was -11.4 mV, consistent with the mammary electrical potentials noted in previous studies. Therefore, no electrochemical gr adient for zinc between the two compartments was apparent, and it is n ot necessary to invoke an active transport mechanism in the mammary gl and to explain the higher zinc concentration in milk than in plasma of most species.