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A Novel Dialysis Procedure Measuring Free Zn²⁺ in Bovine Milk and Plasma^1,2,3,4,

Department of Food Science, North Carolina State University, Raleigh, NC 27695-7624

A dialysis method was developed for measuring free Zn²⁺ concentration in plasma and milk for determination of the electrochemical gradient for 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, calcium and magnesium distribution in milk confirmed the high zinc binding by bovine casein. Zinc in the casein, dialyzed against standards or unknowns, was measured by atomic absorption spectrophotometry. Average 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 equilibrium potential of Zn²⁺ ions across the mammary epithelium, calculated 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 gradient for zinc between the two compartments was apparent, and it is not necessary to invoke an active transport mechanism in the mammary gland to explain the higher zinc concentration in milk than in plasma of most species.

KEY WORDS: • magnesium • calcium • zinc • milk • cows

¹ Portions of this work were previously published in abstract form: Zhang, P. & Allen, J. C. (1992) Zinc activity measurement in bovine milk. J. Dairy Sci. 75 (suppl. 1): 106 (abs.); Zhang, P.-F. & Allen, J. C. (1993) Magnesium and zinc distributions and their activity measurements in bovine milk. Inst. of Food Technol. Ann. Meeting Tech. Prog.: Book of Abstr., p. 122.

² Support for this research was provided by the North Carolina Agricultural Research Service, The North Carolina Dairy Foundation and the Southeast Dairy Foods Research Center.

³ Manuscript no. FSR 94-13 of the NCSU Department of Food Science. The use of trade names in this publication does not imply endorsement by the North Carolina Agricultural Research Service or criticism of similar ones not mentioned.

⁴ The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

⁵ Current address: Department of Physiology, University of Colorado School of Medicine, Box C240, UCHSC, Denver, CO 80262.

⁶ To whom correspondence should be addressed.

Manuscript received 2 August 1994. Revision accepted 19 January 1995.