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Identification of Copper- and Zinc-Binding Ligands in Human and Bovine Milk^1,2,

Department of Biochemistry and Molecular Biology ^* Department of Chemistry, University of North Dakota, Grand Forks, North Dakota 58202

The object of this study was to identify copper and zinc ligands detected during modified gel chromatography (MGC) of bovine and human milk ultrafiltrates. Isolation by anion-exchange chromatography and subsequent proton nuclear magnetic resonance (NMR) spectroscopy analysis demonstrated that the sole major low-molecular-weight ligand binding copper and zinc in bovine milk is citrate. Human milk apparently also contains citrate as a major metal-binding ligand but also contains amino acids, of which primarily glutamate was purified by anion-exchange chromatography. The amino acids bind copper well but are weak zinc-binding ligands. An artifactual MGC peak was seen in the milks, which was shown to be caused by the calcium present in the milk samples. No major differences in zinc-binding capacity were demonstrated between the low-molecular-weight fractions of the two milks. Although citrate may play a role in zinc uptake, it is apparently not the difference between the milks crucial to the acrodermatitis enteropathica individual. The difference in zinc availability between the milks may lie in some other aspect such as binding by proteins, which were noted to bind metal during MGC of nonultrafiltered milks.

KEY WORDS: • acrodermatitis enteropathica • copper-binding ligands • milk • modified gel chromatography • zinc-binding ligands

¹ Any opinion, findings, conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the view of the U.S. Department of Agriculture. This research has been supported in part by a research grant from the U.S. Department of Agriculture/Science and Education Administration, Competitive Research Grants Office, No. 59-2382-0-1-508-0.

² The acquisition of human milk samples was approved from an ethical standpoint by the Institutional Review Board for the use of human subjects.

³ To whom requests for reprints should be sent.

Manuscript received 26 August 1983.