Identification of Copper- and Zinc-Binding Ligands in Human and Bovine Milk

Abstract

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.

Distribution of calcium, magnesium, phosphorus, zinc, manganese, copper and iron between the soluble and colloïdal phases of ewe's and goat 's milk

“…As a result of this distribution, the bioavailability of zinc is much lower than in human milk. In the latter, zinc is largely found in the soluble phase bound to citrate molecules (Lônnerdal et al, 1981;Martin et al, 1984) and lactoferrin molecules (Blakeborough et al, 1983;Flynn and Power, 1985) and is hence more readily absorbed by the intestinal tract of new-born babies. Recent studies (Shen et al, 1995) in human, cow's, goat's and sheep's milks also showed a considerable higher zinc bioavailability for human milk than for the other species.…”

“…Most of the copper is bound to ligands of low molecular mass, largely citrate (Martin et al, 1984) with a very small percentage, not more th an 8%, bound to the serum proteins (Brulé and Fauquant, 1982;Renner et al, 1989). Although these data are taken from studies in cow's milk, Martin et al (1981) reported earlier that citrate acted as a copper-binding ligand in goat's milk as weil.…”

The nutritive and metabolic advantages of homologous milk

“…It is now appreciated that the situation is considerably more complex (Martin et al 1984). In human milk, Zn is distributed evenly between the casein and whey fractions, with about 10% of the total in the fat fraction (Casey et al 1987).…”

Low Molecular Weight Copper-Binding Ligands in Human Bile

“…Hence, the detection of all the major exchangeable as well as nonexchangeable complexes is ensured. The detection limit for some common ligands is 0.1 pmole in the chromatographed sample (10 Bile. Human gall bladder bile was obtained during cholecystectomy, from United Hospital, Grand Forks, North Dakota.…”

“…MGC involves the gel filtration of a biological specimen on a column equilibrated with a chosen metal ion (9,10). Ligands in the chromatographed sample are separated by molecular weight on the gel matrix and the column is monitored for metal binding peaks by atomic absorption spectrophotometry.…”