Characterization and quantitation of a zinc-binding ligand in human milk.

A low-molecular-weight zinc-binding ligand from human milk has been isolated and characterized. The ligand was isolated by chromatography on Dowex 50, Dowex 1, and Sephadex G-15 equilibrated with 0.153 mM Zn(NO3)2. Mass spectroscopy, thin-layer chromatography, and infrared spectroscopy proved that the zinc-binding ligand isolated by this method from human milk is pyridine-2-carboxylic acid, commonly known as picolinic acid. The concentration of picolinic acid in human milk was 308 microM, the concentration in one brand of processed cow's milk was 20 microM, but picolinic acid was undetectable in a second brand of cow's milk and in four different infant formulas. Weanling rats fed supplemental picolinic acid absorbed significantly more dietary zinc and gained significantly more weight than rats fed an unsupplemented diet. The results suggest that the high bioavailability of zinc in human milk results from the presence of picolinic acid, a bidentate chelating ligand which facilitates zinc absorption from the intestine.