Detection of low molecular weight copper(II) and zinc(II) binding ligands in ultrafiltered milks-the citrate connection.

Low molecular weight zinc(II) and copper(II) binding ligands were detected in ultrafiltered human, bovine, and goat milk by the application of the method of modified gel chromatography. Human milk contains at least three detectable low molecular weight copper binders, whereas bovine and goat milk contain at least two. All three milks show two copper binding peaks with the same elution volumes. Zinc chromatograms were less specific than copper. Zinc showed only a single detectable low molecular weight binding ligand common to all three milks. Elution volumes of both zinc (II) and copper(II) citrate and picolinate systems were measured. Elution volumes of both copper II and zinc(II) citrate complexes are identical to elution volumes of an intense peak observed with all three milks; it is reasonable to assume that at least part of this peak corresponds to citrate. Human milk alone has a relatively intense binding peak for copper(II) at the same elution volume as the glutamate complex. Human and goat milk have another low intensity copper(II) binding ligand peak at the same elution volume; a number of amino acid complexes have binding peaks at this position. No peak characteristic of the zinc(II) or copper(II) picolinate systems could be found with any of the milks.