Search

Gel chromatography indicated that most of the zinc in cow's milk was associated with high-molecular-weight fractions, whereas zinc in human milk was associated with low-molecular-weight fractions. A species difference in zinc-binding ligands may explain why symptoms of the genetic disorder of zinc metabolism, acrodermatitis enteropathica, can be alleviated by feeding human but not cow's milk.

The molecular localization of zinc in rat milk and pup intestine was determined by gel chromatography. Commerical gels were found to give low recoveries of zinc and also caused anomalous elution patterns. Treatment of the gel with sodium borohydride corrected these problems. With treated gels, all of the zinc in rat milk eluted with high molecular weight proteins (MW greater than or equal to 30,000). In neonatal intestine, zinc eluted in two major peaks with molecular weights 30,000 and 6,000-8,000, respectively. However, incubating intestinal homogenates at 37 degrees for 30 minutes produced a redistribution of zinc with a new major low molecular weight (less than or equal to 1,500) peak. These results demonstrate the importance of sample preparation and chromatographic conditions when studying trace element ligands and also suggest that rat milk is not analogous to human milk in regard to zinc binding.

The hypothesis that a zinc-binding ligand (ZBL) recently discovered in human milk but absent from cow's milk might be related to zinc nutrition in the neonate was investigated. The zinc-binding characteristics of rat milk were examined to determine if the rat was a suitable model. By gel filtration, rat milk was found to contain a ZBL with characteristics similar to those of the ZBL found in human milk. A similar ZBL was identified in the intestinal mucosa of rats 16 days of age and older but was absent in rats from birth to 16 days. These results support the hypothesis that the ZBL of maternal milk may enhance zinc transport in the neonatal period before the development of intestinal mechanisms for zinc absorption.