| Glycomacropeptide (GMP) found in sweet whey is a sialylated glycopeptide released from the C-terminus of kappa-casein by the action of chymosin during cheese making. Cow GMP has been extensively studied. It has a number of biological activities and is thought to be a potential ingredient for functional foods and pharmaceuticals. There is, however, relatively limited information available concerning structure and composition of goat GMP. Little is known about the effect of pasteurization of milk on the structure and composition of GMP. In addition, it has been reported that hyperthreoninemia, a condition manifested as abnormally high concentrations of serum threonine, is a frequent phenomenon occurring in infants fed with sweet whey-based formulas, and that removal of GMP may help prevent occurrence of the abnormality. This study is comprised of four experiments. The first experiment presents the purification of GMP from goat sweet whey by anion-exchange and hydrophobic interaction chromatography. The second compares GMP isolated from raw and pasteurized goat milk using chemical analysis and sodium dodecyl sulfate (SDS) gel electrophoresis. The third and fourth experiments present analyses of a GMP-depleted sweet whey fractions obtained from goat and bovine sweet whey, respectively, by using anion-exchange chromatography. Amino acid analysis of the GMP purified from goat sweet whey by anion-exchange and hydrophobic interaction chromatography showed that the content of phenylalanine is negligible. The GMP preparation was considered to be suitable as an ingredient for the diet of patients suffering from phenylketonuria, a hereditary disorder of phenylalanine metabolism. Gel electrophoresis of GMP demonstrated that pasteurization generates GMP aggregates with increased mobility on SDS gels, and that the low temperature long time compared to high temperature short time pasteurization is more effective to induce this change. The GMP-depleted whey fractions, containing proteins desorbed from the anion-exchanger, showed a threonine content lower than that of their correspondent sweet whey but similar to that of human milk. The homology of amino acid composition calculated between human milk and the GMP-depleted whey fractions was considerably high. It was concluded that replacement of sweet whey by GMP-depleted whey fraction in sweet whey-based infant formulas may help prevent occurrence of hyperthreoninemia. Research into the potential uses of goat GMP and GMP-depleted whey fraction in the pharmaceutical, cosmetic and functional food sectors may be important, especially in countries where most of caprine milk is used for cheese production. |
