Zinc binding in cow's milk and human milk

Area of Science:

• Nutritional Biochemistry
• Human and Comparative Lactation

Background:

• Zinc is an essential trace element crucial for infant development.
• Milk is a primary source of zinc for human infants.
• Understanding zinc's macromolecular binding in milk is key to assessing bioavailability.

Purpose of the Study:

• To investigate and compare the molecular forms of zinc in cow's milk and human milk.
• To identify the specific milk proteins associated with zinc in each milk type.
• To explore potential implications for zinc bioavailability in infants.

Main Methods:

• Gel filtration chromatography (Sephadex G-75, G-25, G-150) was used to separate milk components by molecular weight.
• Ultracentrifugation was employed to fractionate milk into sediment and supernatant.
• Proteins associated with zinc were identified through these separation techniques.

Main Results:

• In cow's milk, approximately 90% of zinc was recovered as low molecular weight zinc citrate after casein precipitation, while over 95% of total zinc was associated with casein micelles.
• Zinc in cow's milk casein micelles was linked to alpha-casein-kappa-casein and alpha-casein-beta-casein aggregates.
• In human milk, only about 40% of zinc sedimented with ultracentrifugation, with the majority (85%) in the supernatant associated with a high molecular weight complex ( > 150,000 Da) primarily containing lactoferrin.

Conclusions:

• Zinc is bound to distinct macromolecules in cow's milk and human milk.
• Cow's milk zinc primarily associates with casein micelles.
• Human milk zinc predominantly binds to lactoferrin, suggesting its potential role in infant zinc uptake and bioavailability.