- Human milk contains approximately 1% protein.
- Enzymes in milk chop some of these proteins into fragments (peptides).
- Specific peptides have evolved to possess biologically active (bioactive) roles.
- When analyzing human breast milk, 328 putative bioactive peptides were identified.
- From this group of peptides, 41 had anti-bacterial properties.
Milk is a wonderfully complex fluid that is not only nutritious but is also physiologically proactive. Recently, David Dallas and his colleagues from University of California at Davis used a cutting-edge approach to probe the depths of milk composition. The initial results revealed that human breast milk contains proteins which are digested into peptides, some with anti-bacterial properties (1).
Milk contains proteins, sugars, fats, vitamins, and minerals. The relative amounts of each component vary from species to species. Human milk contains approximately 1% protein with greater than 90% contributed by just a few milk proteins: caseins, lactalbumin, immunoglobulins, lactoferrin, serum albumin, and lysozyme. Milk also contains a mixture of enzymes that can chop these proteins into fragments, which together provide a rich source of biologically active (bioactive) peptides. Yes, milk digests itself!
Bioactive peptides are so called because they can induce a measurable response in the person or animal that consumes them. The bioactives of greatest interest are those that have a clear benefit for the consumer and help stimulate health and well-being. In the case of milk-derived peptides, an area that has attracted a lot of interest are bioactive peptides related to the immune system, which we find in relative abundance in milk.
The recent study by Dallas and colleagues (1) focused on identification of peptides that naturally occur in human breast milk, that is, they looked for the peptides that the milk created using its own enzymes. First they adapted a strategy to look for peptides that were hidden beneath the confusion that milk’s complexities can present. This is akin to exploring the depths of the ocean to find hidden treasures. The study revealed 328 peptides that were derived mostly from the major proteins. However, it was clear that these peptides were not generated by a random process, rather they were largely produced by one enzyme named plasmin, an enzyme that is well known for its role in the body, but until now, not so much for its role in milk.
When the 328 peptides were compared to databases of known bioactive peptides, 41 (12.5%) were thought to resemble peptides with anti-microbial properties. Anti-microbial properties of milk may benefit the infant that does not yet possess a fully functional immune system to battle pathogens, but there are also potential benefits for the mother to be taken into consideration. Bacterial infections in the ducts of a mammary gland, which we know as mastitis, may block the ducts and lead to painful inflammation and potentially the shutdown of milk production. One way to combat this, or even prevent it from occurring, would be to generate a frontline anti-bacterial defence mechanism in the milk itself.
With mastitis in mind, Dallas tested the mixture of milk peptides he had collected for their capacity to kill mastitis-inducing bacteria. He found that, when concentrated, the mixture was effective against the two types of bacteria tested, but only when concentrated. His work also suggests that specific peptides within the mixture have anti-bacterial activity and that perhaps even individual peptides, at the concentrations found present in milk, may be effective at fighting bacteria. Once again, milk has found a way to deal with the challenges nature has thrown its way. Dallas has just begun to uncover the bioactives that lie in his breast milk samples. Time will tell what other treasures lie beneath the milky waves.
1. Dallas DC, Guerrero A, Khaldi N, Castillo PA, Martin WF, Smilowitz JT, Bevins CL et al. (2013) Extensive in vivo Human Milk Peptidomics Reveals Specific Proteolysis Yielding Protective Antimicrobial Peptides. J Proteome Res 12: 2295-304.
Prof. Peter Williamson
Associate Professor, Physiology and Genomics
Associate Dean of Research
Faculty of Veterinary Research
University of Sydney, Australia