Bioactive Peptides in Milk: What Do they Do?

May 27, 2020
For infants, milk is more than a source of nutrients. A clear example is the fact that it contains bioactive peptides. Its function is to protect the infant from a more or less hostile environment and, at the same time, directly promote their development.

Although people know the nutritional value of milk proteins, they don’t really know a lot about the functional value of bioactive peptides, which derive from the fragmentation of such proteins. However, during the last decade, experts have recognized its biological importance. In this article, we’ll tell you more about it.

What are bioactive peptides?

To understand the origin of bioactive peptides, you can visualize each protein as a freight train. In these imaginary trains, the carriages correspond to amino acids. In addition, in this simile, there are 20 different types of carriages and each train can be as long as necessary. Thus, the number and combination of carriages in each protein is what makes each protein unique.

Following this same exercise, digestion would be the process of disassembling these freight trains. Peptides are the fragments that result from the digestion of proteins. We should note that the activity of these peptides is based on their composition and inherent amino acid sequence.

How are bioactive peptides in milk generated?

As we discussed above, peptides are segments that lie inactive (or encrypted) within the sequence of the original protein. Digestion releases and activates the peptides to perform various functions.

Bioactive Peptides in Milk: What Do they Do?

Bioactive peptides are derived from milk proteins and can be generated by both the action of digestive enzymes and microbial enzymes. In this regard, a very interesting fact is that breast milk contains both components:

  • Proteolytic enzymes. The most prominent are pepsin, trypsin, and chymotrypsin.
  • A microbial load, that includes a battery of lactobacillus bacteria.
    In general, the size of these active sequences can range from two to 20 amino acid residues.

Where do bioactive peptides carry out their activities?

Some of the bioactive peptides exert their effects directly on the gastrointestinal tract. However, others function in the peripheral organs after they’re absorbed through the intestinal mucosa. Furthermore, experts now know that it’s possible for the same peptide to perform multiple functions.

The functions of bioactive peptides in breast milk

The bioactive peptides derived from milk proteins perform a number of activities that affect the digestive, endocrine, cardiovascular, immune, and nervous systems. Specifically, the beneficial health effects of bioactive peptides include antimicrobial, antioxidant, antithrombotic, antihypertensive, immunomodulatory, and opioid activities, among others.

Immunomodulatory effects

Breastfeeding transmits passive immunity through multiple factors, and the gastrointestinal release of immunostimulatory peptides derived from serum proteins. For example, these are the peptides released by digestion by the enzyme trypsin:

  • ß-casein derivatives. Hexapeptide from residues 54-59 and tripeptide from residues 60-62.
  • Alpha-lactalbumin derivatives. Tripeptide of residues 51-53.

These peptides stimulate the phagocytic activity of human macrophages and stimulate the oxidative burst carried out by human polymorphonuclear leukocytes when they fight bacteria.

Antioxidant effect

Newborns, especially premature infants, are vulnerable to oxidative stress. Furthermore, they’re more susceptible to oxidative stress in diseases associated with prematurity. Such is the case of necrotizing enterocolitis, chronic lung disease, and retinopathy of prematurity (ROP).

Human milk contains many enzymatic and non-enzymatic antioxidants, such as superoxide dismutase, glutathione peroxidase, vitamins E and A, and ß-carotene. Furthermore, the peptides generated from the digestion of milk proteins, by the enzyme pepsin, are powerful antioxidants:

  • Derived from ß-casein. The peptide from seven residues, 154–160, and the peptide from three residues 169-173.
  • Derived from kappa-casein. The peptide from six residues, 31-36, and the peptide from six residues, 53-58.

The proposed antioxidant mechanism of these peptides is primarily through the extinction of free radicals by the amino acid structures of tryptophan and tyrosine residues.

Opioid peptides

Breastfeeding mothers see that their babies calm down after receiving milk. This effect is mainly attributed to the abundance of tryptophan, a precursor to serotonin, in milk proteins.

However, opioid peptides derived from milk proteins also play a role in sleep patterns. Furthermore, it’s likely that these peptides play a considerable role in the development and function of babies’ gastrointestinal tract.

Opioid peptides act by activating or inhibiting opioid receptors in the central nervous system and in peripheral tissues. This includes the gastrointestinal tract and, also, the enteric nervous system.

As of yet, experts have analyzed opioid peptides derived from ß-casein and alpha-lactalbumin designated as ß-casomorphins and alpha-lactorphines.

Opioidergic activity controls gastrointestinal function. In other words, digestive motility, electrolyte transport, and fluid secretion. Also, it regulates gastrointestinal development, for example, facilitating the production and secretion of mucin. Furthermore, these opioids exert analgesic effects, induce sleep, and promote stress adaptation.

Antimicrobial peptides

Furthermore, an increasing number of bioactive peptides with broad antimicrobial activity derived from lactoferrin, have been designated as lactoferricins.

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  • Clare, D. A., & Swaisgood, H. E. (2000). Bioactive milk peptides: A prospectus. Journal of Dairy Science, 83, 1187–1195.
  • López-Expósito, I., & Recio, I. (2006). Antibacterial activity of peptides and folding variants from milk proteins. International Dairy Journal, 16, 1294–1305.