Bioactive Milk Peptides: How Do They Work?
Although the nutritional value of milk proteins is very well known, the functional value of bioactive peptides, which are derived from the fragmentation of these proteins, is not that well known. However, over the last decade, its biological importance has been recognized. In this article, we’ll talk more about that.
What are bioactive peptides?
To understand the origin of bioactive peptides, we can imagine that each protein is a freight train. In these imaginary trains, the cars correspond to amino acids.
In this model there are 20 different types of carriages and each train can be as long as necessary. In this way, the number and combination of wagons in each protein is what makes each protein unique.
Continuing with this exercise in imagination, digestion would be the process of taking these freight trains apart. Peptides are the fragments that result from the digestion of proteins. It is important to highlight that the activity of these peptides is based on their composition and the sequence of inherent amino acids.
How are bioactive milk peptides produced?
As discussed above, peptides are segments that are inactive (or encoded) within the original protein sequence. Digestion releases and activates these peptides so that they perform various functions.
Bioactive peptides are derived from milk proteins and can be generated either by the action of digestive enzymes or by microbial enzymes. It is interesting to know that breast milk itself contains both components:
- Proteolytic enzymes : the most prominent are pepsin, trypsin and chymotrypsin.
- A microbial load, which includes a battery of bacteria of the lactobacilli genus.
In general, the size of these active sequences can range from 2 to 20 amino acid residues.
Where do they work?
Some of the bioactive peptides act directly on the gastrointestinal tract. However, others act on peripheral organs after being absorbed through the intestinal mucosa. Furthermore, it is now known that it is possible that the same peptide could perform multiple functions.
What are the functions of bioactive peptides in breast milk?
Peptides derived from milk proteins have been shown to perform a number of activities that affect the digestive, endocrine, cardiovascular, immune and nervous systems.
More specifically, the effects of bioactive peptides that are beneficial to health include antimicrobial, antioxidant, antithrombotic, antihypertensive, immunomodulatory and opioid activities, among others.
Immunomodulatory effect
Breastfeeding transmits passive immunity through multiple factors, and the gastrointestinal release of immunostimulatory peptides derived from serum proteins. For example, these peptides released through digestion by the trypsin enzyme:
- β-Casein derivatives: the hexapeptide from residues 54-59 and the tripeptide from residues 60-62.
- α-lactalbumin derivatives : the tripeptide from residues 51-53.
These peptides stimulate the phagocytic activity of human macrophages and the oxidative explosion performed by human polymorphonuclear leukocytes when fighting bacteria.
Antioxidant effect
Newborns, especially premature ones, are vulnerable to oxidative stress. Furthermore, in diseases associated with prematurity, there is a greater susceptibility to oxygen stress. This is the case of necrotizing enterocolitis, chronic lung disease and retinopathy of prematurity.
Human milk contains many enzymatic and non-enzymatic antioxidants such as superoxide dismutase, glutathione peroxidase, vitamins E and A, and β-carotene. In addition, the peptides generated from the digestion of milk proteins, through the pepsin enzyme, have a powerful antioxidant activity:
- From β-casein: the 7-residue peptide 154-160 and the 3-residue peptide 169-173.
- From κ-casein: the 6-residue peptide from 31-36 and the 6-residue peptide from 53-58.
The proposed antioxidant mechanism for these peptides is mainly due to the extinction of free radicals through the amino acid structures of tryptophan and tyrosine residues.
Opioid Peptides
Breastfeeding mothers observe daily that babies calm down after receiving the milk. This effect is mainly attributed to the abundance of tryptophan, a precursor of serotonin, in milk proteins.
However, opioid peptides derived from milk proteins also play a role in sleep patterns. Furthermore, these peptides are likely to play a considerable role in the development and function of the gastrointestinal tract in infants.
Opioid peptides act by activating or inhibiting opioid receptors in the central nervous system and peripheral tissues. This includes the gastrointestinal tract or the enteric nervous system.
Until now, opioid peptides derived from β-casein and α-lactalbumin designated as β-casomorphins and α-lactorphins have been studied in detail.
Opioid activity controls gastrointestinal function, that is, digestive motility, electrolyte transport and fluid secretion.
Furthermore, it also regulates gastrointestinal development, facilitating the production and secretion of mucin, for example. These opioids also exert analgesic and sleep-inducing and stress-adaptation effects.
Antimicrobial Peptides
Furthermore, an increasing number of bioactive peptides derived from lactoferrin, with broad antimicrobial activity, have been designated as lactoferricins.
