BIOACTIVE PEPTIDES OF MILK
What are bio active peptides?
Bioactive peptides have been defined as specific protein fragments that have a positive impact on body functions or conditions and may ultimately influence health. The activity of peptides is influenced by their inherent amino acid composition and sequence. The size of active peptide sequences in milk may vary from 2 to 20 amino acid residues, and many peptides are known to exert several bioactive properties.
At present, among the food protein resources available the milk proteins are considered as one of the most important sources of bioactive peptides. Continuous research carried out during the past decade resulted in identification of a great number of peptide sequences with different bioactivities in various milk proteins. Some of the bioactive properties of milk proteins are antihypertensive, antithrombotic, antimicrobial, anti-oxidative, immunomodulatory, and opioid like activities in nature.
Where can I find these bioactive peptides?
Bioactive peptides have been found in fermented dairy products and enzymatic protein hydrolysates. They can also be released during gastrointestinal digestion of milk proteins. Bioactive peptides, as they exist within the sequence of the parent protein molecule are inactive and becomes active when they are released from precursor proteins in the following ways.
(1) By enzymatic hydrolysis of digestive enzymes
(2) By fermentation of milk due to lactic acid bacteria and
(3) By proteolysis caused by enzymes derived from microorganisms or plants.
A successive combination of the above discussed three ways results in generating many active peptides both from caseins and whey proteins of milk.
During digestion, the gastric enzymes such as pepsin, trypsin and chymotrypsin have been shown to liberate a large number of peptides exhibiting anti-hypertensive, antibacterial, immunomodulatory effects from the constituents of whey protein molecules viz. alpha lactalbumin, beta lactoglobulin and to certain extent, the lactoferrin. At present, the blood pressure reducing peptides that inhibit the angiotensin converting enzyme I (ACE) are the most studied ones.
Casein hydrolysates have been shown to produce higher ACE inhibitory activity than whey protein hydrolysates. However, whey peptides too exhibited strong antihypertensive activity. Other minor proteolytic enzymes such as alcalase, thermolysin and subtilisin too proved ACE inhibitory. Whey proteins when successively treated with pepsin and trypsin to simulate gastrointestinal digestion resulted in release of individual whey proteins or whey protein products possessing bioactive properties, including antihypertensive or ACE inhibitory, antibacterial, immunomodulatory and opioid – like in nature.
Many of the industrially employed lactic acid bacteria based starter cultures used in the manufacture of fermented dairy products are highly proteolytic and can release different bioactive peptides from milk proteins through microbial proteolysis. Lactobacillus helveticus strains during proteolysis have been shown capable of releasing anti-hypertensive peptides, the best known of which are casein - derived ACE - inhibitory tripeptides Val - Pro - Pro and Ile - Pro - Pro. The anti-hypertensive properties of these peptides have been demonstrated in many animal and human studies.
The cheese starter bacteria, commercial probiotic bacteria and yogurt bacteria viz. Streptococcus thermophilus and Lactobacillus bulgaricus have been shown to produce different bioactive peptides in milk during fermentation. It is shown that fermentation of milk with a commercial starter culture mixture of five lactic acid bacterial strains followed by hydrolysis with a microbial protease increased ACE inhibitory activity of the hydrolysate and two strong ACE - inhibitory tripeptides (Gly - Thr - Trp and Gly - Val - Trp) were identified. It is generally construed that the benefits obtained from the in vitro studies will be achieved in in-vivo studies too.
Continuous research during the past years, especially based on membrane separation techniques resulted in enrichment and isolation of peptides with a specific molecular weight range. In particular, nanofiltration and ultrafiltration techniques are often employed to fractionate and enrich specific bioactive peptides from casein or whey protein hydrolysates.