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Milk is an excellent source of well balanced nutrients and also exhibits a range of biological activities that influence digestion, metabolic responses to absorbed nutrients, growth and development of specific organs, and resistance to disease. These biological activities are mainly due to the peptides and proteins in milk. However, some of the biological activity of milk protein components is latent, and is released only upon proteolytic action. Bioactive peptides are produced during digestion of milk in the gastrointestinal tract, and also during fermentation and food processing.
Bioactive peptides have been defined as specific protein fragments that have a positive impact on body functions or conditions and may ultimately influence health. Upon oral administration, bioactive peptides, may affect the major body systems—namely, the cardiovascular, digestive, immune and nervous systems. The beneficial health effects may be classified as antimicrobial, antioxidative, antithrombotic, antihypertensive, antimicrobial or immunomodulatory (FitzGerald and Meisel, 2003; Korhonen and Pihlanto, 2003a).
The activity of these biofunctional peptides is based on their inherent amino acid composition and sequence. The size of active sequences may vary from two to twenty amino acid residues, and many peptides are known to have multifunctional properties (Meisel and FitzGerald, 2003) e.g., peptides from the sequence 60-70 of ß-casein show immunostimulatory, opioid and angiotensin I converting enzyme (ACE)-inhibitory activities. This sequence has been defined as a strategic zone (Migliore-Samour and Jolles, 1988; Meisel, 1998). The sequence is protected from proteolysis because of its high hydrophobicity and the presence of proline residues. Other examples of the multifunctionality of milk-derived peptides include the αs1-casein fraction 194-199 showing immunomodulatory and ACE-inhibitory activity, the opioid peptides α- and ß-lactorphin also exhibiting ACE-inhibitory activity and the calcium-binding phosphopeptides (CPPs), which possess immunomodulatory properties (Korhonen and Pihlanto, 2003a).
Source of bioactive peptides
Milk is a rich source of protein. Casein and whey proteins are the two main protein groups in milk, caseins comprises about 80 percent of the total protein content in bovine milk and are divided into α-, ß- and κ-caseins. Whey protein is composed of ß-lactoglobulin, α-lactalbumin, immunoglobulins (IgGs), glycomacropeptides, bovine serum albumin, and minor proteins such as lactoperoxidase, lysozyme and lactoferrin. Each of the subfractions found in casein or whey has its own unique biological properties. Milk proteins can be degraded into numerous peptide fragments by enzymatic proteolysis and serve as source of bioactive peptides.