1. IntroductionThe role of proteins as physiologically active components in the diet is beingincreasingly acknowledged. Many of the proteins that occur naturally in raw foodmaterials exert their physiological action either directly or upon enzymatic hydrolysisin vitro or in vivo. In recent years it has been recognized that dietary proteins providea rich source of biologically active peptides. Such peptides are inactive within thesequence of the parent protein and can be released in three ways: (a) throughhydrolysis by digestive enzymes, (b) through hydrolysis by proteolyticmicroorganisms and (c) through the action of proteolytic enzymes derived frommicroorganisms or plants.Bioactive peptides have been defined as specific protein fragments that have apositive impact on body functions or conditions and may ultimately influence health(Kitts & Weiler, 2003). Upon oral administration, bioactive peptides, may affect themajor body systems namely, the cardiovascular, digestive, immune and nervoussystems (Fig. 1)—depending on their amino acid sequence. For this reason, thepotential of distinct dietary peptide sequences to promote human health by reducingthe risk of chronic diseases or boosting natural immune protection has aroused a lot ofscientific interest over the past few years. These beneficial health effects may beattributed to numerous known peptide sequences exhibiting, e.g., antimicrobial,antioxidative, antithrombotic, antihypertensive and immunomodulatory activities.(FitzGerald & Meisel, 2003).
2. Production of bioactive peptidesbiologically active peptides can be produced from precursor milk proteins inthe following ways: (a) enzymatic hydrolysis by digestive enzymes, (b) fermentationof milk with proteolytic starter cultures, (c) proteolysis by enzymes derived frommicroorganisms or plants. In many studies, combination of (a) and (b) or (a) and (c),has proven effective in generation of short functional peptides (Korhonen & Pihlanto,2003b). Examples of bioactive peptides produced by the above treatments are givenbelow.2.1 Enzymatic hydrolysisThe most common way to produce bioactive peptides is through enzymatichydrolysis of whole protein molecules. Many of the known bioactive peptides havebeen produced using gastrointestinal enzymes, usually pepsin and trypsin.Angiotensin-converting enzyme (ACE)-inhibitory peptides and calcium-bindingphosphopeptides (CPPs), for example, are most commonly produced by trypsin(FitzGerald et al, 2004).Other digestive enzymes and different enzyme combinations of proteinasesincluding alcalase, chymotrypsin, pancreatin, pepsin and thermolysin as well asenzymes from bacterial and fungal sources—have also been utilized to generatebioactive peptides from various proteins (Kilara & Panyam, 2003).
2.2 Microbial fermentationMany industrially utilized dairy starter cultures are highly proteolytic.Bioactive peptides can, thus, be generated by the starter and non-starter bacteria usedin the manufacture of fermented dairy products. The proteolytic system of lactic acidbacteria (LAB), e.g. Lactococcus lactis, Lactobacillus helveticus and Lb. delbrueckiissp. bulgaricus, is already well characterized. This system consists of a cell wall-bound proteinase and a number of distinct intracellular peptidases, includingendopeptidases, aminopeptidases, tripeptidases and dipeptidases (Christensenet al, 1999).Table 1. Examples of bioactive peptides released from milk proteins by variousmicroorganisms and microbial enzymesMicro-organisms usedPrecursorproteinaPeptide sequence BioactivityLactobacillus helveticus,Saccharomyces cerevisiaeb-cn, k-cn Val-Pro-Pro, Ile-Pro-ProACE inhibitory,antihypertensiveLb. delbrueckii subsp.bulgaricus IFO13953k-cnAla-Arg-His-Pro-His-Pro-His-Leu-Ser-Phe-MetAntioxidativeLb. delbrueckii subsp.bulgaricusb-cnSer-Lys-Val-Tyr-Pro-Phe-Pro-Gly Pro-IleACE inhibitoryLb. helveticus ICM 1004cellfreeextractSkim milkhydrolysateVal-Pro-Pro, Ile-Pro-Pro ACE inhibitoryaAbbreviations: cn ¼ casein, ACE ¼ angiotensin I-converting enzyme.
2.2.1 A fermented milk high in bioactive peptides has a bloodpressure–lowering effect in hypertensive subjectsHypertension is a risk factor for cardiovascular diseases, including coronaryheart disease, peripheral arterial disease, and stroke. The renin-angiotensin system isan important regulator of blood pressure. Therefore, drugs that inhibit the renin-angiotensin system, either by inhibiting angiotensin-converting enzyme (ACE; EC184.108.40.206) or by blocking angiotensin (AT1) receptors, are widely used in the treatmentof hypertension. ACE inhibitors have a dual effect on the renin-angiotensin system:they inhibit the production of the vasoconstrictor angiotensin II and they inhibit thedegradation of the vasodilator bradykinin. In addition, ACE inhibitors have otherbeneficial effects in hypertensive patients, for example, in those with cardiac or renalinsufficiency or diabetes. Through fermentation, peptides that have an ACE-inhibiting and thus a blood pressure–lowering effect can be derived from milkproteins.3. Caseins as source of bioactive peptidesCasein is the main proteinaceous component of milk, where it accounts forca. 80% of the total protein inventory. Until recently, the main physiological role ofcasein in the milk system was widely accepted to be a source of amino acids requiredby growth of the neonate. However, the dominant physiological feature of thecasein micelle system has more recently been proven to be the prevention ofpathological calcification of the mammary gland (Holt, 1997). While no specificphysiological property has been proposed for the whole casein system (or its
individual fractions, for that matter), various peptides hidden (or inactive) in theamino-acid sequence have been the subject of increasingly intense studies. Muchwork regarding those peptides, which are known to possess bioactivities, is currentlyunderway regarding their release via selective enzymatic hydrolysis.4. Bioactive peptides in whey proteinsTwo tetrapeptides in the primary structure of whey proteins have potentialopioid activities: Tyr-Gly-Leu-Phe (residues 50-53) from human and bovine α-lactalbumin and Tyr-Leu-Leu-Phe (residues 102-105) from bovine β- lactoglobulin.The corresponding amides - named α and β lactorphin, respectively - werechemically synthesized and their opioid activity was established.5. Physiological effects of bioactive peptides from milk5.1 Effects on the nervous systemIt is a common belief that falling asleep is easier after drinking aglass of milk in the evening, and that babies are soothed after breast or bottlefeeding. Recent studies have provided evidence that peptides exist in dairyproducts which play an active role in the nervous system; known as opioid peptides,they can have agonistic or antagonistic activities
5.2 Effects on the immune systemThe systems involved in the human body’s defenseagainst invaders are rather complex; diet is known to play an important role therein.Research concerning the role of functional peptides in this field is quite recent,but it already seems very promising. The two main activities are theimmunomodulatory one (i.e. stimulation of the immune system) (Table 4) and theantimicrobial one (i.e. inhibition of pathogenic bacteria).5.3 Effects on the nutrition systemSome peptides are able to sequester calcium and other minerals,hence acting as biocarriers—they are called phosphopeptides glycomacropeptide(GMP) may also exhibit a number of nutritional features.6. conclusionBioactive peptides are ubiquitous biomolecules widely abundant and easilyobtainable from food proteins. There is no limit therefore to the number of peptidesthat can be obtained from a single food protein.Each of these peptides may present unique structure and biofunctionalities that canbe exploited in the pharmaceutical industry. As research continues to uncovertechnologies and means to overcome challenges to the use of peptide therapeutics,the prospects of
food-derived bioactive peptides will likely fuel in the pharmaceutical industry anexodus from small molecules and biologics to bioactive peptides.7. DiscussionThe potential health benefits of milk protein-derived peptides have been asubject of growing commercial interest in the context of health-promoting functionalfoods. So far, antihypertensive, mineral-binding and anticariogenic peptides havebeen most studied for their physiological effects. A few commercial developmentshave been launched on the market and this trend is likely to continue alongside withincreasing knowledge about the functionalities of the peptides. The optimalexploitation of bioactive peptides for human nutrition and health possessesan exciting scientific and technological challenge, while at the same time offeringpotential for commercially successful applications. Bioactive peptides can beincorporated in the form of ingredients in functional and novel foods, dietarysupplements and even pharmaceuticals with the purpose of delivering specific healthbenefits.
8. Referencesfunctionality, (945 – 960)Seppo.L, Jauhiainen.T, Poussa,T, Korpela.R. A fermented milk high in bioactivepeptides has a blood pressure–lowering effect in hypertensive subjects.Meisel.H, Frister.H, Sehlimme.E (1989). Biologically active peptides in milkproteins. (267-278).Sofia V. Silva, F. Caseins as source of bioactive peptidesPihlanto. A, Korhonen. H . (2011) . Review Bioactive peptides: Production andBiotechnologyAgyei.D. (2012). Pharmaceutical applications of bioactive peptides.OA