Protein nutrition occupies a very important position in animal nutrition. Traditionally, protein nutrition is essentially an amino acid nutrient. Proteins must be digested into free amino acids before they can be absorbed and utilized. However, recent studies have shown that the amount of amino acid instead of crude protein is limited, and it is also very important and necessary to directly absorb larger molecules of peptides. Therefore, it is difficult to obtain optimal growth effects by adding synthetic amino acids. There are two independent transport mechanisms for the absorption of peptides and amino acids. The absorption of small peptides has the characteristics of fast transport, low energy consumption and low saturation, while amino acids absorb slowly, consume high energy, and saturate the carrier, thus limiting the absorption in the intestine. It is currently believed that it is an important physiological phenomenon that animals absorb dipeptides or tripeptides from the gastrointestinal tract, and a considerable number of amino acids in the circulation are absorbed in the form of oligopeptides. Therefore, peptide nutrition has become a new hot spot in protein nutrition research. In recent years, a variety of biologically active peptides have been isolated from microorganisms, animals and plants, and have antioxidant, hormone, antibiotic, and seasoning functions, as well as changes in the taste of feed, and they are naturally physiologically active regulators of animal bodies. Any adverse effects of the environment can replace certain antibiotics and growth promoters, and can be used as a new type of green nutritional feed additive. Therefore, bioactive peptides have become a hot research field for peptides. This article gives an overview of the physiological functions of biologically active peptides and their application prospects in the feed industry. Biologically active peptides (BAP) refer to a class of polypeptides with a molecular weight of less than 6000D and having multiple biological functions. Its molecular structure is of varying degrees of complexity, ranging from simple dipeptides to circular macromolecular polypeptides, and these polypeptides can be modified by phosphorylation, glycosylation, or acylation. According to its function, bioactive peptides can be roughly classified into four types: physiologically active peptides, antioxidant peptides, flavoring peptides, and nutritional peptides. However, because some peptides have various physiological activities, this classification is only relative. 1. Types of biologically active peptides and their physiological functions 1.1 Physiologically active peptides 1.1.1 Antimicrobial peptides Antimicrobial peptides are usually associated with antibiotic peptides and antiviral peptides produced by bacteria and fungi. Includes cyclic peptides, glycopeptides and lipopeptides such as gramicidin, bacitracin, polymyxins, enramycin , avoparcin, virginiamycin, vancomycin, lactocin, subtilin, nisin, etc. (Kleinkauf et al. , 1998). For example, nisin is a polypeptide containing 34 amino acid residues produced by lactococcus lactis. It is an acidic molecular substance and shows high stability even at low pH such as in the stomach. It inhibits the activity of gram-positive bacteria and has the ability to inhibit the formation of spores by Clostridium and Bacillus. Antimicrobial peptides generally have high thermal stability and are ideal preservative substitutes for animal feed. In addition to the bacteria-derived antimicrobial peptides described above, more than 100 antimicrobial peptides have been isolated from vertebrates. These peptides are small in molecular weight, rich in basic amino acids Lys, Arg, and have amphoteric groups, lipophilic groups that bind to bacterial cell membranes, and hydrophilic groups that make the peptides readily soluble in body fluids, at a concentration of 4 mg/kg (MICs). (Hancock et al., 1998) has a strong antibacterial effect. In addition to microorganisms, animals and plants can produce endogenous antibacterial peptides, food proteins can also be obtained by enzymatic hydrolysis of effective antibacterial peptides, the most interesting of which is antibacterial peptides obtained from milk proteins. Several antibacterial peptides have now been obtained from lactoferricin in lactalbumin, an iron-binding glycoprotein that is considered to be a host antibacterial infection as a prototype protein. Important defense mechanism. The researchers used pepsin to split lactoferrin to purify three antimicrobial peptides that act on enterotoxigenic E. coli, both in cationic form. Two of the peptides inhibited pathogenic bacteria and food spoilage bacteria; the third peptide at a concentration of 2 umol/L inhibited the growth of Listeria monocytogenes. It has been found that these bioactive peptides have a high affinity with the base group and the cell membrane of the microorganism, and kill microorganisms by increasing the permeability of the cell membrane, and can be effective 30 min after exposure to pathogenic bacteria (Doonysius et al., 1997). Antibiotic substitutes. 1.1.2 Neuroactive peptides Neuroactive peptides include endogenous opioids, endorphins, enkephalin, and other regulatory peptides such as somatostatin and Thyrotropin releasing hormone (TRH) et al. (Lewis, 1984). They can act as hormones and neurotransmitters interacting with the μ, δ, and γ-receptors in the body, functioning as an analgesic, regulating respiration and body temperature. Endorphins, for example, can significantly affect the secretion of the stomach and pancreas. Enkephalin inhibits the release of secretin and cholecystokinin and reduces the secretion of water, enzymes and electrolytes in the pancreatic juice. Experimental studies have also found that certain peptides have important regulatory effects on digestive function and feed intake in ruminants (Ruckebusch, 1983). Meisel et al. (1990) showed that in addition to adrenal regulation, the ruminal activity of ruminants was also affected by Exorphins, an opioid bioactive peptide. Webb et al. (1990) pointed out that these peptides can transmit nutrient supply and digestive efficiency information to the central nervous system.
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