The Research Progress of Immunomodulatory Peptides

Immunomodulatory peptides play an important role in immune response. Researchers acquired activity peptides from natural food protein by enzymatic hydrolysis and the most commonly used enzymes are Alcalase. In this paper, the mechanism of immunomodulation was summarized briefly. The peptides charge, hydrophobicity and the length of peptide chain have great influences on the immunomodulating activities of peptides. Introduction It has been reported that more than 1500 different bioactive peptides were found and more than 1250 peptides with different functional significances [1], immunomodulatory peptides was one of them. These bioactive peptides could be used for anti-oxidative, antifungal, antithrombotic, sensory physiological activities and can enhance the nutritional value of food [2]. Immunomodulatory peptides modulate [3] the immune response through improving the antibody production of secretory lgA and epithelial barrier function, modulating apoptosis and TLR4 binding, inducing regulatory T cells and B cells, and controlling cytokine production. The immunomodulatory peptides generated from natural sources usually have varieties of functions but not only for immunomodulatory. Peptide fraction from the larvae of Musca domestica not only has the ability of immunomodulatory, but also plays an important role in anti-tumor [4]. Dipeptidyl peptidase 4 inhibitors could stimulate the immune system 1 and 2 and also be used in the treatment of type 2 diabetes mellitus [5]. It has reveal immunomodulatory peptides not only for immunoregulation, but also could be used for other fields. Preparation of immunomodulatory peptides Enzymatic hydrolysis. Since Jolles acquired a sort of immunomodulatory peptides from milk protein by trypsin for the first time [6], a growing number of people have paid attention to generate peptides from natural sources such as milk, grain and shark meat. It was focused on the use of by-products of food protein bio-processing for immunomodulatory peptides which could enhance the production value and reduce the cost of waste disposal [7]. Immunomodulatory peptides show no activity when contained with the whole protein sequence, but can be released by enzymatic proteolysis. The enzymes used for hydrolysis could be divided into digestive system or microbial origin [8]. Microbial fermentation and food processing could also be used for producing immunomodulatory peptides separated from enzymatic proteolysis. As shown in Table 1, several enzymes had been successfully applied to hydrolysis kinds of natural protein sources to acquire bioactive peptides like whey protein, soy protein, corn protein, oyster protein. It can be found that from these data, the most common enzymes are Alcalase, trypsin and pepsin. The immunomodulatory peptides generated by enzymatic hydrolysis in vitro probably be absorbed more effectively than released from dietary proteins during the digestive process in the gut [9]. In recent years, food-hydrolyzed peptides enriched were added to immunomodulatory diets, like wheyhydrolyzed peptides enriched used to prevent fibrosis. 4th International Conference on Machinery, Materials and Computing Technology (ICMMCT 2016) © 2016. The authors Published by Atlantis Press 1480 Table 1 Immunomodulatory peptides derived from sources Source of peptides Conditions Indicator References enzymes pH T/ °C Time/min Labeorohita roe Pepsin 2 37 120 ABCDEF [10] Trypsin 8 37 150 Alcalase 8 55 180 Alaska pollock frame protein Trypsin 8 20 290 A [11] Whey protein Alcalase 8 50 A [12] Soy proteins Alcalase 8 60 225 AB [13] Oysters protein Protease 7.5 50 300 ABC [14] Corn protein Alkaline 8.5 55 180 G [15] Neutral 7 45 120 Green microalga Pancreatin 7.5 45 240 B [16] A-splenic lymphocyte proliferation, B-phagocytosis capacity of peritoneal macrophages, C-natural killer (NK) cell activity, D-splenic T lymphocyte, E-small intestine mucosal immunity (secretory-IgA), F-serum immunoglobulins (IgA, IgM & IgG), G-antihypertensive activity. Separation and purification. It has been reported that the molecular weight of most immunomodulatory peptides were less than 2 kDa, so the separation and purification of immunomodulatory peptides play an important role in analyzing immunomodulatory peptides activity. Milda obtained peptides of less than 3 kDa with significantly decreased the basal nuclear factor (NF)-κB activity in Caco-2 cells through Ultracel regenerated cellulose ultrafiltration membrane [17]. Chromatographic separation, especially high-performance liquid chromatography (HPLC), and gel electrophoresis were also widely used in the separation and purification of immunomodulatory peptides, but the HPLC was more effective than gel electrophoresis when be used in industry because of different throughput and costing. The mechanism of immunomodulation It is well-known that the composition of amino acid, sequence and special structure influences the immunomodulation of protein peptides. So the factors influenced immunomodulatory activities have aroused increasing interests among researchers. Bioactive peptides were composed of 3 to 20 amino acids and have a positive impact on human health. As show in Table 2, immunomodulatory peptides are usually consist of hydrophobic amino acid, such as Ala, Val, Met, Ple, Ile, Leu, Pro, Trp, is in agreement with report of Easton. However, it has been postulated that overall ability of immunomodulatory must be attributed to comprehensive effects of a lot of factors rather than to the individual effect of the hydrophobicity of amino acid. Three structurally related small molecular weight peptides with limited sequence similarity to frenatin were compared and demonstrated to the C-terminal α-amidation, which plays an important role in immunoregulation [18]. It also has been observed that lower molecular weight and positively charged peptides plays the key role when it stimulated lymphocyte proliferation at much lower concentration [19].