Lysozyme (EC3.2.1.17) is also called Murami dase or N- acetyl murein glycan hydrolase. In 1922, British bacteriologist A. Fleming discovered that there was an enzyme in human saliva and tears that dissolved bacterial cell walls. Because of its bacteriolytic effect, it was named lysozyme. Since then, lysozyme has also been found in various tissues and secretions of human beings and animals, as well as in some plants and microorganisms. With the deepening of research, it is found that lysozyme not only dissolves bacterial cell walls, but also acts on fungal cell walls, and its mechanism is further understood. In recent years, people have applied lysozyme to medical treatment, food preservation, animal husbandry and bioengineering according to its bacteriolytic characteristics, which has certain application value.
1. Types of lysozyme
Lysozyme can be divided into two categories according to different microorganisms, namely bacterial cell wall lysozyme and fungal cell wall lysozyme. There are two kinds of lysozyme in bacterial cell wall, one is cell wall lysozyme that acts on β-1,4 glycosidic bond, and the other is cell wall lysozyme that acts on peptide "tail" and amide part. Fungal cell wall lysozyme includes yeast cell wall lysozyme and mold cell wall lysozyme.
Lysozyme is widely distributed in nature and can be found in human tissues and secretions, as well as in animal tissues, with the largest content in egg white. It also exists in other plant tissues and microbial cells. According to different sources, its nature and mechanism of action are slightly different.
1.1 Egg white lysozyme
Egg white lysozyme accounts for 3.4% ~ 3.5% of the total protein of egg white. As a typical representative of lysozyme, it is the key research object at present and one of the most clearly understood lysozymes. It consists of 18 kinds of 129 amino acid residues, with 4 S-S bonds, a molecular weight of 14 000, an isoelectric point of 11.1, an optimum temperature of 50℃ and an optimum pH value of 6-7. Its chemical properties are very stable, and its structure remains stable when the pH value changes sharply from 1.2 to 11.3. It is also very stable when exposed to heat, and it does not lose its activity when treated at pH 4 ~ 7 and 100℃ for 1min. It is a stable alkaline protein, but it has poor thermal stability under alkaline conditions.
Lysozyme has also been isolated and purified from egg white of other birds, such as quail, guinea fowl and turkey, and its activity is very similar to that of egg white lysozyme, and it is also composed of 129 amino acids. Although the arrangement order is different, the amino acid arrangement of active parts is basically the same.
1.2 Human and Mammalian Lysozyme
Human lysozyme exists in secretions such as tears, saliva, nasal mucus and milk, as well as lymph glands, white blood cells, liver, kidney and lymph tissues. 1ml of tears contains 7mg of lysozyme, and 1ml of milk contains 0.1 ~ 0.5 mg. Human lysozyme is composed of 130 amino acid residues, with 4 S-S bonds and a molecular weight of 14 600. Its bacteriolytic activity is three times higher than that of egg white lysozyme.
For mammalian lysozyme, lysozyme has been isolated from the milk of cattle, pigs, cats, rabbits, monkeys, horses, sheep and other animals. Its chemical properties are similar to those of human lysozyme, but its structure is not clear, and its bacteriolytic activity is far lower than that of human lysozyme by about 3 000 times. Ceng Lin (1999) used agar plate method to determine the content of lysozyme in rabbit colostrum. The results showed that the content of lysozyme in colostrum was (7.96 2.01) μ g/ml, and that in normal milk was (5.01 1.32) μ g/ml. The action mechanism of human and mammal lysozyme is the same as that of egg white lysozyme.
1.3 Plant lysozyme
At present, lysozyme has been isolated from papaya, fig, turnip, barley and other plants, and its molecular weight is about 24 000~29 100. The bacteriolytic activity of plant lysozyme to Micrococcus lysocladium is not more than 1/3 of that of egg white lysozyme, but its decomposition activity to colloidal chitin is 10 times that of egg white lysozyme. Zhou Zewen (1994) isolated lysozyme from Chinese cabbage. The results showed that the specific activity of lysozyme was 3 414. 6U/mg, and the purification multiple was 197.4. Lysozyme in Chinese cabbage is active in a wide range of temperature and pH, the optimum temperature is 60℃ and the optimum pH value is 5.8. Gao Xiangyang (1997) studied the bacteriostatic effect of radish lysozyme on three gram-positive bacteria and Proteus, Escherichia coli, Salmonella typhimurium, Pasteurella multocida, Salmonella pullorum, aerogenes and five fungi, namely Saccharomyces cerevisiae, Mucor racemosus, rhizopus nigricans, Aspergillus niger and Penicillium. At the same time, the bacteriostatic effect of radish lysozyme on six plant pathogenic bacteria, such as cabbage soft rot, citrus ulcer, tomato bacterial wilt, rice stripe disease, rice bacterial blight and tobacco bacterial wilt, was also studied. The results show that radish lysozyme has different antibacterial effects on the above strains.
1.4 Lysozyme produced by microorganisms
At present, lysozyme produced by microorganisms can be divided into seven categories: ① endo-N- acetylhexosaminidase, which is the same as egg white lysozyme and destroys β-1,4 glycosidic bonds in bacterial cell wall peptidoglycan; (2) amidase, which cuts off the N- acetyl muramic acid -L- alanine bond between NAM and peptide "tail" in bacterial cell wall peptidoglycan; ③ Endopeptidase breaks the peptide bond in the "tail" and "bridge" of the peptide; ④ β-1,3,β-1,6 glucanase and mannanase, which decompose the cell wall of yeast cells; ⑤ Chitosanase, acting together with glucanase, can decompose mold and yeast; ⑥ Mannanase phosphate, acting together with glucomannase, can decompose protoplasm; ⑦ Chitosan enzyme mainly decomposes Mucor and Rhizopus.
1.5 Lysozyme Produced by Phage
The enzyme is a specific enzyme, which is induced by phage infection, but it does not exist in uninfected host cells.
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