| Nowadays all kinds of infection illness threaten seriously human. More and more antibitotics are development. But in the recent years strains of microorganisims that are resistant to commonly used antibiotics appeared. The new antimicrobial drus become more and more important to us.Antimicrobial peptides are a kind of important and small molecular immune proteins in cell-free immunity of plants and animals. They have small molecular mass, good thermostability and antigenicity. They have broadspectrum antimicrobial activity. Some of them are active against some viruses, fungi and cancer cell lines, but they are harmless to human normal cells. Recently, some reports on cancer cell lines make people more interested in them. It is possible that antimicrobial peptides may become a kind of new antimicrobial and antitumoral drug. G. Boman, a scientist of Sweden, established the existence of antimicrobial peptides- Cecropins in 1975. Subsequently, many antimicrobial peptides are discovered, including bacteria, fungi, insects, amphibians and mammals etc. They all produce antimicrobial peptides. But every animal has different peptides, such as Magainins, Protegrins,Melitins, Defensins. And the endogenous antimicrobial peptides of plants and animals are typically amphipathic molecules composed of 12 to 45 amino acid residues. These peptides may be produced constitutively or only after infection or injury, and they often contain excess lysine and arginine residues.According to their characters, they were divided into four groups. 1) The cecropins. They are 4KDa peptides and active on both Gram-negative and Gram-positive cells and devoid of cysteines and have the structure of two amphipathic α-helices. 2) The insect defensins. They are 4KDa anti-Gram-positive peptides with six cysteines engaged in three intramolecular disulfide bridges. 3) Small, proline-rich peptides. They are active against Gram- negativebacteria and have been isolated from Hymen-opera, Diptera and Hemoptera. A remarkable feature of the peptides is the presence of an Oglycosylated substitution. 4) Glycine-rich polypeptides. The molecular weights of the peptides range from 10 –30KDa. They are active essentially on Gram-negative bacteria. Frog that lives in all over the world except the Sourth Pole is a specieses of amphibians. Their skins are bare and wet that provide a excellently srvival circums- tance for tye microorganisms. During the processes of the natural selection, theirskins developed recovery system that seceret polypeptide from from the back and belly to resist attack of the nocuousness fators.To date, we can't have a certein theory to explain how the antimicrobial peptides play their roles. But the mechanism of the antimicrobial action of the antimicrobial peptides, including the well-studied peptides melittin, magainin, cecropin and defensins; has been studied in much detail. And the main site of action of the peptides is the cytoplasmic membranes--the peptides tends to assemble to form channels. At the same time, some alternative hypotheses state that the peptides cluster at the membrane surface and cause a cooperative permeabilization of the cytoplasmic membrane. Furthermore, in Gram-negative bacteria, the antimicrobial peptides initially interact with lipopolysacchride, a highly anionic outer-membrane glycolipid, and then disrupt the membrane locally to access the cytoplasmic membrane. These hypotheses of mechanism explain many of the clinically desirable properties of antimicrobial peptides. Antimicrobial peptides have been used in medical, food, industry and plant genetic engineering, and they have great potential in those fields. Because antimicrobial peptides tend to be involved as a localresponse to infection, it has been assumed that they may be limited to the treatment of topical infections. By contrast, a lot of evidence has demonstrated efficacy against systemic infections. This includes α-helical-peptide efficacy against P.aeruginosa peritoneal infections, and extended-helix indolicidin against A...
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