Two novel antimicrobial peptides from skin venoms of spadefoot toad Megophrys minor.

Amphibian skin contains rich bioactive peptides. Especially, a large amount of antimicrobial peptides have been identified from amphibian skin secretions. Antimicrobial peptides display potent cytolytic activities against a range of pathogenic bacteria and fungi and play important defense roles. No antimicrobial peptides have been reported from toads belonging to the family of Pelobatidae. In this work, two novel antimicrobial peptides (Megin 1 and Megin 2) were purified and characterized from the skin venoms of spadefoot toad Megophrys minor (Pelobatidae, Anura, Amphibia). Megin 1 had an amino acid sequence of FLKGCWTKWYSLKPKCPF-NH2, which was composed of 18 amino acid residues and contained an intra-molecular disulfide bridge and an amidated C-terminus. Megin 2 had an amino acid sequence of FFVLKFLLKWAGKVGLEHLACKFKNWC, which was composed of 27 amino acid residues and contained an intra-molecular disulfide bridge. Both Megin 1 and Megin 2 showed potential antimicrobial abilities against bacteria and fungi. The MICs of Megin 1 against Escherichia coli, Bacillus dysenteriae, Staphylococcus aureus, Bacillus subtilis, and Candida albicans were 25, 3, 6.25, 3, and 50 µg·mL(-1), respectively. The corresponding MICs for Megin 2 were 6.25, 1.5, 12.5, 1.5, and 12.5 µg·mL(-1), respectively. They also exerted strong hemolytic activity against human and rabbit red cells. The results suggested that megin peptides in the toad skin of M. minor displayed toxic effects on both eukaryotes and prokaryotes. This was the first report of antimicrobial peptides from amphibians belonging to the family of Pelobatidae.

Transcriptome analysis and identification of genes related to immune function in skin of the Chinese brown frog.

The Chinese brown frog (Rana chensinensis) is a special amphibian in northern China, as it has been used widely in traditional Chinese medicine. The skin of the Chinese brown frog is also a promising resource for producing diverse antimicrobial peptides. To obtain a more comprehensive view of the metabolism and effective pharmacological components of Chinese brown frog skin, we constructed a non-normalized cDNA library from the skin. By sequencing cDNA clones at the 5' end, we obtained 5,976 high-quality EST sequences, which clustered into 512 contigs and 1379 singletons (in all 1,891 clusters). After BLAST searches of the protein and nucleic acid databases in GenBank, we found 46.7% of clusters to have significant similarity to known sequences; 28% matched Xenopus tropicalis ESTs and 29.1% matched Xenopus laevis ESTs. Gene annotation results indicated that genes related to secretion and defensive function, such as ubiquitin, lectin, and proteinase inhibitors, are highly expressed in the skin. Whey acidic-domain proteins are also highly abundant in the skin. Furthermore, both a beta-defensin and a lysozyme are transcribed in the frog skin, providing antibacterial protection. Analyses of gene ontology and KEGG metabolic pathways indicated the physiological roles of Chinese brown frog skin.