Maximin 3

Maximin 3 is a 27-residue-long cationic antimicrobial peptide found in the skin secretion and brain of the Chinese red-belly toad Bombina maxima. The peptide is of biological interest as it possesses anti-HIV activity, not found in the other maximin peptides, in addition to antimicrobial, antitumor and spermicidal activities. The three-dimensional structure of maximin 3 was obtained in a 50/50% water/2,2,2-trifluoroethanol-d3 mixture using two-dimensional NMR spectroscopy. Maximin 3 was found to adopt an α-helical structure from residue G1 to A22, and a coil structure with a helical propensity in the C-terminal tail. The peptide is amphipathic, showing a clear separation between polar and hydrophobic residues. Interactions with sodium dodecyl sulfate micelles, a widely used bacterial membrane-mimicking environment, were modeled using molecular dynamics simulations. The peptide maintained an α-helical conformation, occasionally displaying a flexibility around residues G9 and G16, which is likely responsible for the peptide's low haemolytic activity. It is found to preferentially adopt a position parallel to the micellar surface, establishing a number of hydrophobic and electrostatic interactions with it.

Here we report the first major example of anionic amphibian host defence peptides (HDPs) with anticancer activity. Maximin H5 (MH5N) is a C-terminally amidated, anionic host defence peptide from toads of the Bombina genus, which was shown to possess activity against the glioma cell line, T98G (EC50 = 125 μM). The peptide adopted high levels of α-helical structure (57.3%) in the presence of model cancer membranes (DMPC:DMPS in a molar ratio of 10:1). MH5N also showed a strong ability to penetrate these model membranes (Π = 10.5 mN m-1), which correlated with levels of DMPS (R2 > 0.98). Taken with the high ability of the peptide to lyse these membranes (65.7%), it is proposed that maximin H5 kills cancer cells via membranolytic mechanisms that are promoted by anionic lipid. It was also found that C-terminally deaminated maximin H5 (MH5C) exhibited lower levels of α-helical structure in the presence of cancer membrane mimics (44.8%) along with a reduced ability to penetrate these membranes (Π = 8.1 mN m-1) and induce their lysis (56.6%). These data suggested that the two terminal amide groups of native maximin H5 are required for its optimal membranolytic and anticancer activity.

Amphibian skin is a rich resource of antimicrobial peptides, like maximins and maximin Hs from frog Bombina maxima. Novel cDNA clones encoding a precursor protein, which comprises a novel maximin peptide (maximin 9) and reported maximin H3, were isolated from two constructed skin cDNA libraries of B. maxima. The predicted primary structure of maximin 9 is GIGRKFLGGVKTTFRCGVKDFASKHLY-NH2. A surprising substitution is at position 16, with a free cysteine in maximin 9 rather than usual conserved glycine in other reported maximins. Maximin 9, the homodimer form and its Cys16 to Gly16 mutant were synthesized and their antimicrobial activities were evaluated. Unlike previously reported maximin 3, the tested bacterial and fungal strains were resistant to maximin 9, its homodimer and the Cys16 to Gly16 mutant (with MICs>100 microM). On the other hand, interestingly, while eight clinical Mollicutes strains were generally resistant to maximin 9 homodimer and its Cys16 to Gly16 mutant, most of them are sensitive to maximin 9 at a peptide concentration of 30 microM, especially in the presence of dithiothreitol. These results indicate that the presence of a reactive Cys residue in maximin 9 is important for its antimycoplasma activity. The diversity of antimicrobial peptide cDNA structures encountered in B. maxima skin cDNA libraries and the antimicrobial specificity differences of the peptides may reflect well the species' adaptation to the unique microbial environments.