Improving the Selectivity of Antimicrobial Peptides from Anuran Skin

Anuran skin is known to be a rich source of antimicrobial
peptides although their therapeutic potential is often limited due
to their toxicity against mammalian cells. The analysis of structure–activity
relationships among anuran antimicrobial peptides provided the parameters
to construct the “Mutator” tool for improving their
selectivity for bacterial cells, by suggesting appropriate point substitutions.
Double substitution analogues [K2, K16] of the Xenopus tropicalis peptide XT-7 and [I2, K19] of the Ascaphus truei peptide ascaphin-8 were predicted by this tool to have an increased
‘therapeutic index’ (TI = HC50/MIC for erythrocytes
with respect to bacteria) > 80. The mutated peptides were synthesized
and respectively found to have experimental TI values > 130 for S. aureus or E. coli, a considerable improvement
with respect to TI < 37 for the parent compounds. Circular dichroism
studies of the mutated peptides suggested this may in part be due
to variations in the α-helical structure. For P. aeruginosa, which is more resistant to XT-7, the TI increased in the mutated
peptide from 5 to >270, also due to a significant improvement in
minimal inhibitory concentration. We have shown that the Mutator tool
is capable of suggesting limited variations in natural anuran peptides
capable of increasing peptide selectivity, by decreasing toxicity
against mammalian erythrocytes, in general without compromising antibacterial
activity. The tool is freely available on the Mutator Web server at http://split4.pmfst.hr/mutator/.