Inhibition of Melittin Activity Using a Small Molecule with an Indole Ring

We investigated d-amino acids as potential inhibitors
targeting l-peptide toxins. Among the l- and d-amino acids tested, we found that d-tryptophan (d-Trp) acted as an inhibitor of melittin-induced hemolysis.
We then evaluated various Trp derivatives and found that 5-chlorotryptamine
(5CT) had the largest inhibitory effect on melittin. The indole ring,
amino group, and steric hindrance of an inhibitor played important
roles in the inhibition of melittin activity. Despite the small size
and simple molecular structure of 5CT, its IC50 was approximately
13 μg/mL. Fluorescence quenching, circular dichroism measurements,
and size-exclusion chromatography revealed that 5CT interacted with
Trp19 in melittin and affected the formation of the melittin tetramer
involved in hemolysis. Molecular dynamics simulation of melittin also
indicated that the interaction of 5CT with Trp19 in melittin affected
the formation of the tetramer.