Androctonin, a hydrophilic disulphide-bridged non-haemolytic anti-microbial peptide: a plausible mode of action

Androctonin is a 25-residue non-haemolytic anti-microbial peptide isolated from the scorpion Androctonus australis and contains two disulphide bridges . Androctonin is different from known native anti-microbial peptides, being a relatively hydrophilic and non-amphipathic molecule. This raises the pos sibility that the target of androctonin might not be the bacterial membrane , shown to be a target for most amphipathic lytic peptides. To shed light o n its mode of action on bacteria and its nonhaemolytic activity, we synthes ized androctonin, its fluorescent derivatives and its all-D-amino acid enan tiomer. The enantiomer preserved high activity, suggesting a lipid-peptide interaction between androctonin and bacterial membranes. In Gram-positive a nd (at higher concentrations) Gram-negative bacteria, androctonin induced a n immediate perturbation of the permeability properties of the cytoplasmic membrane of the bacterial energetic state, concomitant with perturbation of the morphology of the cell envelope as revealed by electron microscopy. An droctonin binds only to negatively charged lipid vesicles and induces the l eakage of markers at high concentrations and with a slow kinetics, in contr ast with amphipathic alpha-helical anti-microbial peptides that bind and pe rmeate negatively charged vesicles, and to a smaller extent also zwitterion ic ones. This might explain the selective lytic activity of androctonin tow ards bacteria but not red blood cells. Polarized attenuated total reflectio n-Fourier transform infrared spectroscopy revealed that androctonin adopts a beta-sheet structure in membranes and did not affect the lipid acyl chain order, which supports a detergent-like effect. The small size of androcton in, its hydrophilic character and its physicochemical properties are favour able features for its potential application as a replacement for commercial ly available antibiotics to which bacteria have developed resistance.