SYNTHETIC MELITTIN, ITS ENANTIO, RETRO, AND RETROENANTIO ISOMERS, ANDSELECTED CHIMERIC ANALOGS - THEIR ANTIBACTERIAL, HEMOLYTIC, AND LIPIDBILAYER ACTION

Melittin, the principal toxic component of bee venom, is a cationic, a mphipathic, linear peptide composed of 26 amino acids, which exhibits unique structural and biological characteristics. It has high antimicr obial activity but also has the very detrimental property of killing e ucaryotic cells, as illustrated by the lysis of sheep red cells. Sever al attempts have been made through synthesis of replacement analogs to advance the molecular understanding of the cause of these effects. We have now synthesized retro melittin, an amphipathic alpha-helical ana log with reversal of sequence and therefore of the positions of charge d and apolar residues, notably, the cluster of basic residues Lys(21)- Arg-Lys-Arg(24) near the C-terminus which is now located at positions 3-6 near the amino terminus. This peptide retained high antimicrobial activity against a range of test bacteria, but lost much of its hemoly tic properties. Modification of the N-terminal positive charge by acet ylation did not further alter the antibacterial activity or red cell l ysis. The synthetic retroenantio melittin (all-D isomer) and its acety lated derivative both retained full antibacterial activity, but with c omplete elimination of the hemolytic effect. Therefore, the two effect s of melittin have been separated. Melittin and these analogs promote electrical conductivity in lipid bilayers. Circular dichroism measurem ents showed that all of these peptides-normal, enantio, retro, retroen antio, and their acetylated derivatives-were 80-100% helical in 12-20% hexafluoro-2-propanol, a structure inducing solvent, and they are tho ught to be helical in lipid bilayers and bacterial membranes, Nonhelic al analogs are inactive. It is believed that the helix dipole plays a major part in orienting the peptides in membranes. Active sequences ar e not unique, but sequence plays a role in peptide conformation and ac tivity. Chirality has virtually no role in the antibacterial activity of normal and retro melittin analogs, which leads to the conclusion th at these peptides do not function via a receptor or by enzymatic proce ssing, but by self-aggregation and formation of ion-conducting pores.