The posssible role of enantiodiscrimination in bilirubin toxicity

Conformational study of melectin and antapin antimicrobial peptides in model membrane environments

Antimicrobial peptides have long been considered as promising compounds against drug-resistant pathogens. In this work, we studied the secondary structure of antimicrobial peptides melectin and antapin using electronic (ECD) and vibrational circular dichroism (VCD) spectroscopies that are sensitive to peptide secondary structures. The results from quantitative ECD spectral evaluation by Dichroweb and CDNN program and from the qualitative evaluation of the VCD spectra were compared. The antimicrobial activity of the selected peptides depends on their ability to adopt an amphipathic α-helical conformation on the surface of the bacterial membrane. Hence, solutions of different zwitterionic and negatively charged liposomes and micelles were used to mimic the eukaryotic and bacterial biological membranes. The results show a significant content of α-helical conformation in the solutions of negatively charged liposomes mimicking the bacterial membrane, thus correlating with the antimicrobial activity of the studied peptides. On the other hand in the solutions of zwitterionic liposomes used as models of the eukaryotic membranes, the fraction of α-helical conformation was lower, which corresponds with their moderate hemolytic activity.

Chiral recognition of bilirubin and biliverdin in liposomes and micelles

The structural formula of biologically important chiral pigments bilirubin and biliverdin differs only by one double bond. We showed that this results in dissimilar interactions with two models of membranes: cationic liposomes composed of 3β-[N-(N',N'-dimethylaminoethane)-carbamoyl]cholesterol and zwitterionic micelles from 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS). While the liposomes recognized the P-form of bilirubin, the micelles recognized its M-form. Both recognized the P-form of biliverdin. Our study also comprised ternary systems consisting of the pigments, model membranes and serum albumin (human and bovine). Bilirubin preferentially interacted with the albumins even in the presence of the liposomes. On the other hand, biliverdin preferred the liposomes. Remarkably, the presence of CHAPS completely changed the biliverdin binding to the protein. Because our study was oriented on different chiral interactions, a chiroptical method of electronic circular dichroism was chosen as the principal method to study our systems. As complementary methods, UV-vis absorption and fluorescence emission were used.

A Synthetic mirror image of kalata B1 reveals that cyclotide activity is independent of a protein receptor

Featuring a circular, knotted structure and diverse bioactivities, cyclotides are a fascinating family of peptides that have inspired applications in drug design. Most likely evolved to protect plants against pests and herbivores, cyclotides also exhibit anti-cancer, anti-HIV, and hemolytic activities. In all of these activities, cell membranes appear to play an important role. However, the question of whether the activity of cyclotides depends on the recognition of chiral receptors or is primarily modulated by the lipid-bilayer environment has remained unknown. To determine the importance of lipid membranes on the activity of the prototypic cyclotide, kalata B1, we synthesized its all-D enantiomer and assessed its bioactivities. After the all-D enantiomer had been confirmed by (1)H NMR to be the structural mirror image of the native kalata B1, it was tested for anti-HIV activity, cytotoxicity, and hemolytic properties. The all-D peptide is active in these assays, albeit with less efficiency; this reveals that kalata B1 does not require chiral recognition to be active. The lower activity than the native peptide correlates with a lower affinity for phospholipid bilayers in model membranes. These results exclude a chiral receptor mechanism and support the idea that interaction with phospholipid membranes plays a role in the activity of kalata B1. In addition, studies with mixtures of L and D enantiomers of kalata B1 suggested that biological activity depends on peptide oligomerization at the membrane surface, which determines affinity for membranes by modulating the association-dissociation equilibrium.