Enantioselective semipreparative HPLC separation of PCB metabolites and their absolute structure elucidation using electronic and vibrational circular dichroism

Polychlorinated biphenyls (PCBs) remain one of the most important groups of environmental contaminants. The fate (transformation) as well as the toxicological implications of the different metabolism steps are subject to considerable debate. The aim of this study is to start a comprehensive investigation of atropisomeric PCB metabolites, i.e., hydroxy, methoxy, methylthionyl, and methylsulfonyl PCBs in different biota. For this purpose, enantioselective semipreparative liquid chromatography is used to obtain pure enantiomers of PCB metabolites. Electronic circular dichroism (UV-CD) and vibrational circular dichroism (VCD) in combination with computational techniques were applied to determine their absolute structures. Approximately 18-25 mg of each enantiomer of the following metabolites were obtained using semipreparative HPLC on beta-cyclodextrin-based columns: 4-MeO-CB149, 4-MeS-CB149, 4-MeSO2-CB149, 3-MeS-CB149, and 3-MeSO2-CB149. The enantiomeric purity of the separated enantiomers was in the range of 95.0-99.9%. Rotational angles and absolute configurations were also determined. This study establishes a sound method for future preparation and absolute structure determination of compounds belonging to the same class.     

Enucleation and development of cluster headache: a retrospective study

Background  

Cluster headache (CH) is a neurovascular, primary headache disorder. There are, however, several case reports about patients whose CH started shortly after a structural brain disease or trauma. Motivated by a patient who developed CH 3 weeks after the removal of an eye and by similar case reports, we tested the hypothesis that the removal of an eye is a risk factor for CH.     

Degradation of cyanophycin by Sedimentibacter hongkongensis strain KI and Citrobacter amalonaticus strain G Isolated from an anaerobic bacterial consortium

Using a combination of various enrichment techniques, the strictly anaerobic, gram-positive, endospore-forming bacterium Sedimentibacter hongkongensis strain KI as revealed by 16S rRNA analysis and the gram-negative enterobacterium Citrobacter amalonaticus strain G as revealed by physiological tests were isolated from an anaerobic cyanophycin (CGP)-degrading bacterial consortium. S. hongkongensis strain KI is the first anaerobic bacterium with the ability to hydrolyze CGP to beta-Asp-Arg and beta-Asp-Lys dipeptides, as revealed by electrospray ionization-mass spectrometry and reversed-phase high-performance liquid chromatography analysis. However, these primary accumulated hydrolysis products were only partially used by S. hongkongensis strain KI, and significant growth on CGP did not occur. On the other hand, C. amalonaticus strain G did not degrade CGP but grew on the beta-linked iso-dipeptides formed in vitro by enzymatic CGP degradation or in vivo by metabolic activity of S. hongkongensis strain KI. Dipeptide utilization occurred at the highest rate if both strains were used in cocultivation experiments with CGP, indicating that cooperation between different bacteria occurs in anaerobic natural environments for complete CGP turnover. The amino acids obtained from the cleavage of dipeptides were fermented to ethanol, acetic acid, and succinic acid, as revealed by gas chromatographic analysis and by spectrophotometric enzyme assays.     

A two-alpha-helix extra domain mediates the halophilic character of a plant-type ferredoxin from halophilic archaea

The [2Fe-2S] ferredoxin (HsFdx) of the halophilic archaeon Halobacterium salinarum exhibits a high degree of sequence conservation with plant-type ferredoxins except for an insertion of 30 amino acids near its N-terminus which is extremely rich in acidic amino acids. Unfolding studies reveal that HsFdx has an unfolding temperature of approximately 85 degrees C in 4.3 M NaCl, but of only 50 degrees C in low salinity, revealing its halophilic character. The three-dimensional structure of HsFdx was determined by NMR spectroscopy, resulting in a backbone rmsd of 0.6 A for the diamagnetic regions of the protein. Whereas the overall structure of HsFdx is very similar to that of the plant-type ferredoxins, two additional alpha-helices are found in the acidic extra domain. (15)N NMR relaxation studies indicate that HsFdx is rigid, and the flexibility of residues is similar throughout the molecule. Monitoring protein denaturation by NMR did not reveal differences between the core fold and the acidic domain, suggesting a cooperative unfolding of both parts of the molecule. A mutant of the HsFdx in which the acidic domain is replaced with a short loop of the nonhalophilic Anabaena ferredoxin shows a considerably changed expression pattern. The halophilic wild-type protein is readily expressed in large amounts in H. salinarum, but not in Escherichia coli, whereas the mutant ferredoxin could only be overexpressed in E. coli. The salt concentration was also found to play a critical role for the efficiency of cluster reconstitution: the cluster of HsFdx could be reconstituted only in a solution containing molar concentrations of NaCl, while the reconstitution of the cluster in the mutant protein proceeds efficiently in low salt. These findings suggest that the acidic domain mediates the halophilic character which is reflected in its thermostability, the exclusive expression in H. salinarum, and the ability to efficiently reconstitute the iron-sulfur cluster only at high salt concentrations.     

Reference values for serum silicon in adults

Silicon is an essential nutrient of fundamental importance to human biology. It has been shown that silicon is required for bone, cartilage, and connective tissue formation. However, the assessment of silicon concentration is difficult as reference values are lacking. The aim of the present study was to establish reference values for apparently healthy individuals. Silicon concentrations were determined in serum of 1325 healthy subjects 18-91 years of age using atomic absorption spectrometry. Medians for serum silicon concentrations showed a statistically significant age and sex dependency. In men 18-59 years of age the median was 9.5 micromol/L and decreased to 8.5 micromol/L at 60-74 years of age. In women there was an increase in the median from age 18-29 years (10.00 micromol/L) to 30-44 years (11.10 micromol/L) followed by a decrease in the age group of 45-59 years (9.23 micromol/L). In subjects aged over 74 years the median serum silicon values were 7.70 micromol/L for men and 8.00 micromol/L for women. The most important findings in this study are the decrease of silicon and the course of the silicon concentrations with age, especially in women. The present study is an important prerequisite for studies that aim to identify the health effects and medical implications of silicon.     

Assessing oligomerization of membrane proteins by four-pulse DEER: pH-dependent dimerization of NhaA Na+/H+ antiporter of E. coli

The pH dependence of the structure of the main Na(+)/H(+) antiporter NhaA of Escherichia coli is studied by continuous-wave (CW) and pulse electron paramagnetic resonance (EPR) techniques on singly spin-labeled mutants. Residues 225 and 254 were selected for site-directed spin labeling, as previous work suggested that they are situated in domains undergoing pH-dependent structural changes. A well-defined distance of 4.4 nm between residues H225R1 in neighboring molecules is detected by a modulation in double electron-electron resonance data. This indicates that NhaA exists as a dimer, as previously suggested by a low-resolution electron density map and cross-linking experiments. The modulation depth decreases reversibly when pH is decreased from 8 to 5.8. A quantitative analysis suggests a dimerization equilibrium, which depends moderately on pH. Furthermore, the mobility and polarity of the environment of a spin label attached to residue 225 change only slightly with changing pH, while no other changes are detected by CW EPR. As antiporter activity of NhaA changes drastically in the studied pH range, residues 225 and 254 are probably located not in the sensor or ion translocation sites themselves but in domains that convey the signal from the pH sensor to the translocation site.     

Identification of a novel pharmacophore for peptide toxins interacting with K+ channels

KappaM-conotoxin RIIIK blocks TSha1 K+ channels from trout with high affinity by interacting with the ion channel pore. As opposed to many other peptides targeting K+ channels, kappaM-RIIIK does not possess a functional dyad. In this study we combine thermodynamic mutant cycle analysis and docking calculations to derive the binding mode of kappaM-conotoxin RIIIK to the TSha1 channel. The final model reveals a novel pharmacophore, where no positively charged side chain occludes the channel pore. Instead the positive-charged residues of the toxin form a basic ring; kappaM-RIIIK is anchored to the K+ channel via electrostatic interactions of this basic ring with the loop and pore helix residues of the channel. The channel amino acid Glu-354 is likely to be a fundamental determinant of the selectivity of kappaM-RIIIK for the TSha1 channel. The Cgamma-OH of Hyp-15 is in contact with the carbonyls of the selectivity filter, disturbing the charge distribution pattern necessary for the coordination of K+ ions. This novel, experimentally based pharmacophore model proves the existence of diverse binding modes of peptidic toxins to K+ channels and underlines the role of intermolecular electrostatic interactions involving channel loop side chains in determining the selectivity of toxins for specific K+ channel types.     

Characterization of Lck-binding elements in the herpesviral regulatory Tip protein

Herpesvirus saimiri encodes a tyrosine kinase interacting protein (Tip) that binds to T-cell-specific tyrosine kinase Lck via multiple sequence motifs and controls its activity. The regulation of Lck by Tip represents a key mechanism in the transformation of human T-lymphocytes during herpesviral infection. In this study, the interaction of Tip with the regulatory SH3 and SH2 domains of Lck was investigated by biophysical and computational techniques. NMR spectroscopy of isotopically labeled Tip(140-191) revealed that the interaction with the LckSH3 domain is not restricted to the classical proline-rich motif, but also involves the C-terminally adjacent residues which pack into a hydrophobic pocket on the surface of the SH3 domain, thus playing a likely role in mediating binding specificity. Fluorescence binding studies of Tip further demonstrate that Tyr127 in its phosphorylated form represents a strong ligand of the LckSH2 domain, indicating the presence of an additional Lck interaction motif. In contrast, Tyr114, known to be essential for STAT-3 binding, does not interact with the LckSH2 domain, showing that the tyrosines in Tip exhibit distinct binding specificity. The existence of numerous interaction sites between Tip and the regulatory domains of Lck implies a complex regulatory mechanism and may have evolved to allow a gradual regulation of Lck activity in different pathogenic states.     

APOBEC3G incorporation into human immunodeficiency virus type 1 particles

APOBEC3G is promiscuous with respect to its antiretroviral effect, requiring that it be packaged into diverse retrovirus particles. Here, we show that most virally encoded human immunodeficiency virus type 1 particle components are dispensable for APOPEC3G incorporation. However, replacement of the nucleocapsid (NC) Gag domain with a leucine zipper abolished APOBEC3G incorporation. Moreover, coprecipitation analysis showed that APOBEC3G-Gag interaction requires NC and nonspecific RNA. These observations suggest that APOBEC3G exploits an essential property of retroviruses, namely, RNA packaging, to infiltrate particles. Because it is, therefore, difficult to evolve specific sequences that confer escape from APOBEC3G, these findings may explain why lentiviruses evolved an activity that induces its destruction.