Benzophenone-3: Comprehensive review of the toxicological and human evidence with meta-analysis of human biomonitoring studies.

BACKGROUND: Benzophenone-3 (BP-3) and its major metabolite benzophenone-1 (BP-1) are widely used as UV filters in sunscreens and cosmetics to prevent sunburn and skin damage, or as stabilizers to prevent photodegradation in many commercial products. As a result, their presence is ubiquitous in the environment, wildlife and humans. Based on endocrine disruption concerns, international regulatory agencies are performing a closer evaluation. OBJECTIVE AND METHODS: This work aimed to comprehensively review the available human relevant evidence for safety issues in MEDLINE/PubMed in order to create a structured database of studies, as well as to conduct an integrative analysis as part of the Human Biomonitoring for Europe (HBM4EU) Initiative. RESULTS: A total of 1,635 titles and abstracts were screened and 254 references were evaluated and tabulated in detail, and classified in different categories: i) exposure sources and predictors; ii) human biomonitoring (HBM) exposure levels to perform a meta-analysis; iii) toxicokinetic data in both experimental animals and humans; iv) in vitro and in vivo rodent toxicity studies; and v) human data on effect biomarkers and health outcomes. Our integrative analysis showed that internal peak BP-3 concentrations achieved after a single whole-body application of a commercially available sunscreen (4% w/w) may overlap with concentrations eliciting endocrine disrupting effects in vitro, and with internal concentrations causing in vivo adverse female reproductive effects in rodents that were supported by still limited human data. The adverse effects in rodents included prolonged estrous cycle, altered uterine estrogen receptor gene expression, endometrium hyperplasia and altered proliferation and histology of the mammary gland, while human data indicated menstrual cycle hormonal alterations and increased risk of uterine fibroids and endometriosis. Among the modes of action reported (estrogenic, anti-androgenic, thyroid, etc.), BP-3 and especially BP-1 showed estrogenic activity at human-relevant concentrations, in agreement with the observed alterations in female reproductive endpoints. The meta-analysis of HBM studies identified a higher concern for North Americans, showing urinary BP-3 concentrations on average 10 and 20 times higher than European and Asian populations, respectively. DISCUSSION AND CONCLUSIONS: Our work supports that these benzophenones present endocrine disrupting properties, endorsing recent European regulatory efforts to limit human exposure. The reproducible and comprehensive database generated may constitute a point of departure in future risk assessments to support regulatory initiatives. Meanwhile, individuals should not refrain from sunscreen use. Commercially available formulations using inorganic UV filters that are practically not absorbed into systemic circulation may be recommended to susceptible populations.

Tying Up a Loose End: On the Role of the C-Terminal CCHHRAG Fragment of the Metalloregulator CueR.

The transcriptional regulator CueR is activated by the binding of CuI , AgI , or AuI to two cysteinates in a near-linear fashion. The C-terminal CCHHRAG sequence in Escherichia coli CueR present potential additional metal binding ligands and here we explore the effect of deleting this fragment on the binding of AgI to CueR. CD spectroscopic and ESI-MS data indicate that the high AgI -binding affinity of WT-CueR is significantly reduced in Δ7C-CueR.[111 Ag PAC spectroscopy demonstrates that the WT-CueR metal site structure (AgS2 ) is conserved, but less populated in the truncated variant. Thus, the function of the C-terminal fragment may be to stabilize the two-coordinate metal site for cognate monovalent metal ions. In a broader perspective this is an example of residues beyond the second coordination sphere affecting metal site physicochemical properties while leaving the structure unperturbed.

A study on the secondary structure of the metalloregulatory protein CueR: effect of pH, metal ions and DNA.

The response of CueR towards environmental changes in solution was investigated. CueR is a bacterial metal ion selective transcriptional metalloregulator protein, which controls the concentration of copper ions in the cell. Although several articles have been devoted to the discussion of the structural and functional features of this protein, CueR has not previously been extensively characterized in solution. Here, we studied the effect of change in pH, temperature, and the presence of specific or non-specific binding partners on the secondary structure of CueR with circular dichroism (CD) spectroscopy. A rather peculiar reversible pH-dependent secondary structure transformation was observed, elucidated and supplemented with pKa estimation by PROPKA and CpHMD simulations suggesting an important role of His(76) and His(94) in this process. CD experiments revealed that the presence of DNA prevents this structural switch, suggesting that DNA locks CueR in the α-helical-rich form. In contrast to the non-cognate metal ions HgII, CdII and ZnII, the presence of the cognate AgI ion affects the secondary structure of CueR, most probably by stabilizing the metal ion and DNA-binding domains of the protein.

Flexibility of the CueR Metal Site Probed by Instantaneous Change of Element and Oxidation State from AgI to CdII.

Selectivity for monovalent metal ions is an important facet of the function of the metalloregulatory protein CueR. 111 Ag perturbed angular correlation of γ-rays (PAC) spectroscopy probes the metal site structure and the relaxation accompanying the instantaneous change from AgI to CdII upon 111 Ag radioactive decay. That is, a change from AgI , which activates transcription, to CdII , which does not. In the frozen state (-196 °C) two nuclear quadrupole interactions (NQIs) are observed; one (NQI1 ) agrees well with two coordinating thiolates and an additional longer contact to the S77 backbone carbonyl, and the other (NQI2 ) reflects that CdII has attracted additional ligand(s). At 1 °C only NQI2 is observed, demonstrating that relaxation to this structure occurs within ≈10 ns of the decay of 111 Ag. Thus, transformation from AgI to CdII rapidly disrupts the functional linear bis(thiolato)AgI metal site structure. This inherent metal site flexibility may be central to CueR function, leading to remodelling into a non-functional structure upon binding of non-cognate metal ions. In a broader perspective, 111 Ag PAC spectroscopy may be applied to probe the flexibility of protein metal sites.

C-terminal Cysteines of CueR Act as Auxiliary Metal Site Ligands upon HgII Binding-A Mechanism To Prevent Transcriptional Activation by Divalent Metal Ions?

Intracellular CuI is controlled by the transcriptional regulator CueR, which effectively discriminates between monovalent and divalent metal ions. It is intriguing that HgII does not activate transcription, as bis-thiolate metal sites exhibit high affinity for HgII . Here the binding of HgII to CueR and a truncated variant, ΔC7-CueR, without the last 7 amino acids at the C-terminus including a conserved CCHH motif is explored. ESI-MS demonstrates that up to two HgII bind to CueR, while ΔC7-CueR accommodates only one HgII . 199m Hg PAC and UV absorption spectroscopy indicate HgS2 structure at both the functional and the CCHH metal site. However, at sub-equimolar concentrations of HgII at pH 8.0, the metal binding site displays an equilibrium between HgS2 and HgS3 , involving cysteines from both sites. We hypothesize that the C-terminal CCHH motif provides auxiliary ligands that coordinate to HgII and thereby prevents activation of transcription.

Purification of proteins with native terminal sequences using a Ni(II)-cleavable C-terminal hexahistidine affinity tag.

The role of the termini of protein sequences is often perturbed by remnant amino acids after the specific protease cleavage of the affinity tags and/or by the amino acids encoded by the plasmid at/around the restriction enzyme sites used to insert the genes. Here we describe a method for affinity purification of a metallonuclease with its precisely determined native termini. First, the gene encoding the target protein is inserted into a newly designed cloning site, which contains two self-eliminating BsmBI restriction enzyme sites. As a consequence, the engineered DNA code of Ni(II)-sensitive Ser-X-His-X motif is fused to the 3'-end of the inserted gene followed by the gene of an affinity tag for protein purification purpose. The C-terminal segment starting from Ser mentioned above is cleaved off from purified protein by a Ni(II)-induced protease-like action. The success of the purification and cleavage was confirmed by gel electrophoresis and mass spectrometry, while structural integrity of the purified protein was checked by circular dichroism spectroscopy. Our new protein expression DNA construct is an advantageous tool for protein purification, when the complete removal of affinity or other tags, without any remaining amino acid residue is essential. The described procedure can easily be generalized and combined with various affinity tags at the C-terminus for chromatographic applications.

Ferrocenyl GNA Nucleosides: A Bridge between Organic and Organometallic Xeno-nucleic Acids.

The enantioselective synthesis and electrochemistry of the first ferrocenyl GNA nucleosides is reported. These compounds were obtained by a Sharpless asymmetric dihydroxylation reaction of [3-(N1-thyminyl)-1-(ferrocenyl)]propene as S,R and R,S enantiomers in about 70 % yield with enantiomeric excesses of >99 % and 71 %, respectively. The absolute configurations of the chiral carbon atoms in the nucleosides were assigned by single-crystal X-ray diffraction analysis of the methyl derivatives in the solid state. The compounds were also studied with circular dichroism (CD) spectroscopy in solution. The enantiomeric relationship between the S,R and R,S isomers was confirmed by the near-mirror-image CD spectra. The redox properties of the nucleosides and their methylated derivatives were investigated using cyclic voltammetry. The cyclic voltammograms revealed reversible redox processes for the entire series of compounds at potentials of -25 mV (for nonmethylated derivatives) and 75 mV (for methylated derivatives) versus the ferrocene/ferrocenium reference redox couple.

Intrinsic protein disorder could be overlooked in cocrystallization conditions: An SRCD case study.

X-ray diffractometry dominates protein studies, as it can provide 3D structures of these diverse macromolecules or their molecular complexes with interacting partners: substrates, inhibitors, and/or cofactors. Here, we show that under cocrystallization conditions the results could reflect induced protein folds instead of the (partially) disordered original structures. The analysis of synchrotron radiation circular dichroism spectra revealed that the Im7 immunity protein stabilizes the native-like solution structure of unfolded NColE7 nuclease mutants via complex formation. This is consistent with the fact that among the several available crystal structures with its inhibitor or substrate, all NColE7 structures are virtually the same. Our results draw attention to the possible structural consequence of protein modifications, which is often hidden by compensational effects of intermolecular interactions. The growing evidence on the importance of protein intrinsic disorder thus, demands more extensive complementary experiments in solution phase with the unligated form of the protein of interest.

Advanced purification strategy for CueR, a cysteine containing copper(I) and DNA binding protein.

Metal ion regulation is essential for living organisms. In prokaryotes metal ion dependent transcriptional factors, the so-called metalloregulatory proteins play a fundamental role in controlling the concentration of metal ions. These proteins recognize metal ions with an outstanding selectivity. A detailed understanding of their function may be exploited in potential health, environmental and analytical applications. Members of the MerR protein family sense a broad range of mostly late transition and heavy metal ions through their cysteine thiolates. The air sensitivity of latter groups makes the expression and purification of such proteins challenging. Here we describe a method for the purification of the copper-regulatory CueR protein under optimized conditions. In order to avoid protein precipitation and/or eventual aggregation and to get rid of the co-purifying Escherichia coli elongation factor, our procedure consisted of four steps supplemented by DNA digestion. Subsequent anion exchange on Sepharose FF Q 16/10, affinity chromatography on Heparin FF 16/10, second anion exchange on Source 30 Q 16/13 and gel filtration on Superdex 75 26/60 resulted in large amounts of pure CueR protein without any affinity tag. Structure and functionality tests performed with mass spectrometry, circular dichroism spectroscopy and electrophoretic gel mobility shift assays approved the success of the purification procedure.