Interplay between manganese and zinc homeostasis in the human pathogen Streptococcus pneumoniae

ICP-MS analysis of Streptococcus pneumoniae reveals a high cell-associated Mn(II) concentration that is comparable to that of Zn(II). Stressing these cells with 100–200 μM Zn(II) leads to a slow-growth phenotype and a total Mn(II) concentration that is reduced, with no decrease of other metal ions. Supplementation of the growth media with as little as 10 μM Mn(II) fully restores the growth defect and cell-associated Mn(II) to normal levels. DNA microarray analysis reveals that zinc stress induces the expected upregulation of czcD (encoding a zinc effluxer), but also a pleiotropic transcriptional response suggestive of mild cell wall stress. Genes encoding a nitric oxide (NO) detoxification system (nmlR) and the Mn(II) uptake system (psaBCA) are also induced. We conclude that Zn(II) toxicity results in a cytoplasmic Mn(II) deficiency, possibly caused by competition at the Mn(II) uptake transporter protein PsaA.

The metalloregulatory zinc site in Streptococcus pneumoniae AdcR, a zinc-activated MarR family repressor

Streptococcus pneumoniae D39 AdcR (adhesin competence repressor) is the first metal-sensing member of the MarR (multiple antibiotic resistance repressor) family to be characterized. Expression profiling with a ΔadcR strain grown in liquid culture (brain-heart infusion) under microaerobic conditions revealed upregulation of 13 genes, including adcR and adcCBA, encoding a high-affinity ABC uptake system for zinc, and genes encoding cell-surface zinc-binding pneumococcal histidine triad (Pht) proteins and AdcAII (Lmb, laminin binding). The ΔadcR, H108Q and H112Q adcR mutant allelic strains grown in 0.2 mM Zn(II) exhibit a slow-growth phenotype and an approximately twofold increase in cell-associated Zn(II). Apo- and Zn(II)-bound AdcR are homodimers in solution and binding to a 28-mer DNA containing an adc operator is strongly stimulated by Zn(II) with K(DNA-Zn)=2.4 × 10(8) M(-1) (pH 6.0, 0.2 M NaCl, 25 °C). AdcR binds two Zn(II) per dimer, with stepwise Zn(II) affinities K(Zn1) and K(Zn2) of ≥10(9) M(-1) at pH 6.0 and ≥10(12) M(-1) at pH 8.0, and one to three lower affinity Zn(II) depending on the pH. X-ray absorption spectroscopy of the high-affinity site reveals a pentacoordinate N/O complex and no cysteine coordination, the latter finding corroborated by wild type-like functional properties of C30A AdcR. Alanine substitution of conserved residues His42 in the DNA-binding domain, and His108 and His112 in the C-terminal regulatory domain, abolish high-affinity Zn(II) binding and greatly reduce Zn(II)-activated binding to DNA. NMR studies reveal that these mutants adopt the same folded conformation as dimeric wild type apo-AdcR, but fail to conformationally switch upon Zn(II) binding. These studies implicate His42, His108 and H112 as metalloregulatory zinc ligands in S. pneumoniae AdcR.

Inflammatory hyperalgesia induces essential bioactive lipid production in the spinal cord

Lipid molecules play an important role in regulating the sensitivity of sensory neurons and enhancing pain perception, and growing evidence indicates that the effect occurs both at the site of injury and in the spinal cord. Using high-throughput mass spectrometry methodology, we sought to determine the contribution of spinal bioactive lipid species to inflammation-induced hyperalgesia in rats. Quantitative analysis of CSF and spinal cord tissue for eicosanoids, ethanolamides and fatty acids revealed the presence of 102 distinct lipid species. After induction of peripheral inflammation by intra-plantar injection of carrageenan to the ipsilateral hind paw, lipid changes in cyclooxygenase (COX) and 12-lipoxygenase (12-LOX) signaling pathways peaked at 4 h in the CSF. In contrast, changes occurred in a temporally disparate manner in the spinal cord with LOX-derived hepoxilins followed by COX-derived prostaglandin E(2), and subsequently the ethanolamine anandamide. Systemic treatment with the mu opioid agonist morphine, the COX inhibitor ketorolac, or the LOX inhibitor nordihydroguaiaretic acid significantly reduced tactile allodynia, while their effects on the lipid metabolites were different. Morphine did not alter the lipid profile in the presence or absence of carrageenan inflammation. Ketorolac caused a global reduction in eicosanoid metabolism in naïve animals that remained suppressed following injection of carrageenan. Nordihydroguaiaretic acid-treated animals also displayed reduced basal levels of COX and 12-LOX metabolites, but only 12-LOX metabolites remained decreased after carrageenan treatment. These findings suggest that both COX and 12-LOX play an important role in the induction of carrageenan-mediated hyperalgesia through these pathways.

A macrophage cell model for selective metalloproteinase inhibitor design

The desire to inhibit zinc-dependent matrix metalloproteinases (MMPs) has, over the course of the last 30 years, led to the development of a plethora of MMP inhibitors that bind directly to the active-site metal. With one exception, all of these drugs have failed in clinical trials, due to many factors, including an apparent lack of specificity for MMPs. To address the question of whether these inhibitors are selective for MMPs in a biological setting, a cell-based screening method is presented to compare the relative activities of zinc, heme iron, and non-heme iron enzymes in the presence of these compounds using the RAW264.7 macrophage cell line. We screened nine different zinc-binding groups (ZBGs), four established MMP inhibitors (MMPis), and two novel MMP inhibitors developed in our laboratory to determine their selectivities against five different metalloenzymes. Using this model, we identified two nitrogen donor compounds--2,2'-dipyridylamine (DPA) and triazacyclononane (TACN)--as the most selective ZBGs for zinc metalloenzyme inhibitor development. We also demonstrated that the model could predict known nonspecific interactions of some of the most commonly used MMPis, and could also give cross-reactivity information for newly developed MMPis. This work demonstrates the utility of cell-based assays in both the design and the screening of novel metalloenzyme inhibitors.

The design of inhibitors for medicinally relevant metalloproteins

A number of metalloproteins are important medicinal targets for conditions ranging from pathogenic infections to cancer. Many but not all of these metalloproteins contain a zinc(II) ion in the protein active site. Small-molecule inhibitors of these metalloproteins are designed to bind directly to the active site metal ions. In this review several metalloproteins of interest are discussed, including matrix metalloproteinases (MMPs), histone deacetylases (HDACs), anthrax lethal factor (LF), and others. Different strategies that have been employed to design effective inhibitors against these proteins are described, with an effort to highlight the strengths and drawbacks of each approach. An emphasis is placed on examining the bioinorganic chemistry of these metal active sites and how a better understanding of the coordination chemistry in these systems may lead to improved inhibitors. It is hoped that this review will help inspire medicinal, biological, and inorganic chemists to tackle this important problem by considering all aspects of metalloprotein inhibitor design.

Model Complexes of Cobalt-Substituted Matrix Metalloproteinases: Tools for Inhibitor Design

The tetrahedral cobalt(II) complex [(Tp(Ph,Me))CoCl] (Tp(Ph,Me) = hydrotris(3,5-phenylmethylpyrazolyl)borate) was combined with several hydroxypyridinone, hydroxypyridinethione, pyrone, and thiopyrone ligands to form the corresponding [(Tp(Ph,Me))Co(L)] complexes. X-ray crystal structures of these complexes were obtained to determine the mode of binding for each ligand L. The structures show that the [(Tp(Ph,Me))Co(L)] complexes are pentacoordinate complexes, with a general tendency toward square pyramidal geometry. The electronic, EPR, and paramagnetic NMR spectroscopy of the [(Tp(Ph,Me))Co(L)] complexes have been examined. The frozen-solution EPR spectra are indicative of pentacoordination in frozen solution, while the NMR indicates some dynamics in ligand binding. The findings presented here suggest that [(Tp(Ph,Me))Co(L)] complexes can be used as spectroscopic references for investigating the mode of inhibitor binding in metalloproteinases of medicinal interest. Potential limitations when using cobalt(II) model complexes are also discussed.

A New Role for Old Ligands: Discerning Chelators for Zinc Metalloproteinases

In an effort to identify promising non-hydroxamate inhibitors of matrix metalloproteinases (MMPs) several new zinc-binding groups (ZBGs) based on pyridine-derived or aza-macrocycle chelators have been examined. Fluorescence-based enzyme assays have been used to determine the IC50 values for these ZBGs against MMP-1, MMP-3, and anthrax lethal factor (LF). Many of these ligands were found to be remarkably potent, with IC50 values as much as 185-fold lower than that found for acetohydroxamic acid. These ligands are proposed to be more selective "warheads" for the inhibition of metalloenzymes that contain Zn2+ versus other metal ions at their active site.

A novel heterocyclic atom exchange reaction with Lawesson's reagent: a one-pot synthesis of dithiomaltol

A one-pot reaction of maltol with Lawesson's reagent generates dithiomaltol, a thiopyran-4-thione, via an unusual heterocyclic atom exchange (HCAE) reaction; only pyrones with proton or aliphatic substituents undergo the HCAE substitution.

Investigations of the reactions of monochloramine and dichloramine with selected phenols: examination of humic acid models and water contaminants

Our paper reports on the reactivities and orientations of two common phenols, phenol (2) and m-cresol (3), and some of their chlorinated intermediates with aqueous monochloramine, NH2Cl, and dichloramine, NHCl2. We also examined the further reactivity of 2,4,6-trichlorophenol (4) with the chloramines. The phenols are an important area of investigation because they are substituents in the humic acids and are common contaminants in water. m-Cresol (3) was found to be more reactive than phenol (2)with both chlorinating agents. Both NH2Cl and NHCl2were sufficiently reactive to chlorinate all positions ortho and para to the hydroxyl groups. Mono- and dichloramine showed the same orientation with 2 but different orientations in their reactions with the substituent phenols. Indophenol (as its salt) was formed to a minor extent at high pH but not at pH 9. Both NH2Cl and NHCl2 rapidly replaced the parachlorine in 2,4,6-trichlorophenol (4) to give a mixture of 2,6-dichloro-1,4-benzoquinone-4-(N-chloro) imine (5) and 2,6-dichloro-1,4-benzoquinone (18). Similar reactions occurwith 2,4,6-trichloro-m-cresol (17) and 2,4,6-trichloro-3-methoxyphenol (29). The products for 17 were confirmed by mass spectrometry (El and Cl), 1H NMR, 13C NMR, and IR; the products for 29 were confirmed by mass spectrometry (El and Cl) and IR. An ion radical mechanism is suggested to account for the chlorine replacement by the chloramines. [No side chain oxidation of the methyl group in 17 in H20 or ether occurred, with or without ultraviolet radiation.] Both 5 and 18 underwent further chlorination with NH2Cl or NHCl2. Imine 5 did not function as a chlorinated agent.

Using Model Complexes To Augment and Advance Metalloproteinase Inhibitor Design

The tetrahedral zinc complex [(Tp(Ph,Me))ZnOH] (Tp(Ph,Me) = hydrotris(3,5-phenylmethylpyrazolyl)borate) was combined with 2-thenylmercaptan, ethyl 4,4,4-trifluoroacetoacetate, salicylic acid, salicylamide, thiosalicylic acid, thiosalicylamide, methyl salicylate, methyl thiosalicyliate, and 2-hydroxyacetophenone to form the corresponding [(Tp(Ph,Me))Zn(ZBG)] complexes (ZBG = zinc-binding group). X-ray crystal structures of these complexes were obtained to determine the mode of binding for each ZBG, several of which had been previously studied with SAR by NMR (structure-activity relationship by nuclear magnetic resonance) as potential ligands for use in matrix metalloproteinase inhibitors. The [(Tp(Ph,Me))Zn(ZBG)] complexes show that hydrogen bonding and donor atom acidity have a pronounced effect on the mode of binding for this series of ligands. The results of these studies give valuable insight into how ligand protonation state and intramolecular hydrogen bonds can influence the coordination mode of metal-binding proteinase inhibitors. The findings here suggest that model-based approaches can be used to augment drug discovery methods applied to metalloproteins and can aid second-generation drug design.