Roles of metal ions in nucleases

The hydrolysis of phosphodiester bonds by metallonucleases is crucial to most aspects of nucleic acid processing. In recent years, studies of the classical restriction endonucleases have given way to the characterization of metallonucleases with widely divergent active site motifs. These developments fuel debates regarding the roles of metal ions in these enzymes. It is fortuitous that the current literature also includes the increased application of a variety of computational techniques to test the roles of metal ions in nucleic acid hydrolysis by these systems. This includes recent proposals and indirect evidence that these enzymes utilize metal ion movement in these reactions.     

Biosorption of metal ions byAzotobacter vinelandii

Azotobacter vinelandii was better than eitherDerxia gummosa orRhizobium trifolii for sorption of UO ₂ ²⁺ . Its maximum binding capacity was 0.25 mmol UO ₂ ²⁺ /g dry biomass with an affinity constant of 333 l/mmol at pH 4.1 according to the Langmuir model. In a semisynthetic medium,A. vinelandii showed the highest sorption capacity in the early stationary phase. The binding of UO ₂ ²⁺ , Cu²⁺, Ca²⁺ and Zn²⁺ was affected by the pH of the solution. With HCl as eluent, virtually all the sorbed UO ₂ ²⁺ was released. The presence of Cu²⁺, Cd²⁺, Ca²⁺, and Zn²⁺ inhibited the UO ₂ ²⁺ biosorption whereas Mg²⁺ and K⁺ had no effect.     

Toxic metal ions in photoautotrophic organisms

We summarize the contemporary understanding of the effects of metal stress on various photosynthetic processes in photoautotrophic organisms and of the defence strategies employed by these organisms to avoid such stress. Cadmium is in the centre of interest of this review, as a non-essential element and important environmental pollutant, but Al, Pb, Hg, As, Cu, and Zn are also considered. Toxic metal ions pollute the environment through anthropogenic activities and affect the quality of plant crop. They represent one of the main abiotic stress factors influencing the health of plants and, as a secondary effect, of animals including man. The review summarizes the generally accepted answers to the questions: How do the toxic metal ions enter the photosynthetic organisms? How are they accumulated in plants? Which mechanisms do plants develop to tolerate metal stress and protect themselves?     

Antibody-based sensors for heavy metal ions

Competitive immunoassays for Cd(II), Co(II), Pb(II) and U(VI) were developed using identical reagents in two different assay formats, a competitive microwell format and an immunosensor format with the KinExA™ 3000. Four different monoclonal antibodies specific for complexes of EDTA–Cd(II), DTPA–Co(II), 2,9-dicarboxyl-1,10-phenanthroline–U(VI), or cyclohexyl–DTPA–Pb(II) were incubated with the appropriate soluble metal–chelate complex. In the microwell assay format, the immobilized version of the metal–chelate complex was present simultaneously in the assay mixture. In the KinExA format, the antibody was allowed to pre-equilibrate with the soluble metal-chelate complex, then the incubation mixture was rapidly passed through a microcolumn containing the immobilized metal-chelate complex. In all four assays, the KinExA format yielded an assay with 10–1000-fold greater sensitivity. The enhanced sensitivity of the KinExA format is most likely due to the differences in the affinity of the monoclonal antibodies for the soluble versus the immobilized metal–chelate complex. The KinExA 3000 instrument and the Cd(II)-specific antibody were used to construct a prototype assay that could correctly assess the concentration of cadmium spiked into a groundwater sample. Mean analytical recovery of added Cd(II) was 114.25±11.37%. The precision of the assay was satisfactory; coefficients of variation were 0.81–7.77% and 3.62–14.16% for within run and between run precision, respectively.     

Interactions between metal ions and carbohydrates: the coordination behavior of neutral erythritol to transition metal ions

The single crystals of coordinated complexes of neutral erythritol (C4H10O4) with various transition metal ions were synthesized and studied using FT-IR and single crystal X-ray diffraction analysis. Two CuCl2-erythritol complexes (denoted as CuE(I) and CuE(II)) were obtained. In CuE(I), Cu2+ coordinates with two chloride ions and four OH groups from two erythritol molecules. Two copper centers are linked by one erythritol molecule to form a zigzag chain. For CuE(II), each Cu2+ coordinates with two OH groups from an erythritol molecule and two chloride ions. The crystal of CuE(II) contains complexed and free erythritol, the dimers of [Cu2Cl4(C4H10O4)] further form a [Cu2Cl4(C4H10O4)]infinity chain via secondary Cu...Cl bonds, both the dimer unit of [Cu2Cl4.(C4H10O4)] and non-coordinated C4H10O4 unit exist side by side in the crystal. MnCl2-erythritol complex whose structure is similar to CuE(I) is also acquired. The OH groups of erythritol act as ligand to coordinate to metal ions on one hand, one the other hand, OH groups form hydrogen bonds network that link chain and layer together to build three-dimensional structures.     

DNA hydrolysis by rare-earth metal ions.

Plasmid DNA and poly(dA) are cleaved by rare-earth(III) ions at pH 7-8 and 50 degrees C. The cleavage has been confirmed by prompt conversion of supercoiled pBR 322 plasmid DNA (Form I) to a relaxed Form II. Furthermore, degradation of poly(dA) to shorter oligonucleotides is clearly evidenced by HPLC. A possible application of the metal ions (and their complexes) to artificial nucleases is indicated.     

Interaction of DNA with divalent metal ions

The interaction between the native DNA macromolecules and Ca2+, Mn2+, Cu2+ ions in solutions of low ionic strength (10(-3) M Na+) is studied using the methods of differential UV spectroscopy and CD spectroscopy. It is shown that the transition metal ions Mn2+ exercise binding to the nitrogen bases of DNA at concentrations approximately 5 x 10(-6) M and form chelates with guanine of N7-Me(2+)-O6 type. Only at high concentrations in solution (5 x 10(-3) M) do Ca2+ ions interact with the nitrogen bases of native DNA. In the process of binding to Ca2+ and Mn2+ the DNA conformation experiences some changes under which the secondary structure of the biopolymer is within the B-form family. The DNA transition to the new conformation is revealed by its binding to Cu2+ ions.     

Strain-specific prion-protein conformation determined by metal ions

In animals infected with a transmissible spongiform encephalopathy, or prion disease, conformational isomers (known as PrPSc proteins) of the wild-type, host-encoded cellular prion protein (PrPc) accumulate. The infectious agents, prions, are composed mainly of these conformational isomers, with distinct prion isolates or strains being associated with different PrPSc conformations and patterns of glycosylation. Here we show that two different human PrPSc types, seen in clinically distinct subtypes of classical Creutzfeldt-Jakob disease, can be interconverted in vitro by altering their metal-ion occupancy. The dependence of PrPSc conformation on the binding of copper and zinc represents a new mechanism for post-translational modification of PrP and for the generation of multiple prion strains, with widespread implications for both the molecular classification and the pathogenesis of prion diseases in humans and animals.     

Potential carcinogenicity of some transition metal ions

Potential carcinogenicity of some transition metal ions was tested using a direct-current polarography method. The measurements were based on the reduction of tested compounds in an anhydrous solution using α-lipoic acid as the detection compound. The potential carcinogenicity was expressed in terms of the parameter tg α, which is known to directly correlate with the carcinogenicity of tested compounds. For the metal ions tested, tg α was found to decrease in the following sequence: Fe(III) > Pb(II) > V(IV) > Fe(II) > Mn(II) > Cu(II). Zero values of tg α were found for Cd(II) and Mn(III).     

Cytotoxicity of copper ions released from metal

The aim of this work is to contribute to the elucidation of the cytotoxic process caused by the copper ions released from the biomaterials. Clonal cell lines UMR106 were used in the experiments. Copper ions were obtained from two different sources: copper salts and metal dissolution. Experiments carried out with constant ion concentrations (copper salts) were compared with those with concentrations that vary with time and location (dissolution of the metal). Present results and others previously reported could be interpreted through mathematical models that describe: (1) the variation of concentration of copper ions with time and location within a biofilm and (2) the variation of the killing rate with the concentration of the toxic ion and time. The large number of dead cells found near the copper sample with an average ion concentration below the toxic limit could be interpreted bearing in mind that these cells should be exposed to a local concentration higher than this limit. A logarithmic dependence between the number of cells and exposure time was found for nearly constant ion concentrations. Apparent discrepancies, observed when these results and those of different researchers were contrasted, could be explained considering the dissimilar experimental conditions such as the source of the ions and their local concentration at real time.     

Aggregation of fibrinogen molecules by metal ions

The ability of metal ions to cause physical aggregation of neutral solutions of bovine fibrinogen has been studied. Three categories were found: (a) ions (such as Ca²⁺, Mg²⁺ and Mn²⁺) which did not cause aggregation even when present in 1–100 mm concentrations: (b) ions (such as Fe²⁺, Cu²⁺ and Ni²⁺) which caused aggregation in the 0–10 mm concentration range, (c) ions (such as Hg²⁺, Zn²⁺, Cr³⁺, La³⁺) which caused aggregation in the 0–1000 μm concentration range. Aggregation occurs immediately the metal ion is brought into contact with the fibrinogen, and product formation reaches a steady state within 5 min. With the exception of Zn²⁺, all the ions that caused aggregation exhibited a threshold concentration below which no observable aggregation took place. The threshold concentration for Hg²⁺, the most effective ion studied, was 6 μm. Addition of excess EDTA caused resolubilization of the aggregated fibrinogen due to removal of the metal ions. Aggregation is thus thought to be a physical process initiated by binding of metal ions to those carboxyl groups in fibrinogen responsible for keeping the monomers apart in solution. The aggregation does not involve prior proteolytic degradation of the fibrinogen.