Bioinorganic chemistry最新文献

The nature and strength of chelation ofl-DOPA with the metal ions, namely, Zn(II), Cu(II), Ni(II), Co(II), Fe(II, III), Mn(II), Ca(II), Mg(II), and Al(III) in both presence and absence of ATP were investigated by thermodynamic equilibrium studies at a constant ionic strength (0.12 M NaCl) and temperature, 25 ± 0.1°C, using the potentiometric pH method. Copper(II), Fe(III), and Al(III) interacted strongly withl-DOPA with the stability constants (logK_ML, 18.47 ± 0.13, 18.39 ± 0.31, and 19.60 ± 0.20, respectively. In the combined presence of ATP the metal ions were predominantly coordinated with ATP in the pH range 3–5, followed by interaction withl-DOPA above pH = 5 and the formation of the mixed ligand chelates. The stabilities (logK_MLA) for the chelation ofl-DOPA with the metal-ATP (1:1) systems varied from 2.32 ± 0.15 to 13.24 ± 0.20 for the different metals. Electronic absorption spectral studies of the metal-l-DOPA systems were undertaken to determine the relative involvement of the different donor groups of the ligand in metal binding. On the basis of the spectral and equilibrium data, the nature and extent of metal binding by the pyrocatechol and the alanine ends of thel-DOPA molecule over the pH range 3–10 are discussed.

The kinetics of dissociation of Zn²⁺ from the metalloenzyme carbonic anhydrase was measured over a range of pH, temperature, and acetate concentration. The rate of dissociation is extremely slow at neutral pH (t_l/2 ≈ 3 years, 4°C), but increases in almost direct proportion to the hydrogen ion concentration and is enhanced in the presence of 1,10-phenanthroline or acetate. The thermodynamic stability of the zinc-apoenzyme complex was determined over a range of pH from rate data on binding and dissociation (stability constants 10⁹–10¹¹ M⁻¹, 25°C). The great stability of the complex and slow exchange of the apoenzyme ligand is attributed, at least in part, to the rigidity of the multidentate protein ligand.

The joint administration of 2 ppm of arsenic as arsenite and of 2 ppm of selenium as selenite in the drinking water of inbred female C₃H/St mice increases the incidence of spontaneous mammary adenocarcinoma significantly over the simultaneous controls and animals receiving 2 ppm As as arsenite or 2 ppm Se as selenite alone, indicating that arsenic abolishes the anticarcinogenic effect of selenium. Arsenite caused a significant increase of the tumor growth rates and raised the incidence of multiple tumors. The tumor growth rates in the As-Se group were also increased, but the incidence of multiple tumors was lower than in the 2 ppm As group.

A complex of platinum and human transferrin has been formed by appropriately combining apotransferrin (metal free protein) and potassiumchloroplatinate (K₂PtCl₄). Atomic absorption spectroscopy indicated that both primary binding sites on the protein participated in the complex. Electron paramagnetic resonance (EPR) examination showed that the bound platinum was not paramagnetic, and thus it is highly probable that the Pt ion is in the +2 oxidation state. The results suggest a possible mechanism for physiological distribution of third-transition-series metal.

O-Furfurylhydroxylamine (FHA) was synthesized and found to react with cytosine derivatives in the presence of sodium bisulfite. Conversion of cytidine to sodiumN⁴-furfuryloxy-5,6-dihydrocytidine-6-sulfonate by 1 M FHA in aqueous methanolic or ethanolic sodium bisulfite took place with a half-life of 11 min at pH 5 and room temperature. Adenine, guanine, thymine, and uracil derivatives did not react with FHA. The reaction of OsO₄/pyridine with a model compound.N⁶-furfuryadenine (kinetin), resulted in addition of the OsO₄ to the furyl group. Calf-thymus DNA and polycytidylic acid were treated with FHA and bisulfite under denaturing conditions. Perchloric acid hydrolysis of the DNA showed that 80% of the cytosines were modified after 1 day. Modification of the cytosine nucleotides in the nucleic acids was followed by treatment with mercuric acetate or osmium tetroxide. The resulting polymers contained three mercury atoms or two osmium atoms per cytosine nucleotide. The potential usefulness of these reactions for electron microscopic studies of nucleic acids is discussed.

Osmium tetraoxide, in the presence of ligands such as pyridine and bipyridine, adds across the etheno bridge of 1N⁶-etheno-9-methyladenine and poly-1 N⁶-ethenoadenylic acid. The Os:P ratio in the labeled polynucleotide was 1 when bipyridine was used as the stabilizing ligands. A similar study with polycytidylic acid, which had been partially modified with chloroacetaldehyde so that some bases were converted to 3 N⁴-ethenocytosine, gave an Os:P ration of 1.3. Calf-thymus DNA, in which the adenine and cytosine bases were modified by chloroacetaldehyde, gave an Os:P ratio of 1 after 24 h. These results suggest that 3 N⁴-ethenocytosine will add two Os labels.

To gain information about the influence of the oxidation state of vanadium on its metabolic behavior, different ⁴⁸V-labelled vanadium compounds, such as cationic VO²⁺(V), VO²⁺(IV), V³⁺(III), and anionic V₄O_12³⁻(V), VS₄³⁻(V) species were prepared and intravenously injected into rats. The ⁴⁸V radioactivity was measured in whole tissues as well as in nuclei, mitochondria, lysosomes, microsomes, and cytosols from liver and kidney homogenates. The distribution of ⁴⁸V redioactivity between the plasma components was investigated using gel filtration of the {48}V-labeled plasma. The findings indicate that there are common pathways of the different chemical forms of vanadium in animals. The similarities are referred to the distribution in different tissues and their intracellular distribution as well as to the transport in the blood, in which ⁴⁸V was always found in the plasma bound to transferrin. The results obtained tend to exclude a possible influence of the oxidation state of vanadium on its metabolism and support the existence in the body of two mechanisms of conversion of different chemical forms of vanadium ions to one with the same valence.

The possible relationship between the water binding by bacterial endospores and their dormancy and heat resistances has been examined in terms of the coordination characteristics of the spore-bound calcium. Stabilities of the calcium complexes of typical cytoplasmic and structural spore components were determined by potentiometric equilibrium pH measurements in model systems consisting of DPA, glycine, alanine, glutamic acid, alanyl-glutamic acid, triglycine, and tetraglycine. The Ca⁺⁺-form and H⁺-form spores of Clostridium botulinum 33A were investigated in vivo with respect to their water sorption and heat-resistance characteristics.

The results suggest that the complexing of calcium and Ca(II)-DPA may be biologically significant for spore resistance and dormancy at the following three levels: (1) complexing with spore cytoplasmic pool constituents consistent with the idea of a metal-chelate cross-linked cytoplasm or spore cement stabilizing the essential biological macromolecules, (2) complexing with structural components of the spore as indicated by the interaction with model peptides, and (3) coordination with water to produce an apparently dehydrated environment in the spore as evident from the much greater watersorption capacity of the Ca⁺⁺-form spores vs the much smaller water sorption of the H⁺-form spores.

Interestingly enough, DPA itself, in the absence of metal ion, showed some interaction with di-, tri-, and tetrapeptides and a weak but detectable interaction with amino acids. Although the exact mode of the DPA-peptide interaction is not clear, it is attractive to speculate about its possible involvement in the control of spore dormancy and resistance.

The rates of formation of a number of metallocarboxypeptidases from metal ions and bovine apocarboxypeptidase A (CPA) have been measured directly and by a competitive method. Rates were determined within pH = 6–8 by utilising the pH change attending metal-ion incorporation, employing indicator and stopped-flow. Second-order rate constants (k_f, M⁻¹ s⁻¹ at 25°C, I = 1 M NaCl, pH = 7, Tris = 25 μM) were 1.7 × 10⁵ (Mn²⁺), 3 × 10⁴ (Co²⁺), 5 × 10³ (Ni²⁺), 7 × 10⁵ (Zn²⁺), and 9 × 10⁵ (Cd²⁺). Relative incorporation rate constants were determined at 25°, pH = 7.0, Tris = 0.1 M, by competing two metal ions for a deficiency of apoprotein and analyzing the products by differential enzyme activity. Agreement between the two methods was reasonable. Rate constants for dissociation of CoCPA, NiCPA, and ZnCPA were measured by loss of enzyme activity on addition of the metal ion scavenger EDTA. Values of k_d at 25°, I = 1.0 M NaCl, pH = 7.0 were 8 × 10⁻³, 3 × 10⁻⁵, and 4 × 10⁻⁴ s⁻¹, respectively, Values of K obtained kinetically (k_f/k_d) were in good agreement with those determined by activity measurements of equilibrated solutions. Results are compared with those of bovine apocarbonic anhydrase, where generally significantly slower rates are encountered.