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PubMed2002

Some aspects of NaBH(4) reduction in NMP.

In our solvent optimization study of NaBH(4) reduction, NMP was found to enhance the reactivity. A chemoselective debromination of the bromide and sulfonates can be attained in the new borohydride reagent system: NaBH(4)-LiOTf-NMP. This mixed system worked as an alternative to NaBH(3)CN and Bu(3)SnH for the S(N)2 type displacement of alkylbromide and sulfonate. Also mentioned is an expedient reduction of an azide group into amine by NaBH(4) in NMP without any additive, which offers a convenient protocol for the direct transformation of halides into amines via azide in one flask. Some examples of other reductions were also presented.

Azides↗
PubMed2001

Crystal structure of manganese catalase from Lactobacillus plantarum.

BACKGROUND: Catalases are important antioxidant metalloenzymes that catalyze disproportionation of hydrogen peroxide, forming dioxygen and water. Two families of catalases are known, one having a heme cofactor, and the other, a structurally distinct family containing nonheme manganese. We have solved the structure of the mesophilic manganese catalase from Lactobacillus plantarum and its azide-inhibited complex. RESULTS: The crystal structure of the native enzyme has been solved at 1.8 A resolution by molecular replacement, and the azide complex of the native protein has been solved at 1.4 A resolution. The hexameric structure of the holoenzyme is stabilized by extensive intersubunit contacts, including a beta zipper and a structural calcium ion crosslinking neighboring subunits. Each subunit contains a dimanganese active site, accessed by a single substrate channel lined by charged residues. The manganese ions are linked by a mu1,3-bridging glutamate carboxylate and two mu-bridging solvent oxygens that electronically couple the metal centers. The active site region includes two residues (Arg147 and Glu178) that appear to be unique to the Lactobacillus plantarum catalase. CONCLUSIONS: A comparison of L. plantarum and T. thermophilus catalase structures reveals the existence of two distinct structural classes, differing in monomer design and the organization of their active sites, within the manganese catalase family. These differences have important implications for catalysis and may reflect distinct biological functions for the two enzymes, with the L. plantarum enzyme serving as a catalase, while the T. thermophilus enzyme may function as a catalase/peroxidase.

Azides↗
PubMed2001

Isolation and characterization of mitochondrial F(1)-ATPase from crayfish (Orconectes virilis) gills.

A soluble F(1)-ATPase was isolated from the mitochondria of crayfish (Orconectes virilis) gill tissue. The maximal mitochondrial disruption rate (95%) was obtained by sonicating for 4 min at pH 8.6. A 15-fold purification was estimated. The properties for both soluble and membrane-bound enzyme were studied. Both enzyme forms were stable at 4 to -70 degrees C when kept in 20% glycerol. Soluble F(1)-ATPase was more stable at room temperature than membrane-bound enzyme. It displayed a narrower pH profile (pK(1) =6.58, pK(2)=7.68) and more acid pH optimum (7.13) than membrane-bound enzyme (pK(1)=6.42, pK(2)=8.55, optimum pH 7.49). The anion-stimulated activities were in the order HCO(3)(-)>SO(4)(2-)>Cl(-). The apparent K(a) values for soluble enzyme were 11.4, 11.2, and 10.9 mM, respectively, but the K(a) of HCO(3)(-) for membrane-bound enzyme (14.9 mM) was higher than for soluble enzyme. Oligomycin and DCCD inhibited membrane-bound F(1)-ATPase with I(50) of 18.6 ng/ml and 2.2 microM, respectively, but were ineffective in inhibiting soluble enzyme. Both enzyme forms shared identical sensitivity to DIDS (I(50)=12.5 microM) and vanadate (I(50)=9.0 mM). Soluble ATPase was significantly more sensitive to pCMB (I(50)=0.15 microM) and NO(3)(-) (I(50)=28.6 mM) than membrane-bound enzyme (I(50)=1.04 microM pCMB and 81.5 mM NO(3)(-)). In addition, soluble F(1)-ATPase was slightly more sensitive to azide (I(50)=91.8 microM) and NBD-Cl (I(50)=9.18 microM) than membrane-bound enzyme (I(50)=111.6 microM azide and 12.88 microM NBD-Cl). These data suggest a conformational change transmission between F(0) and F(1) sectors and slight conformational differences between soluble F(1) and membrane-bound F(1). In addition, an unmodified F(0) stabilizes F(1) and decreases F(1) sensitivities to inhibitors and modulators.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed1998

Development of new tester strains derived from E. coli WP2uvrA for the determination of mutational specificity.

We have developed a set of multipurpose tester strains (WP3101 to WP3106) derived from E. coli WP2uvrA for the detection and classification of mutagens. Six kinds of F' plasmid (lacI, lacZ, proAB+) in strains CC101-CC106, each of which carried a different lacZ allele, were transferred to a delta(lac-pro) derivative of WP2uvrA. Assays for transitions and transversions are based upon Lac+ reversion of a specific mutation located within the lacZ gene on an F' plasmid in strains WP3101-WP3106. In addition, the trpE65(ochre) allele in the same strains is available for Trp+ reversion assays. Using the new tester strains, we investigated the mutational specificities of various chemical mutagens. Base analog mutagens and alkylating mutagens induced specific types of base substitutions. G:C-->A:T transitions and G:C-->T:A transversions predominated in mutagenesis induced by 4-nitroquinoline 1-oxide. Only a slight increase in G:C-->T:A transversions was observed in cells treated with 2-(2-furyl)-3-(5-nitro-2-furyl)acrylamide (AF-2), although the potent mutagenicity of AF-2 was detected in a concurrent Trp+ reversion assay in the same strain. Sodium azide, on the other hand, was negative in the Trp+ reversion assay but specifically induced G:C-->A:T transitions. Present finding suggested that target sites for AF-2- and azide-induced lesions may largely depend on sequence context.

4-Nitroquinoline-1-oxide↗
PubMed2005

Multiphenotypic whole-cell sensor for viability screening.

Here, we describe the fabrication of whole mammalian cell biosensors for the optical monitoring of cell viability. Three phenotypes were examined for their response to the addition of two model chemotoxins: sodium hypochlorite and sodium azide, and one model biotoxin, concanavalin A. Two sensing platforms containing cells, hydrogel microspheres, or hydrogel arrays, were also explored. Step changes in viability in response to small doses of sodium hypochlorite were seen nearly instantaneously in all cell lines, in solution, microspheres, and microarrays. Linear detection of sodium azide by entrapped hepatocytes was 0-10 microM, whereas the linear detection range for macrophages and endothelial cells was 0-75 microM. Macrophages and hepatocytes have a greater sensitivity, as indicated by a 40% change in fluorescence over the linear range, whereas endothelial cells show only a 15% change in fluorescence over the linear range. Using photoreaction injection molding, we were also able to generate a multiphenotype sensor that enables the measurement of the toxic effect of 100 microg/mL concanavalin A on macrophages and hepatocytes, but not on endothelial cells.

Animals↗
PubMedDate not supplied

N-(3-azidophenyl)-N-methyl-N'-([4-1H]- and [4-3H]-1-naphthyl)guanidine. A potent and selective ligand designed as a photoaffinity label for the phencyclidine site of the N-methyl-D-aspartate receptor.

A novel radiolabeled photoaffinity ligand has been synthesized for the phencyclidine (PCP) site of the N-methyl-D-aspartate (NMDA) receptor. N-(3-Azidophenyl)-N-methyl-N'-([4-3H]-1-naphthyl)guanidine (13) was prepared with a specific activity of 25 Ci/mmol by diazotization of N-(3-aminophenyl)-N-methyl-N'-([4-3H]-1-naphthyl)guanidine (12) followed by treatment with sodium azide. Guanidine 12 was obtained by catalytic tritiation of N-(4-bromo-1-naphthyl)-N'-methyl-N'-(3-nitrophenyl)guanidine (11). The nontritiated analog 5 of 13 was prepared beginning with N-methyl-N'-1-naphthyl-N-(3-nitrophenyl)guanidine (9). The guanidines 9 and 11 were prepared in moderate yield by the aluminum chloride-catalyzed reaction of N-methyl-3-nitroaniline hydrochloride with 1-naphthylcyanamide and 4-bromo-1-naphthylcyanamide, respectively. Azide 5 showed high selectivity and affinity (IC50 = 100 nM vs [3H]MK801; 3000 nM vs [3H]ditolylguanidine) for the PCP site of the NMDA receptor in guinea pig brain homogenate. Photolabeling experiments with 13, however, failed to radiolabel a significant amount of receptor polypeptide.

Affinity Labels↗
PubMedDate not supplied

Soluble camptothecin derivatives prepared by click cycloaddition chemistry on functional aliphatic polyesters.

Aliphatic polyesters are of interest as biomaterials and drug-delivery vehicles, as their ability to degrade under physiological conditions provides a mechanism for both drug release and clearance of the polymer from the body. Presented here is the synthesis of a polyester-drug graft copolymer conjugate, enabled by click cycloaddition of azide-functionalized camptothecin derivatives with alkyne-functionalized aliphatic polyesters. Further grafting of residual alkyne groups with azide-terminated poly(ethylene oxide) gave a water-soluble polyester-camptothecin conjugate. Control over PEGylation and drug loading, inherent to the graft copolymer design, opens versatile routes to new materials with potential utility in polymer therapeutics.

Azides↗
PubMed1993

Catalytic properties of horseradish peroxidase reconstituted with the 8-(hydroxymethyl)- and 8-formylheme derivatives.

Recent studies suggest that 8-(thiomethyl)- and 8-formylheme modifications may be present in, respectively, lactoperoxidase and myeloperoxidase. To examine whether these heme modifications contribute to the unusual catalytic properties of the mammalian peroxidases, we have reconstituted apo-horseradish peroxidase (HRP) with 8-(hydroxymethyl)heme (8HM-HRP) and 8-formylheme (8F-HRP) and have characterized the reconstituted enzymes. Native HRP and 8HM-HRP have identical spectra in the ferric, compound I, and compound II states. In contrast, the Soret band of 8F-HRP is at 417 rather than 402 nm and that of its compound II species is at 436 rather than 416 nm. Compound I was observed as a transient species with 8F-HRP. The rate of formation of compound I was the same for native and 8HM-HRP, but the pseudo-first-order constant for decay of compound I was 0.021 s-1 for 8HM-HRP and 0.010 s-1 for native HRP. The rates of oxidation of guaiacol, iodide, and thioanisole are the same for native HRP and 8HM-HRP but are significantly slower for 8F-HRP. The stereospecificity of thioanisole oxidation is the same for native and 8HM-HRP, but differs for 8F-HRP. For guaiacol, which was studied in detail, Km = 2.3 mM and kcat = 33 s-1 for 8F-HRP versus Km = 1.8 mM and kcat = 104 s-1 for native HRP. 8HM-HRP oxidizes ethylhydrazine and azide to the ethyl and azidyl radicals, respectively, and is simultaneously inactivated. 8F-HRP is also slowly inactivated by ethylhydrazine and azide.(ABSTRACT TRUNCATED AT 250 WORDS)

Anisoles↗
PubMed1994

Changes in the catalytic properties of p-hydroxybenzoate hydroxylase caused by the mutation Asn300Asp.

By site-directed mutagenesis, we have changed Asn300 to Asp in p-hydroxybenzoate hydroxylase (PHBH; EC 1.14.13.2) from Pseudomonas aeruginosa. In the wild-type (WT) enzyme, residue 300 is in contact with the isoalloxazine ring of the active-site FAD; in the Asn300Asp mutant, this side chain has moved by about 5 A, altering the protein structure [Lah, M.S., Palfey, B.A., Schreuder, H.A., & Ludwig, M.L. (1994) Biochemistry (following paper in this issue)]. The structural changes are responsible for profound catalytic and dynamic effects. The flavin of PHBH is reduced by NADPH in the first half of catalysis. The mutation has decreased this rate 330-fold, apparently by affecting the reactive orientation of the isoalloxazine and pyridine rings. Furthermore, the redox potential of the flavin is lower in the mutant enzyme than in WT by 20-40 mV. The reduced flavin of PHBH reacts with O2 to form a flavin C(4a)-hydroperoxide, which is the species that transfers oxygen to the aromatic substrate. Previous studies indicated that the enzyme promotes the hydroxylation reaction in part by activating the substrate through lowering the phenolic pKa. The Asn300Asp mutant does not lower the substrate pKa. As a consequence of this, and also an enhanced stability of the flavin C(4a)-hydroperoxide, the hydroxylation is 50-fold slower in the mutant than in WT. However, despite the slow rate of the hydroxylation reaction, no H2O2 is formed by the competitive elimination reaction. The kinetic stability of the flavin C(4a)-hydroxide formed by the hydroxylation was also enhanced by the mutation. By studying the effects of the inhibitor azide on the oxidative sequence, we were able to conclude that the inhibitory site is readily accessible to solvent; azide binding at a second site slowly displaces the substrate from the reduced enzyme. The mutation has profoundly slowed the rates of ligand binding to the enzyme. Kinetic studies of binding indicated the presence of several enzyme conformations. Thus, the mutation of this one residue interferes with the orientation of pyridine nucleotide and flavin during reduction, stabilizes flavin C(4a) intermediates, prevents substrate ionization, and alters the rates and strengths of ligand binding.

4-Hydroxybenzoate-3-Monooxygenase↗
PubMed2000

Glutamate 325 is a general acid-base catalyst in the reaction catalyzed by fructose-2,6-bisphosphatase.

A bifunctional enzyme, fructose-6-phosphate, 2-kinase:fructose-2, 6-bisphosphatase, catalyzes synthesis and hydrolysis of fructose 2, 6-bisphosphate. The phosphatase reaction occurs in two steps: the formation of a phosphoenzyme intermediate and release of beta-D-fructose 6-phosphate, followed by hydrolysis of the phosphoenzyme. The objective of this study was to determine whether E325 in the Fru 2,6-Pase active site is an acid-base catalyst. The pH-rate profile for k(cat) for the wild-type enzyme exhibits pK values of 5.6 and 9.1. The pH dependence of k(cat) for the E325A mutant enzyme gives an increase in the acidic pK from 5.6 to 6.1. Formate, acetate, propionate, and azide accelerate the rate of hydrolysis of the E325A mutant enzyme, but not of the wild-type enzyme. Azide and formate, the smallest of the weak acids tested, are the most potent activators. The k(cat) vs pH profile of the E325A mutant enzyme in the presence of formate is similar to that of the wild-type enzyme. Taken together, these data are consistent with E325 serving an acid-base role in the phosphatase reaction. The exogenous low MW weak acids act as a replacement general base in the hydrolysis of the phosphoenzyme intermediate, rescuing some of the activity lost upon eliminating the glutamate side chain.

Alanine↗
PubMed1987

Products of the inactivation of ribonucleoside diphosphate reductase from Escherichia coli with 2'-azido-2'-deoxyuridine 5'-diphosphate.

Ribonucleoside diphosphate reductase (RDPR) from Escherichia coli was completely inactivated by 1 equiv of the mechanism-based inhibitor 2'-azido-2'-deoxyuridine 5'-diphosphate (N3UDP). Incubation of RDPR with [3'-3H]N3UDP resulted in 0.2 mol of 3H released to solvent per mole of enzyme inactivated, indicating that cleavage of the 3' carbon-hydrogen bond occurred in the reaction. Incubation of RDPR with [beta-32P]N3UDP resulted in stoichiometric production of inorganic pyrophosphate. One equivalent of uracil was eliminated from N3UDP, but no azide release was detected. Analysis of the reaction of RDPR with [15N3]N3UDP by mass spectrometry revealed that the azide moiety was converted to 0.9 mol of nitrogen gas per mole of enzyme inactivated. The tyrosyl radical of the B2 subunit was destroyed during the inactivation by N3UDP as reported previously [Sjöberg, B.-M., Gräslund, A., & Eckstein, F. (1983) J. Biol. Chem. 258, 8060-8067], while the specific activity of the B1 subunit was reduced by half. Incubation of [5'-3H]N3UDP with RDPR resulted in stoichiometric covalent radiolabeling of the enzyme. Separation of the enzyme's subunits by chromatofocusing revealed that the modification was specific for the B1 subunit.

Affinity Labels↗
PubMed1988

Active site of dopamine beta-hydroxylase. Comparison of enzyme derivatives containing four and eight copper atoms per tetramer using potentiometry and EPR spectroscopy.

Potentiometric titrations, continuous wave EPR, and microwave power saturation measurements have been used to examine 8-Cu and 4-Cu forms of native dopamine beta-hydroxylase and its azide derivative. The formation curve for the binding of Cu2+ to the apoenzyme is best fit by assuming two independent binding sites per subunit, with pK' values of 8.90 and 7.35 at pH 5.0. On the other hand, only minor differences are observed in either continuous wave EPR spectra or power saturation behavior of the 8- and 4-Cu forms of the native enzyme or of its azide derivative. The intensity of the EPR spectra of all derivatives integrates to greater than 95% of the total copper, and the temperature dependence of P1/2 shows no evidence for any S = 1 state of the copper ions in the enzyme. These results suggest a lower limit of ca. 7 A for the separation between the two copper ions per subunit and thus rule out a type 3 site in the oxidized enzyme. The data are most consistent with Cu(II) sites consisting of two or three N (imidazole) and one or two O donor ligands in the coordination sphere. The similarity in EPR spectra and power saturation of 8- and 4-Cu derivatives suggests that the difference in Cu-binding constants may be due either to differences in the identity of an axial ligand or to solvation effects in the active site.

Animals↗
PubMed1980

High-pressure proton nuclear magnetic resonance studies of hemoproteins. Pressure-induced structural change in heme environments of myoglobin, hemoglobin, and horseradish peroxidase.

Hyperfine shifted proton NMR spectra of metmyoglobin, methemoglobin, and their complexes with azide, imidazole, and cyanide as well as the spectrum of native horseradish peroxidase were obtained at high pressures up to 2000 atm with a specially designed high-pressure cell for 220-MHz superconducting NMR spectrometer. For the azide complexes of metmyoglobin, in all of which the iron atoms are in thermal spin equilibrium between high- and low-spin states, the increased pressure shifted their heme methyl proton signals to the upfield side. For the cyanide complexes of metmyoglobin and methemoglobin and for the fluoride complex of metmyoglobin, which are in purely low- and high-spin states, respectively, the spectra were almost insensitive to changes in pressure up to 2000 atm. The heme methyl proton signals of aquometmyoglobin, its formate complex, and horseradish peroxidase showed appreciable upfield shifts upon pressurization. These results were interpreted to indicate that the primary effect of pressure on the hemoprotein structure is to shift the spin equilibrium in favor of the low-spin form. Hemichrome formation of methemoglobin at high pressures was also observed, and the effect of pressure on the heme environmental structure of deoxyhemoglobin and deoxymyoglobin was also discussed.

Animals↗
PubMed1978

Photogenerated reagents for membrane labeling. 1. Phenylnitrene formed within the lipid bilayer.

Phenylnitrene generated photochemically from phenyl azide that is bound to artificial phospholipid vesicles labels the fatty acid chains of the lipids in low yield. The labeling yield varies from approximately 3.3% with soybean lecithin (which is highly unsaturated) to approximately 0.25% with dimyristoyllecithin (which is completely saturated). Labeling is largely eliminated by reduced glutathione in the aqueous phase. Nitrenes are evidently unsatisfactory reagents for the labeling either of lipids or by analogy of the hydrophobic portions of membrane proteins. This is mainly because the long lifetimes and electrophilic character of nitrenes will lead to the preferential labeling of extrinsic membrane components. Phenyl azide itself is further compromised as a lipophilic reagent by its rather low partition coefficient into lipid bilayers, as measured by equilibrium dialysis.

Azides↗
PubMed1977

Metal sites of copper-zinc superoxide dismutase.

Silver-copper and silver-cobalt proteins have been prepared in which Ag+ resides in the native copper site of superoxide dismutase and either Cu2+ of Co2+ reside in the zinc site. The electron paramagnetic resonance (EPR) spectrum of the copper and the visible absorption spectrum of the cobalt greatly resemble those of either Cu4 of Cu2,Cu2,Co2 proteins, respectively, in which the copper of the native copper sites has been reduced. It was found that, unlike cyanide, azide anion would not perturb the EPR spectrum of Ag2,Cu2 protein. Since azide produces the same perturbation upon the EPR spectrum of native and Cu2 proteins, it must bind to the copper and not the zinc of superoxide dismutase. A model of the metal sites of the enzyme has been fitted to a 3-A electron-density map using an interactive molecular graphics display. The model shows that histidine-61, which appears to bind both copper and zinc, does not lie in the plane of the copper and its three other histidine ligands, but occupies a position intermediate between planar and axial. This feature probably accounts for the rhombicity of the EPR spectrum and the activity of the enzyme.

Azides↗
PubMed1976

Kinetics of reaction of anions with methemerythrin derivatives.

The kinetics of anation of methemerythrin over a wide range of pH and concentration of anions have been studied at 25 degrees C. The azide and thiocyanate ions have been most intensively investigated but experiments with fluoride and chloride are also reported. The replacement of anion in methemerythrin-anionic adducts by other anions has also been studied. Except for replacement of met-fluoride by azide, all replacements can be explained by a dissociative mechanism via the aquated species. Anations are second-order and an associative mechanism is preferred. The second-order rate constant decreases with increasing anion concentrations (from 20 muM to 20 mM). This is attributed to the effect of a secondary anion binding site. The behavior of octameric and monomeric forms of the protein toward thiocyanate is identical. A comparison of results with simple Fe(III) complexes and certain metalloproteins is made.

Animals↗
PubMed2003

Catalytic and structural effects of amino acid substitution at histidine 30 in human manganese superoxide dismutase: insertion of valine C gamma into the substrate access channel.

Catalysis of the disproportionation of superoxide by human manganese superoxide dismutase (MnSOD) is characterized by an initial burst of catalysis followed by a much slower region that is zero order in superoxide and due to a product inhibition by peroxide anion. We have prepared site-specific mutants with replacements at His30, the side chain of which lies along the substrate access channel and is about 5.8 A from the metal. Using pulse radiolysis to generate superoxide, we have determined that kcat/K(m) was decreased and product inhibition increased for H30V MnSOD, both by 1-2 orders of magnitude, compared with wild type, H30N, and H30Q MnSOD. These effects are not attributed to the redox potentials, which are similar for all of these variants. An investigation of the crystal structure of H30V Mn(III)SOD compared with wild type, H30Q, and H30N Mn(III)SOD showed the positions of two gamma carbons of Val30 in the active site; Cgamma1 overlaps Cgamma of His30 in wild type, and Cgamma2 extends into the substrate access channel and occupies the approximate position of a water molecule in the wild type. The data suggest that Cgamma2 of the Val side chain has significantly interrupted catalysis by this overlap into the access channel with possible overlap with the substrate-product binding site. This is supported by comparison of the crystal structure of H30V MnSOD with that of azide bound to Mn(III)SOD from Thermus thermophilus and by visible absorption spectra showing that azide binding to the metal in H30V Mn(III)SOD is abolished. Moreover, the presence of Val30 caused a 100-fold decrease in the rate constant for dissociation of the product-inhibited complex compared with wild type.

Amino Acid Substitution↗
PubMed2003

Electrochemical characterization of purified Rhus vernicifera laccase: voltammetric evidence for a sequential four-electron transfer.

Rhus vernicifera (Rv) laccase was purified to electrophoretic homogeneity by hydrophobic interaction chromatography. A comprehensive study of the direct electrochemistry of Rv laccase covalently immobilized at a gold electrode using alkanethiol monolayers was undertaken. The observed midpoint potential was 410 mV versus the normal hydrogen electrode (NHE), consistent with reduction potentials obtained by potentiometric titration for the T1 copper site. Evidence is presented for a concerted 4-electron reversible process at slow scan rates (v) on the basis of peak current ratios (i(pa)/i(pc)). Catalytic currents were observed in the presence of the biological substrate oxygen, indicating that laccase activity is retained throughout the immobilization process. Electrochemical characteristics of the immobilized laccase were essentially invariant across the pH range 5.5-8.5 and the temperature range 5-35 degrees C. The purified enzyme displayed a pH optimum of 9.0, when assayed spectrophotometrically with syringaldazine as a substrate. Inhibition of the laccase activity with azide or fluoride showed an I(50)(NaN(3)) of 2.5 mM and an I(50)(NaF) of 18.5 mM. Electrochemistry in the presence of azide reduces the anodic current by ca. one-half, consistent with the 4-electron process decreasing to a 2-electron process. However, fluoride has no effect on anaerobic electrochemistry. These electrochemical results suggest that the pH dependence of laccase activity is related to the effects of pH on the structure or binding of the substrate.

Catalysis↗