Interaction of flavodoxin with cyanobacterial thylakoids

Flavodoxin from the cyanobacterium Anabaena PCC 7119 has been shown to mediate, under illumination, the transfer of electrons from the thylakoidal membranes that were isolated from the same organism, to both the enzyme ferredoxin-NADP⁺ reductase and cytochrome c. Chemical cross-linking of ferredoxin or flavodoxin to the photosynthetic membranes provides a preparation that is active in cytochrome c photoreduction without the addition of external protein carrier. NADP⁺ photoreduction, albeit diminished, was observed only after addition of exogenous electron carrier protein. Immunoblotting analysis of the chemical adduct reveals that flavodoxin binds to a 10 kDa polypeptide subunit in the cyanobacterial Photosystem I which appears to act as its physiological partner in the electron transfer process.Abbreviations Fd ferredoxin - Fld flavodoxin - cyt c cytochrome c - EDC 1-ethyl-3-(3-dimethyl-aminopropyl) carbodiimide - PS I Photosystem I     

Effects of NH4 +, NO3 − and HCO3 − on apoplast pH in the outer cortex of root zones of maize, as measured by the fluorescence ratio of fluorescein boronic acid

A fluorimetric ratio technique was elaborated to measure apoplastic pH in the outer root cortex of maize (Zea mays L.) grown hydroponically. A newly synthesized fluorescent probe, fluorescein boronic acid (pK_a = 5.48), which covalently binds to the cell wall of the outer cell layers, was used. Under conditions of saturating ion concentrations the apoplastic pH was determined along the root axis ranging from 1 to 30 mm behind the root tip. Apoplastic pH was recorded for root segment areas (1 mm²), and pH values of high statistical significance were obtained. With an external solution of pH 5, the apoplastic pH was about pH 5.1 in the division zone, between pH 4.8 and 4.9 in the elongation region and about pH 4.9 in the root hair zone. At an external pH of 8.6, the difference between the external pH and the apoplastic pH was considerably more, with a pH of 5.2–5.3 in all root zones. Addition of 1 mM NH₄ ⁺ caused a small apoplastic pH decrease (0.05 of a pH unit) in all root zones. Apoplastic alkalization upon application of 6 mM NO₃ ⁻ was highest (0.3 of a pH unit) in the zone where root hairs emerge; in the division and early elongation zones, apoplastic pH increased only transiently. In the presence of 10 mM HCO₃ ⁻, NO₃ ⁻ elicited a higher and persistent alkalization (0.06–0.25 of a pH unit) in all root zones. Application of fusicoccin reduced apoplastic pH from 4.85 to 4.75 in the elongation zone, while inhibition of the H⁺-ATPase with vanadate alkalized the apoplast in the root hair zone from pH 5.4 to 5.6. The observed pH differences along the root axis upon differential N supply and application of HCO₃ ⁻ provide evidence that this new pH technique is a useful tool with which to measure apoplastic pH, and in future may permit measurements at microsites at the cell level by use of microscope imaging. Received: 26 August 1998 / Accepted: 4 May 1999     

Effects of nonspecific ion-protein interactions on the redox chemistry of cytochrome c

 The effects of the ionic atmosphere on the enthalpic and entropic contributions to the reduction potential of native (state III) beef heart cytochrome c have been determined through variable-temperature direct electrochemistry experiments. At neutral or slightly alkaline pH values, from 5 to 50  °C, the reduction enthalpy and entropy become less negative with decreasing ionic strength. The reduction entropy extrapolated at null ionic strength is approximately zero, indicating that, in the absence of the screening effects of the salt ions on the network of the electrostatic interactions at the protein-solvent interface, the solvation properties and the conformational flexibility of the two redox states are comparable. The moderate decrease in E°′ observed with increasing ionic strength [ΔE°′_IS =(E°′) _{I =0.1 M}–(E°′) _{I =0 M}=–0.035 V at 25  °C], once the compensating enthalpic and entropic effects of the salt-induced changes in the hydrogen bonding within the hydration sphere of the molecule in the two redox states are factorized out, results in being ultimately determined by the stabilizing enthalpic effect of the negatively charged ionic atmosphere on the ferri form. At pH 9, the ionic strength dependence of the reduction termodynamics of cytochrome c follows distinctive patterns, possibly as a result of specific binding of the hydroxide ion to the protein. A decrease in ionic strength at constant pH, as well as a pH increase at constant ionic strength, induces a depression of the temperature of the transition from the low-T to high-T conformer of cytochrome c, which suggests that a temperature-induced decrease in the pK _a for a residue deprotonation is the key event of this conformational change. Received: 7 April 1999 / Accepted: 19 July 1999     

Investigation of exchange couplings in [Fe3S4]+ clusters by electron spin-lattice relaxation

3 S₄]⁺, S=1/2, composed of three, antiferromagnetically coupled high-spin ferric ions) by continuous wave (CW) and pulsed EPR techniques: Azotobacter vinelandii ferredoxin I, Desulfovibrio gigas ferredoxin II, and the 3Fe forms of Pyrococcus furiosus ferredoxin and aconitase. The 35 GHz (Q-band) CW EPR signals are simulated to yield experimental g tensors, which either had not been reported, or had been reported only at X-band microwave frequency. Pulsed X- and Q-band EPR techniques are used to determine electron spin-lattice (T ₁, longitudinal) relaxation times at several positions on the samples' EPR envelope over the temperature range 2–4.2 K. The T ₁ values vary sharply across the EPR envelope, a reflection of the fact that the envelope results from a distribution in cluster properties, as seen earlier as a distribution in g ₃ values and in ⁵⁷ Fe hyperfine interactions, as detected by electron nuclear double resonance spectroscopy. The temperature dependence of 1/T ₁ is analyzed in terms of the Orbach mechanism, with relaxation dominated by resonant two-phonon transitions to a doublet excited state at ∼20 cm⁻¹ above the doublet ground state for all four of these 3Fe proteins. The experimental EPR data are combined with previously reported ⁵⁷Fe hyperfine data to determine electronic spin exchange-coupling within the clusters, following the model of Kent et al. Their model defines the coupling parameters as follows: J ₁₃=J, J ₁₂=J(1+ε′), J ₂₃=J(1+ε), where J _ij is the isotropic exchange coupling between ferric ions i and j, and ε and ε′ are measures of coupling inequivalence. We have extended their theory to include the effects of ε′≠0 and thus derived an exact expression for the energy of the doublet excited state for any ε, ε′. This excited state energy corresponds roughly to ε J and is in the range 5–10 cm⁻¹ for each of these four 3Fe proteins. This magnitude of the product ε J, determined by our time-domain relaxation studies in the temperature range 2–4 K, is the same as that obtained from three other distinct types of study: CW EPR studies of spin relaxation in the range 5.5–50 K, NMR studies in the range 293–303 K, and static susceptibility measurements in the range 1.8–200 K. We suggest that an apparent disagreement as to the individual values of J and ε be resolved in favor of the values obtained by susceptibility and NMR (J≳200 cm⁻¹ and ε≳0.02 cm⁻¹ ), as opposed to a smaller J and larger ε as suggested in CW EPR studies. However, we note that this resolution casts doubt on the accepted theoretical model for describing the distribution in magnetic properties of 3Fe clusters. Received: 23 December 1999 / Accepted: 8 March 2000     

Modulation of Bacillus pasteurii cytochrome c 553 reduction potential by structural and solution parameters

 Direct cyclic voltammetry and ¹H NMR spectroscopy have been combined to investigate the electrochemical and spectroscopic properties of cytochrome c ₅₅₃ isolated from the alkaliphilic soil bacterium Bacillus pasteurii. A quasi-reversible diffusion-controlled redox process is exhibited by cytochrome c ₅₅₃ at a pyrolitic graphite edge microelectrode. The temperature dependence of the reduction potential, measured using a non-isothermal electrochemical cell, revealed a discontinuity at 308 K. The thermodynamic parameters determined in the low-temperature range (275–308 K;ΔS°′=–162.7±1.2 J mol^–1 K^–1, ΔH°′=–53.0±0.5 kJ mol^–1, ΔG°′=–4.5±0.1 kJ mol^–1, E°′=+47.0±0.6 mV) indicate the presence of large enthalpic and entropic effects, leading, respectively, to stabilization and destabilization of the reduced form of cytochrome c ₅₅₃. Both effects are more accentuated in the high-temperature range (308–323 K;ΔS°′=–294.1±8.4 J mol^–1 K^–1, ΔH°′=–93.4±3.1 kJ mol^–1, ΔG°′=–5.8±0.6 kJ mol^–1, E°′=+60.3±5.8 mV), with the net result being a slight increase of the standard reduction potential. These thermodynamic parameters are interpreted using the compensation theory of hydration of biopolymers as indicating the extrusion, upon reduction, of water molecules from the hydration sphere of the cytochrome. The low-T and high-T conformers differ by the number of water molecules in the solvation sphere: in the high-T conformer, the number of water molecules extruded upon reduction increases, as compared to the low-T conformer. The ionic strength dependence of the reduction potential at 298 K, treated within the frame of extended Debye-Hückel theory, yields values of E ^°′ _(I=0) =–25.4±1.4 mV, z _red=–11.3, and z _ox=–10.3. The pH dependence of the reduction potential at 298 K shows a plateau in the pH range 7–10 and an increase at more acidic pH, allowing the calculation of pK _O=5.5 and pK _R=5.7, together with the estimate of the reduction potentials of completely protonated (+71 mV) and deprotonated (+58 mV) forms of cytochrome c ₅₅₃. ¹H NMR spectra of the oxidized paramagnetic cytochrome c ₅₅₃ indicate the presence of a His-Met axial coordination of the low-spin (S=1/2) heme iron, which is maintained in the temperature interval 288–340 K at pH 7 and in the pH range 4.8–10.0 at 298 K. The temperature dependence of the hyperfine-shifted signals shows both Curie-type and anti-Curie-type behavior, with marked deviations from linearity, interpreted as indicating the presence of a fast equilibrium between the low-T and high-T conformers, having slightly different heme electronic structures resulting from the T-induced conformational change. Increasing the NaCl concentration in the range 0–0.2 M causes a slight change of the ¹H NMR chemical shifts of the hyperfine-shifted signals, with no influence on their linewidth. The calculated lower limit value of the apparent affinity constant for specific ion binding is estimated as 5.2±1.1 M^–1. The pH dependence of the isotropically shifted ¹H NMR signals of the oxidized cytochrome displays at least one ionization step with pK _O=5.7. The thermodynamic and spectroscopic data indicate a large solvent-derived entropic effect as the main cause for the observed low reduction potential of B. pasteurii cytochrome c ₅₅₃. Received: 9 January 1998 / Accepted: 8 April 1998     

High rates of microbial sulphate reduction in a mesophilic ethanol-fed expanded-granular-sludge-blanket reactor

In a mesophilic (30–35 °C), sulphidogenic, ethanol-fed expanded-granular-sludge-blanket reactor, sulphate, at loading rates of up to 10.0–12.0 g Sl⁻¹␣day⁻¹, was removed with an average efficiency of more than 80%. The pH was between 7.7 and 8.3 and the maximal total dissolved sulphide concentration was up to 20 mM S (650 mg S/l). The alkaline pH was maintained by either a pH-control unit with sodium hydroxide or by stripping part of the sulphide and CO₂ from the recycle with nitrogen gas. The superficial upstream liquid velocity (v _up) was 3.0–4.5 m/h. The ratio of ethanol to sulphur was near stoichiometry. At alkaline pH, the activity of the acetotrophic sulphate-reducing bacteria, growing on acetate, was strongly enhanced, whereas at pH below 7.7 the acetotrophic sulphate-reducing bacteria were inhibited by aqueous H₂S. With regard to the removal efficiency and operational stability, external stripping with N₂ and pH control were equally successful. Received: 2 December 1996 / Received revision: 13 March 1997 / Accepted: 15 March 1997     

Resonance Raman characterization of the di-heme protein cytochrome c4 from Pseudomonas stutzeri

 Di-heme Pseudomonas stutzeri cytochrome c ₄ has been characterized by electronic absorption and resonance Raman spectroscopies in the ferric and ferrous forms at pH 7.5 and at room temperature. The data indicate that the two hemes are inequivalent. It is proposed that the N-terminal contains a more relaxed heme as a consequence of the relative orientation of the methionine and histidine ligands with respect to the N-Fe-N directions of the heme plane. This causes a weakening of the Fe-S bond with concomitant partial dissociation of the methionine and the formation of an Fe-aquo bond. Heme group relaxation is further accompanied by less distortion of the heme group than that associated with cytochrome c, expansion of the "core" and a negative shift of the redox potential. Received: 17 December 1996 / Accepted: 6 March 1997     

Pyranose 2-oxidase from Phanerochaete chrysosporium– further biochemical characterisation

Pyranose 2-oxidase (P2O) was purified 43-fold to apparent homogeneity from the basidiomycete Phanerochaete chrysosporium using liquid chromatography on phenyl Sepharose, Mono Q (twice) and phenyl Superose. The native enzyme has a molecular mass of about 250 kDa (based on native PAGE) and is composed of four identical subunits of 65 kDa. It contains three isoforms of isoelectric point (pI) 5.0, 5.05 and 5.15 and does not appear to be a glycoprotein. P2O is optimally stable at pH 8.0 and up to 60 °C. It is active over a broad pH range (5.0–9.0) with maximum activity at pH 8.0–8.5 and at 55 °C, and a broad substrate specificity. d-Glucose is the preferred substrate, but 1-β-aurothioglucose, 6-deoxy-d-glucose, l-sorbose, d-xylose, 5-thioglucose, d-glucono-1,5-lactone, maltose and 2-deoxy-d-glucose are also oxidised at relatively high rates. A Ping Pong Bi Bi mechanism was demonstrated for the P2O reaction at pH 8.0, with a catalytic constant (k _cat) of 111.0 s⁻¹ and an affinity constant (K _m) of 1.43 mM for d-glucose and 83.2 μM for oxygen. Whereas the steady-state kinetics for glucose oxidation were unaffected by the medium at pH ≥ 7.0, at low pH both pH and buffer composition affected the P2O kinetics with the k _cat/K _m value decreasing with decreasing pH. The greatest effect was observed in acetate buffer (0.1 M, pH 4.5), where the k _cat decreased to 60.9 s⁻¹ and the K _m increased to 240 mM. The activity of P2O was completely inhibited by 10 mM HgCl₂, AgNO₃ and ZnCl₂, and 50% by lead acetate, CuCl₂ and MnCl₂. Received: 28 August 1996 / Received revision: 25 November 1996 / Accepted: 29 November 1996     

The Relative Rates of Thiol–Thioester Exchange and Hydrolysis for Alkyl and Aryl Thioalkanoates in Water

This article reports rate constants for thiol–thioester exchange (k _ex), and for acid-mediated (k _a), base-mediated (k _b), and pH-independent (k _w) hydrolysis of S-methyl thioacetate and S-phenyl 5-dimethylamino-5-oxo-thiopentanoate—model alkyl and aryl thioalkanoates, respectively—in water. Reactions such as thiol–thioester exchange or aminolysis could have generated molecular complexity on early Earth, but for thioesters to have played important roles in the origin of life, constructive reactions would have needed to compete effectively with hydrolysis under prebiotic conditions. Knowledge of the kinetics of competition between exchange and hydrolysis is also useful in the optimization of systems where exchange is used in applications such as self-assembly or reversible binding. For the alkyl thioester S-methyl thioacetate, which has been synthesized in simulated prebiotic hydrothermal vents, k _a = 1.5 × 10⁻⁵ M⁻¹ s⁻¹, k _b = 1.6 × 10⁻¹ M⁻¹ s⁻¹, and k _w = 3.6 × 10⁻⁸ s⁻¹. At pH 7 and 23°C, the half-life for hydrolysis is 155 days. The second-order rate constant for thiol–thioester exchange between S-methyl thioacetate and 2-sulfonatoethanethiolate is k _ex = 1.7 M⁻¹ s⁻¹. At pH 7 and 23°C, with [R″S(H)] = 1 mM, the half-life of the exchange reaction is 38 h. These results confirm that conditions (pH, temperature, pK _a of the thiol) exist where prebiotically relevant thioesters can survive hydrolysis in water for long periods of time and rates of thiol–thioester exchange exceed those of hydrolysis by several orders of magnitude.     

A prokaryotic sucrose synthase gene (susA) isolated from a filamentous nitrogen-fixing cyanobacterium encodes a protein similar to those of plants

Sucrose synthase (SS), a key enzyme in plant carbohydrate metabolism, has recently been isolated from Anabaena sp. strain PCC 7119, and biochemically characterized; two forms (SS-I and SS-II) were detected (Porchia et al. 1999, Planta 210: 34–40). The present study describes the first isolation and characterization of a prokaryotic SS gene, susA, encoding SS-II from that strain of Anabaena. A 7 kbp DNA fragment containing an open reading frame (EMBL accession number AJ010639) with about 30–40% amino acid identity with plant SSs was isolated from an Anabaena subgenomic library. The putative SS gene was demonstrated to encode an SS protein by expression in Escherichia coli. The biochemical properties of the recombinant enzyme were identical to those of the enzyme purified from the cyanobacterial cells. The deduced amino acid sequence of the Anabaena SS diverged from every plant SS reported. The occurrence of SS in cyanobacteria of different taxonomic groups was investigated. The enzyme occurs in several filamentous nitrogen-fixing cyanobacteria but not in two species of unicellular, non-diazotrophic cyanobacteria. Received: 5 January 2000 / Accepted: 7 March 2000     

pH dependence of the enantioselective excited-state quenching of Λ,Δ-Tb(III) and Λ,Δ-Eu(III)tris(pyridine-2,6-dicarboxylate) chelates by ferricytochrome c from horse heart and ferricytochrome c-550 from Paracoccus versutus

 The pH dependence of the dynamic quenching of the luminescence from Tb(III) and Eu(III) tris(pyridine-2,6-dicarboxylate≡DPA) chelates by the title proteins is studied. For Tb(DPA)₃ ^3– also the quenching by the Lys 14→Glu and Lys99→Glu mutants of cytochrome c-550 (cytc-550) is investigated. The rate constants for quenching of the electronically excited Λ and Δ enantiomers of the luminophore by equine cytochrome c show a sharp decrease upon increasing the pH from 7 to 10, which can be described phenomenologically by deprotonation of a single acidic group with pK _a of 9.2±0.1 for Eu and 9.4±0.1 for Tb. These values are similar to that found for the alkaline transition of the protein. The alkaline conformer(s) of the protein at pH>10 is found to be a very inefficient quencher of the lanthanide luminescence. For Tb, but not for Eu, a significant lowering of the degree of enantioselectivity (E _q) in the quenching is found along with a reduction of the quenching rates. For cytc-550, the decrease of the quenching rate constants with increasing pH is described by pK _a=9.8±0.1 and for the two mutants the same value is obtained. For the cytc-550 proteins the change of the quenching rates does not correlate with the alkaline transition, for which a pK _a of 11.2 has been reported by other workers. For all proteins, the reduction of the quenching rates at high pH is ascribed to a reduction of the binding affinity of the excited lanthanide complex to the surface area of the protein near the exposed heme edge, caused by deprotonation of (presumably) several lysine residues. Received: 3 April 1998 / Accepted: 15 June 1998     

Redox chemistry of biological tungsten: an EPR study of the aldehyde oxidoreductase from Pyrococcus furiosus

 Aldehyde:ferredoxin oxidoreductase (AOR) from the hyperthermophilic archaeon Pyrococcus furiosus is a homodimeric protein. Each subunit carries one [4Fe-4S] cubane and a novel tungsten cofactor containing two pterins. A single iron atom bridges between the subunits. AOR has previously been studied with EPR spectroscopy in an inactive form known as the red tungsten protein (RTP): reduced RTP exhibits complex EPR interaction signals. We have now investigated the active enzyme AOR with EPR, and we have found an S = 1/2 plus S = 3/2 spin mixture from a non-interacting [4Fe-4S]¹⁺ cluster in the reduced enzyme. Oxidized AOR affords EPR signals typical for W(V) with g–values of 1.982, 1.953, and 1.885. The W(V) signals disappear at a reduction potential E _m,7.5 of +180 mV. This unexpectedly high value indicates that the active-site redox chemistry is based on the pterin part of the cofactor. Received: 18 December 1995 / Accepted: 26 March 1996     

Hemoprotein models based on a covalent helix-heme-helix sandwich. 3. Coordination properties, reactivity and catalytic application of Fe(III)- and Fe(II)-mimochrome I

 The coordination state of Fe(III)- and Fe(II)-mimochrome I, a covalent peptide-deuteroheme sandwich involving two nonapeptides bearing a histidine residue in a central position, was studied by UV-visible, EPR, and resonance Raman spectroscopy. The ferric and ferrous states of this new iron species mainly exist, at pH 7, in a low-spin hexacoordinate form with two axial histidine ligands coming from the peptide chains. A minor amount of high-spin form for the ferric state is also present at pH 7. However, it is mainly high-spin at pH 2 or in DMSO. Fe(II)-mimochrome I binds CO with an affinity comparable to that of myoglobin and hemoglobin. Fe(III)-mimochrome I reacts with alkylhydroxylamine and arylhydrazines, leading to the corresponding Fe(II)-nitrosoalkyl and Fe(III)-σ-aryl complexes, respectively. These reactions were greatly dependent on the solvent used and on the pH, and were much slower than the corresponding reactions performed by deuterohemin in the presence of excess imidazole. All these results indicate that the reactivity of iron-mimochrome I is controlled by the binding of the peptide chains to the iron. The reactivity shown by this complex at neutral pH is intermediate between that observed for iron porphyrins in the presence of excess imidazole and that of hemoproteins characterized by a strong bis-histidine axial coordination, such as cytochrome b ₅. Fe(III)-mimochrome I is able to catalyze styrene epoxidation by using a [Fe(III)-mimochrome I]/[H₂O₂]/[stryrene] ratio of 1 : 10 : 2000 in phosphate buffer solution (pH 7.2) containing 2% CTAB both under strictly anaerobic conditions and in the presence of oxygen, at 0  °C. Received: 26 May 1998 / Accepted: 20 August 1998     

Electron and proton transfer in the <Emphasis Type="Italic">ba</Emphasis><Subscript>3</Subscript> oxidase from <Emphasis Type="Italic">Thermus thermophilus</Emphasis>

The ba ₃-type cytochrome c oxidase from Thermus thermophilus is phylogenetically very distant from the aa ₃–type cytochrome c oxidases. Nevertheless, both types of oxidases have the same number of redox-active metal sites and the reduction of O₂ to water is catalysed at a haem a ₃-Cu_B catalytic site. The three-dimensional structure of the ba ₃ oxidase reveals three possible proton-conducting pathways showing very low homology compared to those of the mitochondrial, Rhodobacter sphaeroides and Paracoccus denitrificans aa ₃ oxidases. In this study we investigated the oxidative part of the catalytic cycle of the ba ₃ -cytochrome c oxidase using the flow-flash method. After flash-induced dissociation of CO from the fully reduced enzyme in the presence of oxygen we observed rapid oxidation of cytochrome b (k ≅ 6.8 × 10⁴ s⁻¹) and formation of the peroxy (P_R) intermediate. In the next step a proton was taken up from solution with a rate constant of ~1.7 × 10⁴ s⁻¹, associated with formation of the ferryl (F) intermediate, simultaneous with transient reduction of haem b. Finally, the enzyme was oxidized with a rate constant of ~1,100 s⁻¹, accompanied by additional proton uptake. The total proton uptake stoichiometry in the oxidative part of the catalytic cycle was ~1.5 protons per enzyme molecule. The results support the earlier proposal that the P_R and F intermediate spectra are similar (Siletsky et al. Biochim Biophys Acta 1767:138, 2007) and show that even though the architecture of the proton-conducting pathways is different in the ba ₃ oxidases, the proton-uptake reactions occur over the same time scales as in the aa ₃-type oxidases. Smirnova and Zaslavsky contributed equally to the work described in this paper.     

Physicochemical Characterization of NPC 1161C,A Novel Antimalarial 8-Aminoquinoline,in Solution and Solid State

NPC 1161C is a novel antimalarial drug of interest because of its superior curative and prophylactic activity, and favorable toxicity profile against in vivo and in vitro models of malaria, pneumocystis carinii pneumonia, and leishmaniasis. The preformulation studies performed included determination of pK_as, aqueous and pH solubility, cosolvent solubility, log P, pH stability, thermal analysis, and preliminary hygroscopicity studies. The mean pK_a1, pK_a2, and pK_a3 were determined to be 10.12, 4.07, and 1.88, respectively. The aqueous solubility was found to be 2.4 × 10⁻⁴ M having a saturated solution pH of 4.3–5.0 and a low intrinsic solubility of 1.6 × 10⁻⁶ M. A mathematical model of the pH-solubility profile was derived from pH 2.2 to 8.0. An exponential decrease in solubility was observed with increasing pH. The excess solid phase in equilibrium with the solution in aqueous buffers was determined to be the free-base form of the drug. A significant increase in solubility was observed with all the cosolvents studied, in both unbuffered and buffered systems. Mean log P of the salt and the free base were estimated to be 2.18 and 3.70, respectively. The compound had poor stability at pH 7.0 at 37°C, with a t ₉₀ of 3.58 days. Thermal analysis of the drug using DSC and TGA revealed that the drug is present as a semi-crystalline powder, which transformed into the amorphous state after melting. The drug was also found to sublime at higher temperatures. Determination of physicochemical properties of NPC 1161C provided useful information for the development of a dosage form and preclinical evaluation.     

Bioluminescence induction response and survival of the bioreporter bacterium Pseudomonas fluorescens HK44 in nutrient-deprived conditions

Pseudomonas fluorescens HK44 is a bioluminescent bioreporter synthesizing light in the presence of naphthalene or salicylate. Upon immobilization, HK44 is useful as an in situ or on-line biosensor of bioavailable naphthalene and salicylate in waste streams or contaminated fields. The bioreporting efficacy of alginate/SrCl₂-immobilized HK44 was investigated in simulated groundwater with different pH regimes. When induced with complex (salicylate plus auxiliary energy supplements) and simple (salicylate as the sole energy supplement) inducer solutions, the specific light response was steadier at pH 6 than at pH 7 in a 35-day study. There was no bioluminescence response from cells incubated in groundwater samples with pH below 6. The rate of the luminescence reaction was stable at pH 6 irrespective of the type of inducer solution, indicating the robust physiological status of the bioreporter bacteria. In addition, the quantity of light synthesized was at least one order of magnitude higher with complex inducer solution than with simple inducer solution. The numbers of viable and cultivable cells remained constant in groundwater at pH 6 and 7 (approx. 10⁷ g⁻¹ beads). The numbers decreased by four orders of magnitude (10⁷ to 10³) to zero in groundwaters with pH below 6. This study suggested that HK44 is useful for long-term biosensor applications in moderately acidic to neutral groundwater conditions. Received: 6 August 1996 / Received revision: 12 December 1996 / Accepted: 4 January 1997     

The NAD-linked soluble hydrogenase from Alcaligenes eutrophus H16: detection and characterization of EPR signals deriving from nickel and flavin

 In this study we confirmed the previous observation that the cytoplasmic NAD-linked hydrogenase of Alcaligenes eutrophus H16 is EPR-silent in the oxidized state. We also demonstrated the presence of significant Ni-EPR signals when the enzyme was either reduced with the natural electron carrier NADH (5–10 mM) or carefully titrated with sodium dithionite to an intermediate, narrow redox potential range (–280 to –350 mV). Reduction with NADH under argon atmosphere led to a complex EPR spectrum at 80 K with g values at 2.28, 2.20, 2.14, 2.10, 2.05, 2.01 and 2.00. This spectrum could be differentiated by special light/dark treatments into three distinct signals: (1) the "classical" Ni-C signal with g values at 2.20, 2.14 and 2.01, observed with many hydrogenases in the reduced, active state; (2) the light-induced signal (Ni-L) with g values at 2.28, 2.10 and 2.05 and (3) a flavin radical (FMN semiquinone) signal at g = 2.00. The assignment of the Ni-EPR signal was clearly confirmed by EPR spectra of hydrogenase labeled with ⁶¹Ni (nuclear spin I = 3/2) yielding a broadening of the Ni spectra at all g values and a resolved ⁶¹Ni hyperfine splitting into four lines of the low field edge in the case of the light-induced Ni-EPR signal. The redox potentials determined at pH 7.0 for the described redox components were: for FMN –170 mV (midpoint potential, E_m, for appearance), –200 mV (EPR signal intensity maximum) and –230 mV (E_m for disappearance); for the Ni centre (Ni-C), –290 mV (E_m for appearance), –305 mV (signal intensity maximum) and –325 mV (E_m for disappearance). Exposure of the NADH-reduced hydrogenase to carbon monoxide led to an apparent Ni-CO species indicated by a novel rhombic EPR signal with g values at 2.35, 2.08 and 2.01. Received: 19 July 1995 / Accepted: 10 September 1995     

Kinetic studies of electron transfer in the cyt b 5 : metHb system

 The kinetics of methemoglobin reduction by cytochrome b ₅ has been studied by stopped-flow and saturation transfer NMR. A forward rate constant k _f = 2.44×10⁴ M^–1 s^–1 and a reverse rate constant k _b = 540 M^–1s^–1 have been observed at 10 mm, pH 6.20, 25  °C. The ratio k _f/k _b = k _eq = 43.6 is in good agreement with the equilibrium constant calculated from the electrochemical potential between cyt b ₅ and methemoglobin. A bimolecular collisional mechanism is proposed for the electron transfer from cyt b ₅ to methemoglobin based on the kinetic data analysis. The dependence of the rate constants on ionic strengths supports such collisional mechanism. It is also found that the reaction rate strongly depends on the conformations of methemoglobin. Received: 20 February 1996 / Accepted: 4 June 1996     

Effects of hypercapnia on blood-gas and acid-base status in the white sturgeon, Acipenser transmontanus

The effect of environmental hypercapnia on respiratory and acid-base variables was studied in white sturgeon, Acipenser transmontanus. Blood PCO₂, PO₂, pH, hemoglobin concentration, and plasma lactate, glucose, catecholamines and cortisol were measured first under normocapnia (water PCO₂ < 0.5 Torr, 1 Torr = 133.32 Pa), then under hypercapnia (25–35 Torr) and a final return to normocapnia at 19 ± 0.5 °C. Acute (≤ 2h) hypercapnia significantly increased arterial PCO₂ (8-fold increase), ventilation frequency (2-fold increase), plasma HCO₃ ⁻ (2.3-fold) and decreased arterial pH (to 7.15 ± 0.02). After 24 h, norepinephrine, epinephrine and cortisol, were significantly increased, and arterial pH reached its nadir (7.10 ± 0.03). During the 72- and 96-h-periods, arterial PCO₂ (24 ± 4.4 Torr) and ventilatory frequency (105 ± 5 breaths min⁻¹) stabilized, HCO₃ ⁻ reached its apparent maximum (23.6 ± 0.0 mmol⁻¹), glucose decreased by 32%, and pH increased significantly to 7.31 + 0.03. The return to normocapnia completely restored arterial PCO₂ (2.5 ± 0.14 Torr), HCO₃ ⁻ (7.4 ± 0.59 mmol · l⁻¹), ventilation frequency (71 ± 7 breaths · min⁻¹), and pH (7.75 ± 0.04). Overall, hypercapnia produced a respiratory acidosis, hyperventilation, a transient norepinephrine “spike”, and increased plasma catecholamines, cortisol, and arterial PO₂. The respiratory acidosis was only partially compensated (35% pH restoration) 96 h after the onset of hypercapnia and resulted in a significantly decreased blood-O₂ affinity (Bohr effect), as determined by construction of in vitro blood O₂ equilibrium curves at 15 °C and 20 °C. Prolonged exposure to hypercapnia may lead to acid-base disturbances and negatively affect growth of white sturgeon. Accepted: 17 August 1997