Cortisol metabolism in vitro—III. Inhibition of microsomal 6β—hydroxylase and cytosolic 4-ene-reductase

The in vitro metabolism of cortisol in human liver fractions is highly complex and variable. Cytosolic metabolism proceeds predominantly via A-ring reduction (to give 3,5β-tetrahydrocortisol; 3,5β-THF), while microsomal incubations generate upto 7 metabolites, including 6β-hydroxycortisol (6β-OHF), and 6β-hydroxycortisone (6β-OHE), products of the cytochrome P450 (CYP) 3A subfamily. The aim of the present study was, therefore, to examine two of the main enzymes involved in cortisol metabolism, namely, microsomal 6β-hydroxylase and cytosolic 4-ene-reductase. In particular, we wished to assess the substrate specificity of these enzymes and identify compounds with inhibitory potential. Incubations for 30 min containing [³H]cortisol, potential inhibitors, microsomal or cytosolic protein (3 mg), and co-factors were followed by radiometric HPLC analysis. The K_m value for 6β-OHF and 6β-OHE formation was 15.2 ± 2.1 μM (mean ± SD; n = 4) and the V_max value 6.43 ± 0.45 pmol/min/mg microsomal protein. The most potent inhibitor of cortisol 6β-hydroxylase was ketoconazole (K_i = 0.9 ± 0.4 μM; N = 4), followed by gestodene (K_i = 5.6 ± 0.6 μM) and cyclosporine (K_i = 6.8 ± 1.4 μM). Both betamethasone and dexamethasone produced some inhibition (K_i = 31.3 and 54.5 μ, respectively). However, substrates for CYP2C (tolbutamide), CYP2D (quinidine), and CYP1A (theophylline) were essentially non-inhibitory. The K_m value for cortisol 4-ene-reductase was 26.5 ± 11.2 μM (n = 4) and the V_max value 107.7 ± 46.0 pmol/min/mg cytosolic protein. The most potent inhibitors were androstendione (K_i = 17.8 ± 3.3 μM) and gestodene (K_i = 23.8 ± 3.8 μM). Although both compounds have identical A-rings to cortisol, and undergo reduction, inhibition was non-competitive.     

Influence of nickel (II) and chromium (VI) on the laboratory scale rotating biological contactor

High concentration of heavy metals is toxic for most microorganisms and cause strict damage in wastewater treatment operations and often a physico-chemical pretreatment prior to biological treatment is considered necessary. However, in this study it has been shown that biological systems can adapt to Ni (II) and Cr (VI) when their concentration is below 10 and 20 mg/L, respectively. The aim of this study was to evaluate the effect of Ni (II) and Cr (VI) on the lab-scale rotating biological contactor process. It was found that, addition of Ni (II) up to 10 mg/L did not reduce the chemical oxygen demand removal efficiency and on the contrary concentrations below 10 mg/L improved the performance. The influent Ni (II) concentration of 1 mg/L was the concentration where the treatment efficiency produced a maximum COD removal of 86.5%. Moreover, Ni (II) concentration above 10 mg/L was relatively toxic to the system and produced lower treatment efficiencies than the baseline study without Ni (II). Turbidity and suspended solids removals were not stimulated to a great extent with nickel. Addition of Ni (II) did not seem to affect the pH of the system during treatment. The dissolved oxygen concentration did not drop below 4 mg/L at all concentrations of Ni (II) indicating aerobic conditions prevailed in the system. Experiments conducted with Cr (VI) revealed that addition of Cr (VI) up to 20 mg/L did not reduce the COD removal efficiency and on the contrary concentrations below 20 mg/L improved the performance. The influent Cr (VI) concentration of 1 mg/L was the concentration where the treatment efficiency produced a maximum COD removal of 88%. Turbidity and SS removals were more efficient at 5 mg/L Cr (VI) concentration, rather than 1 mg/L, which lead to the conclusion that 5 mg/L Cr (VI) concentration is the optimum concentration, in terms of COD, turbidity and SS removals. Similar with Ni (II) experiments, addition of Cr (VI) did not significantly affect the pH value of the effluent. The DO concentration remained above 5 mg/L.     

Biochemical and biophysical studies on cytochrome aa3 VIII. Effect of cyanide on the catalytic activity

1. 1. Cyanide inhibits the catalytic activity of cytochrome aa₃ in both polarographic and spectrophotometric assay systems with an apparent velocity constant of 4·10³ M⁻¹·s⁻¹ and a K_i that varies from 0.1 to 1.0 μM at 22 °C, pH 7·3.

2. 2. When cyanide is added to the ascorbate-cytochrome c-cytochromeaa₃−O₂ system a biphasic reduction of cytochrome c occurs corresponding to an initial K_i of 0.8 μM and a final K_i of about 0.1 μM for the cytochrome aa₃−cyanide reaction.

3. 3. The inhibited species (a²+a₃³⁺HCN) is formed when a²⁺a₃³⁺ reacts with HCN, when a²⁺a₃²⁺HCN reacts with oxygen, or when a³⁺a₃³⁺HCN (cyano-cytochrome aa₃) is reduced. Cyanide dissociates from a²⁺a₃³⁺HCN at a rate of 2·10⁻³ s⁻¹ at 22 °C, pH 7.3.

4. 4. The results are interpreted in terms of a scheme in which one mole of cyanide binds more tightly and more rapidly to a²⁺a₃³⁺ than to a³⁺a₃³⁺.

Sorptive removal of Methylene blue from aqueous solution using palm kernel fibre: Effect of fibre dose

The use of palm kernel fibre, a readily available agricultural waste product for the sorption of Methylene blue from aqueous solution and the possible mechanism of sorption has been investigated at various fibre doses. The extent of dye removal and the rate of sorption were analyzed using two kinetic rate models (pseudo-first and pseudo-second-order kinetic models) and two diffusion models (intraparticle and external mass transfer models).

Analysis of the kinetic data at different sorbent dose revealed that the pseudo-first order kinetics fitted to the kinetic data only in the first 5 min of sorption and then deviated from the experimental data. The pseudo-second-order kinetic model was found to better fit the experimental data with high correlation coefficients at the various fibre dose used. The dye sorption was confirmed to follow the pseudo-second-order model by investigating the relationship between the amount of dye sorbed and the change in hydrogen ion concentration of the dye solution and also the dependence of dye uptake with solution temperature. It was found that the change in hydrogen ion concentration and increase in sorption temperature were directly related to the amount of dye sorbed, and activation energy was calculated to be −39.57 kJ/mol, indicating that the dye uptake is chemisorption, involving valence forces through sharing or exchange of electrons between sorbent and sorbate as covalent forces.

The intraparticle diffusion plots showed three sections indicating that intraparticle diffusion is not solely rate controlling. The intraparticle diffusion and mass transfer rate constants where observed to be well correlated with sorbent dose in the first 5 min of sorption, indicating sorption process is complex. It was found that at low sorbent dose the mass transfer is the main rate controlling parameter. However at high sorbent dose, intraparticle diffusion becomes rate controlling.     

Bioaccumulation of copper(II), lead(II) and chromium(VI) by growing Aspergillus niger

The effect of copper(II), lead(II) and chromium(VI) ions on the growth and bioaccumulation properties of Aspergillus niger was investigated as a function of initial pH and initial metal ion concentration. The optimum pH values for growth and metal ion accumulation were determined as 5.0, 4.5 and 3.5 for copper(II), lead(II) and chromium(VI) ions, respectively. Although all metal ion concentrations caused an inhibition effect on the growth of A. niger, it was capable of removing of copper(II) and lead(II) with a maximum specific uptake capacity of 15.6 and 34.4 mg g⁻¹ at 100 mg dm⁻³ initial copper(II) and lead(II) concentration, respectively. Growth of A. niger was highly effected by chromium(VI) ions and inhibited by 75 mg dm⁻³ initial chromium(VI) concentration since some inhibition occurred at lower concentrations.     

Biological characterization of A-ring steroids

Hydroxylated 2,19-methylene-bridged androstenediones were designed as potential mimics of enzyme oxidized intermediates of androstenedione. These compounds exhibited competitive inhibition with low micromolar affinities for aromatase. These inhibitory constants (K_i values) were 10 times greater than the 2,19-methylene-bridged androstenedione constant (K_i = 35–70 nM). However, expansion of the 2,19-carbon bridge to ethylene increased aromatase affinity by 10-fold (K_i = 2 nM). Substitution pf a methylene group with oxygen and sulfur in this expanded bridge resulted in K_i values of 7 and 20 nM, respectively. When the substituent was an NH group, the apparent inhibitory kinetics changed from competitive to uncompetitive. All of these analogs exhibited time-dependent inhibition of aromatase activity following preincubation of the inhibitor with human placental microsomes prior to measuring residual enzyme activity. Part of this inhibition was NADPH cofactor-dependent for the 2,19-methyleneoxy- but not for the 2,19-ethylene-bridged androstenedione. The time-dependent inhibition for these four analogs was very rapid since they exhibited τ₅₀ values, the t_1/2 for enzyme inhibition at infinite inhibitor concentration, of 1 to 3 min. These A-ring-bridged androstenedione analogs represent a novel series of potent steroidal aromatase inhibitors. The restrained A-ring bridge containing CH₂, O, S, or NH could effectively coordinate with the heme of the P450 aromatase to allow the tight-binding affinities reflected by their nanomolar K_i values.     

Fuscin, an inhibitor of mitochondrial SH-dependent transport-linked functions

1. 1. Fuscin, a mould metabolite, is a colored quinonoid compound which reacts readily with −SH groups to give colorless addition derivatives.

2. 2. Binding of fuscin to mitochondria has been monitored spectrophotometrically. Fuscin binding is prevented by −SH reagents such as N-ehylmaleimide, N-Methylmaleimide, mersalyl or p-chloromercuribenzoate. Conversely, fuscin prevents the binding of −SH reagents as shown with N-[¹⁴C]ethylmaleimide. Once bound to mitochondria, fuscin is not removable by washing of mitochondria.

3. 3. High affinity-fuscin binding sites (K_d = 1 μM, N = 4–8 nmoles/mg protein) are present in whole mitochondria obtained from rat heart, rat liver, pigeon heart or yeast (Candida utilis). They are lost upon sonication but are still present in digitonin inner membrane + matrix vesicles. On the other hand, lysis of mitochondria by Triton X-100 does not increase the number of high affinity binding sites indicating that all these sites are accessible to fuscin in whole mitochondria. The number of fuscin high affinity sites appears to correlate with the glutathione content of mitochondrial preparations.

4. 4. Fuscin as well as N-ethylmaleimide and avenaciolide are penetrant SH-reagents;

5. 5. Fuscin interferes with the ADP-stimulated respiration of mitochondria on NAD-linked substrates, several functions of the mitochondrial respiratory apparatus being inhibited by fuscin in a non-competitive manner, but to various extents: (a) The electron transfer chain (K_i in the range of 0.1 mM); (b) the lipoamide dehydrogenase system (K_i = 5–10 μM); (c) the transport systems of phosphate (K_i ≈ 20 μM) and of glutamate (K_i = 3–5 μM); (d) the ADP transport, indirectly (K_i ≈ 10 μM).

6. 6. Like N-ethylmaleimide, fuscin inhibits the glutamate-OH⁻ carrier, the inhibition of that carrier bringing about an apparent increase of aspartate entry in glutamate-loaded mitochondria by the glutamate-aspartate carrier.

7. 7. The inhibition of phosphate transport by fuscin probably accounts for the inhibition of the reduction of endogenous NAD by succinate in intact pigeon heart mitochondria.

8. 8. By binding the −SH groups of mitochondrial membrane specifically unmasked by addition of micromolar amounts of ADP, fuscin, like N-ethylmaleimide, prevents the functioning of ADP translocation.

9. 9. Because of their specific and analogous effects on some well defined mitochondrial functions such as glutamate transport and ADP transport, fuscin and N-ethylmaleimide can be distinguished from other −SH reagents. The lipophilic nature of fuscin and N-ethylmaleimide which accounts for the accessbility of these compounds to hydrophobic sites in the mitochondrial membrane or on the matrix side of this membrane may be partly responsible for their characteristic inhibitory effects on mitochondrial functions.

Kinetic evidence for the involvement of a common enzyme in the microsomal reduction of retinal and androstenedione in rat liver

Androst-4-ene-3,17-dione (androstenedione) was found to be a potent competitive inhibitor of the NADH-supported reduction of retinal in rat hepatic microsomes (K_i 42 μM, K_m/K_i ratio 1.1). Similarly, the NADH-mediated reduction of androstenedione was inhibited in mixed fashion by retinal (K_i 12 μM, K_m/K_i ratio 0.34). In subsequent experiments the cofactor NADH exhibited an identical K_m (8 μM) in the microsomal reductions of both substrates. Acidic pH markedly stimulated the microsomal reduction of androstenedione to testosterone and was also found to enhance retinal reduction to retinol, although the latter reaction exhibited a district pH optimum between 6.0 and 6.5. These results suggest that a common enzyme may participate in the reduction of both substrates but at least one other enzyme probably participates in hepatic microsomal testosterone production.     

Action of endogenous steroid inhibitors of brain aromatase relative to fadrozole

To study mechanisms of aromatase inhibition in brain cells, a highly effective non-steroidal aromatase inhibitor (Fadrozole; 4-[5,6,7,8-tetra-hydroimidazo-(1,5-a)-pyridin-5-yl] benzonitrile HCl; CGS 16949A) was compared with endogenous C-19 steroids, known to be formed in the preoptic area, which inhibit oestrogen formation. Using a sensitive in vitro tritiated water assay for aromatase activity in avian (dove) preoptic tissue, the order of potency, with testosterone as substrate was: Fadrozole (K_i < 1 × 10⁻⁹ M) > 4-androstenedione 5-androstanedione > 5-dihydrotestosterone (K_i = 6 × 10⁻⁸ M) > 5β-androstanedione > 5β-dihydrotestosterone (K_i = 3.5 × 10⁻⁷ M) > 5-androstane-3, 17β-diol (K_i = 5 × 10⁻⁶ M) > 5β-androstane-3β,17β-diol. Five other steroids, 5β-androstane-3,17β-diol, 5-androstane-3β,17β-diol, progesterone, oestradiol and oestrone, showed no inhibition at 10⁻⁴ M. The kinetics indicate that endogenous C-19 steroids show similar competitive inhibition of the aromatase as Fadrozole. Mouse (BALB/c) preoptic aromatase was also inhibited by Fadrozole. We conclude that endogenous C-19 metabolites of testosterone are effective inhibitors of the brain aromatase, and suggest that they bind competitively at the same active site as Fadrozole.     

Effects of Phosphate,HEDTA, and Light Sources on Cr(VI) Retention by Goethite

Iron hydrous hydro(oxide) has been regarded as an important sorbent for Cr(VI) in soil systems due to its wide distribution. However, many factors, such as phosphate (P), organic ligands, and light sources, could influence Cr(VI) retention by the soil components. The existence of inorganic or organic ligands not only competes with solution Cr(VI) for surface sites, but also results in releasing sorbed Cr(VI). Although organic matter can reduce Cr(VI) to less toxic Cr(III), the reduction rate is extremely slow. The objective of this study was to evaluate the influence of P on Cr(VI) sorption by goethite. The reduction of Cr(VI) by N-hydroxyethyl-ethylenediamine-triacetic acid (HEDTA) and goethite under different intensity of light was also investigated. Competitive sorption experiment indicated that P had lower inhibition of Cr(VI) sorption when the initial Cr(VI) concentration was higher than P. Goethite suspensions could catalyze Cr(VI) reduction under growth chamber light. Goethite accompanied with light could also accelerate Cr(VI) reduction by HEDTA. This phenomenon could be evidenced by the formation of Cr(III) and decreasing desorption of retained Cr(VI) by P.     

Sorption of heavy metals from electroplating effluent using immobilized biomass Trichoderma viride in a continuous packed-bed column

The sorption of heavy metals ions by immobilized Trichoderma viride biomass in a packed-bed column was studied. Fungal biomass T. viride was immobilized to Ca-alginate used for removal of Cr(VI), Ni(II) and Zn(II) ions from synthetic solutions and electroplating effluent. The experiments were conducted to study the effect of important design parameters such as bed height, flow rate and initial concentration of metal ions. The maximum sorption capacity was observed at flow rate 5 ml/min, bed height 20 cm and metal ions concentration 50 mg/L with immobilized biomass. Whereas, breakthrough time and saturation time decreased with increase flow rate and metal ions concentration and an inverse condition was found in bed height. The bed depth service time (BDST) Adams-Bohart model was used to analyze the experimental data. The regeneration efficiency was observed 40.1%, 75% and 53% for Cr(VI), Ni(II) and Zn(II) without any significant alteration in sorption capacity after 5th sorption-desorption cycles.     

Competitive product inhibition of aromatase by natural estrogens

In order to better understand the function of aromatase, we carried out kinetic analyses to asses the ability of natural estrogens, estrone (E₁), estradiol (E₂), 16-OHE₁, and estriol (E₃), to inhibit aromatization. Human placental microsomes (50 μg protein) were incubated for 5 min at 37°C with [1β-³H]testosterone (1.24 × 10³ dpm ³H/ng, 35–150 nM) or [1β-³H,4-¹⁴C]androstenedione (3.05 × 10³ dpm ³H/ng, ³H/¹⁴C = 19.3, 7–65 nM) as substrate in the presence of NADPH, with and without natural estrogens as putative inhibitors. Aromatase activity was assessed by tritium released to water from the 1β-position of the substrates. Natural estrogens showed competitive product inhibition against androgen aromatization. The K_i of E₁, E₂, 16-OHE₁, and E₃ for testosterone aromatization was 1.5, 2.2, 95, and 162 μM, respectively, where the K_m of aromatase was 61.8 ± 2.0 nM (n = 5) for testosterone. The K_i of E₁, E₂, 16-OHE₁, and E₃ for androstenedione aromatization was 10.6, 5.5, 252, and 1182 μM, respectively, where the K_m of aromatase was 35.4 ± 4.1 nM (n = 4) for androstenedione. These results show that estrogens inhibit the process of andrigen aromatization and indicate that natural estrogens regulate their own synthesis by the product inhibition mechanism in vivo. Since natural estrogens bind to the active site of human placental aromatase P-450 complex as competitive inhibitors, natural estrogens might be further metabolized by aromatase. This suggests that human placental estrogen 2-hydroxylase activity is catalyzed by the active site of aromatase cytochrome P-450 and also agrees with the fact that the level of catecholestrogens in maternal plasma increases during pregnancy. The relative affinities and concentration of androgens and estrogens would control estrogen and catecholestrogen biosynthesis by aromatase.     

Inhibition of chitosan-immobilized urease by slow-binding inhibitors: Ni, F and acetohydroxamic acid

The inhibitions by Ni²⁺ and F⁻ ions and by acetohydroxamic acid of jack bean urease covalently immobilized on chitosan membrane was studied (pH 7.0, 25°C) and compared with those of the native enzyme. The reaction progress curves of the immobilized urease-catalyzed hydrolysis of urea were recorded in the absence and presence of the inhibitors. They revealed that the inhibitions are of the competitive slow-binding type similar to those of native urease. The immobilization weakened the inhibitory effect of the inhibitors on urease as measured by the inhibition constants K_i^*. The increase in their values: 17.9-fold for Ni²⁺, 26.5-fold for F⁻ and 1.7-fold for acetohydroxamic acid, was accounted for by environmental effects generated by heterogeneity of the urease–chitosan system: (1) mass transfer limitations imposed on substrate and reaction product in the external solution, and (2) the increase in local pH on the membrane produced by both the enzymatic reaction and the electric charge of the support. By relating the K_M/K_i^* ratio to the electrostatic potential of chitosan it was found that while the reduced Ni²⁺ inhibition is mainly brought about by the potential, inhibition by acetohydroxamic acid is independent of the potential, and the acid inhibits urease in its non-ionic form. The reduction in F⁻ inhibition was ascribed to the increased pH in the local environment of the immobilized enzyme.     

Sorption of Cr(VI), Cu(II) and Pb(II) by growing and non-growing cells of a bacterial consortium

This paper reports the sorption of three metallic ions, namely Cr(VI), Cu(II) and Pb(II) in aqueous solution by a consortium culture (CC) comprising an acclimatised mixed bacterial culture collected from point and non-point sources. Metal sorption capability of growing and non-growing cells at initial pH of between 3 and 8 in the 1-100mg/L concentration range were studied based on Q(max) and K(f) values of the Langmuir and linearised Freundlich isotherm models, respectively. Maximal metal loading was generally observed to be dependent on the initial pH. Growing cells displayed significant maximal loading (Q(max)) for Pb(II) (238.09 mg/g) and Cu(II) (178.87 mg/g) at pH 6 and at pH 7 for Cr(VI) (90.91 mg/g) compared to non-growing cells (p < 0.05). At the pH range of 6-8, growing cells showed higher loading capacity compared to non-growing cells i.e. 38-52% for Cr, 17-28% for Cu and 3-17% for Pb. At lower metal concentrations and at more acidic pH (3-4) however, non-growing cells had higher metal loading capacity than growing cells. The metal sorption capacity for both populations were as follows: Pb(II) > Cu(II) > Cr(VI).     

Removal of lead (Pb(2+)) by the cyanobacterium Gloeocapsa sp

Pb²⁺ removal ability of the viable-freshwater cyanobacterium Gloeocapsa sp. was studied in batch experiments. Gloeocapsa sp. was cultured in the Medium 18 with pH adjusted to 3, 4, 5, 6 and 7. Growth was subsequently determined based on the increase of chlorophyll-a content. Gloeocapsa sp. was able to grow at all pH levels tested, except at pH 3. Removal of Pb²⁺ was then further studied under pH 4. The results showed that Pb²⁺ concentration in the range of 0–20 mg L⁻¹ was not inhibitory to Gloeocapsa sp. growth but reduced its Pb²⁺ removal efficiency (by 4.5% when Pb²⁺ concentration increased from 2.5 to 20 mg L⁻¹). Pb²⁺ removal characteristics followed the Langmuir adsorption isotherm with the maximum removal capacity (q_max) of 232.56 mg g⁻¹. Adsorption of Pb²⁺ by this cyanobacterium followed the second order rate reaction and intraparticle diffusion was likely the rate-determining step. The initial rate of Pb²⁺adsorption during intraparticle diffusion was slower under light than under dark conditions, indicating that light probably slowed down the initial rate of intraparticle diffusion through the repulsion effects on cell membrane.     

The Oxidation of Phenol by Ferrate(VI) Andferrate(V). A Pulse Radiolysis and Stopped-Flow Study

Potassium ferrate. K₂FeO₄, is found to oxidize phenol in aqueous solution (5.5 ± pH ± 10) by a process which is second order in both reactants; -d[Fe^vt]/dt=ki[Fe^VI][phenol], kI = 10⁷M^-1s^-1. Product analysis by HPLC showed a mixture of hydroxylated products, principally paraquinone. and biphenols that indicate that oxidation of phenol occurs by both one-electron and two-electron pathways. The two-electron oxidant. producing both para- and ortho-hydroxylated phenols is considered to be ferrate(V) which is itself produced by the initial one-electron reduction of ferrate(VI). The rate of ferrate(V) reaction with phenol was determined by pre-mix stopped flow pulse-radiolysis and found to be k7 = (3.8 ± 0.4)± 10⁵M^-1s^-1.     

Equilibrium and structure of thallium(III)–ethylenediamine complexes in pyridine solution and in solid

The formation of three [Tl(en)_n]³⁺ complexes (n=1–3) in a pyridine solvent has been established by means of ²⁰⁵Tl and ¹H NMR. Their stepwise stability constants based on concentrations, K_n=[Tl(en)_n ³⁺]/{[Tl(en)_n−1 ³⁺]·[en]}, at 298 K in 0.5 M NaClO₄ ionic medium in pyridine, were calculated from ²⁰⁵Tl NMR integrals: log K₁=7.6±0.7; log K₂=5.2±0.5 and log K₃=2.64±0.05. Linear correlation between both the ²⁰⁵Tl NMR shifts and spin–spin coupling ²⁰⁵Tl–¹H versus the stability constants has been found and discussed. A single crystal with the composition [Tl(en)₃](ClO₄)₃ was synthesized and its structure determined by X-ray diffraction. The Tl³⁺ ion is coordinated by three ethylenediamine ligands via six N-donor atoms in a distorted octahedral fashion.     

Kinetics and mechanism of methylene blue sorption onto palm kernel fibre

The kinetics and mechanism of the sorptive removal of methylene blue dye from aqueous solution using palm kernel fibre as adsorbent have been investigated. Batch kinetic experiments were performed and system variables investigated includes pH and initial dye concentration. The kinetic data were fitted to the pseudo-first, pseudo-second, intraparticle diffusion and mass transfer models. The pseudo-first order reaction kinetics fitted to the experimental data only in the first 5 min of sorption and then deviated, while the pseudo-second order kinetic model was found to fit the experimental data for the entire sorption period with high coefficient of determination. Equations were developed using the pseudo-second order model, which predicts the amounts of methylene blue at any contact time and initial concentration within the given range. This suggests that the sorption of methylene blue onto palm kernel fibre follows a chemical activation mechanism. A mathematical relationship was also drawn between the equilibrium sorption capacity and the change in pH (ΔH⁺) at the end of the kinetic experiments with varying initial dye concentration, supporting the fact that chemical reaction (ion exchange) occurred and is important in the rate determining step. Mass transfer was found to be favoured at high concentrations while intraparticle diffusion was favoured at low concentrations.     

Kinetic and Equilibrium Studies of Cr(VI) Biosorption by Dead <Emphasis Type="Italic">Bacillus licheniformis</Emphasis> Biomass

Many studies have been carried out on the biosorption capacity of different kinds of biomass. However, reports on the kinetic and equilibrium study of the biosorption process are limited. In our experiments, the removal of Cr(VI) from aqueous solution was investigated in a batch system by sorption on the dead cells of Bacillus licheniformis isolated from metal-polluted soils. Equilibrium and kinetic experiments were performed at various initial metal concentrations, pH, contact time, and temperatures. The biomass exhibited the highest Cr(VI) uptake capacity at 50°C, pH 2.5 and with the initial Cr(VI) concentration of 300 mg/g. The Langmuir and Freundlich models were considered to identify the isotherm that could better describe the equilibrium adsorption of Cr(VI) onto biomass. The Langmuir model fitted our experimental data better than the Freundlich model. The suitability of the pseudo first-order and pseudo second-order kinetic models for the sorption of Cr(VI) onto Bacillus licheniformis was also discussed. It is better to apply the pseudo second-kinetic model to describe the sorption system.     

Determination of effective diffusion coefficient of substrate in gel particles with immobilized biocatalyst

A method of determining of the effective diffusion coefficient of substrate in a particle, where the diffusion and consumption of substrate by biocatalytic reaction are present simultaneously, was designed and experimentally verified. The method is based on measuring the overall rate of heterogeneous biocatalytic reaction in particles of varying diameter. The effective diffusion coefficient, D_e, was determined by fitting the measured reaction rates with the solution of the reaction-diffusion equation. The method is tailored for cases where the enzyme reaction is governed by the Michaelis-Menten kinetics. The value of K_m required for the solution of the mathematical model was adopted from the measurement of the kinetics of free cells, whereas the rate parameter, k₂, was optimized together with D_e. As an experimental model, the sucrose hydrolysis catalyzed by Ca-alginate-entrapped yeast cells was examined. The particle diameter varied in the range of 1.2–3.9 mm and the initial reaction rates were measured in a batch-stirred reactor at a sucrose concentration of 100 m . The D_e of sucrose at 30°C was found to be 2.9 · 10⁻¹⁰ m²s⁻¹.