Thermal Stability of Glucose Oxidase from Penicillium adametzii

The thermal stability of glucose oxidase was studied at temperatures between 50 and 70°C by kinetic and spectroscopic (circular dichroism) methods. The stability of glucose oxidase was shown to depend on the medium pH, protein concentration, and the presence of protectors in the solution. At low protein concentrations (<15 g/ml) and pH > 5.5, the rate constants k _in, s^–1, for thermal inactivation of glucose oxidase were high. Circular dichroic spectra suggested an essential role of structures in stabilizing the protein globule. At a concentration of 15 g protein/ml, the activation energy E _Aof thermal inactivation of glucose oxidase in aqueous solution was estimated at 79.1 kcal/mol. Other thermodynamic activation parameters estimated at 60°C had the following values: H= 78.4 kcal/mol, G= 25.5 kcal/mol, and S= 161.9 entropy units. The thermal inactivation of glucose oxidase was inhibited by KCl, polyethylene glycols, and polyols. Among polyols, the best was sorbitol, which stabilized glucose oxidase without affecting its activity. Ethanol, phenol, and citrate exerted destabilizing effects.     

Thermodynamics and kinetics of the helix-coil transition of oligomers containing GC base pairs

Absorbance-temperature profiles have been determined for the following self-complementary oligonucleotides or equimolar paris of complementary oligonucleotides containing GC base pairs: A₂GCU₂, A₃GCU₃, A₄GCU₄, A₆CG + CGU₆, A₈CG + CGU₈, A₄G₂ + C₂U₄, A₅G₂ + C₂U₅, A₄G₃ + C₃U₄, and A₅G₃ + C₃U₅. In all cases cooperative melting transitions indicate double-helix formation. As was found previously, the stability of GC containing oligomer helices is much higher than that of AU helices of corresponding length. Moreover, helices with the same length and base composition but different sequences also have quite different stabilites. The melting curves were andlyzed using a zipper model and the thermodynamic parameters for the AU pairs determined previously. The effect of single-strand stacking was considered separately. According to this model, the formation of a GC pair from unstacked single strands is associated with an ethalpy change of ?15 kcal/mole. Due to the high degree of single-strand stacking at room temperature the enthalpy change for the formation of GC pairs from unstacked single strands is only ?5 to ?6 kcal/mole. (The corresponding parameters for AU pairs are ?10.7 kcal/mole and ?5 to ?6 kcal/mole.) The sequence dependence of helix stability seems to be primarily entropic since no differences in ΔH were seen among the sequence isomers. The kinetics of helix formation was investigated for the same molecules using the temperature jump technique. Recombination of strands is second order with rate constants in the range of 10⁵ to 10⁷M^?1 sec^?1 depending on the chain length and the nucleotide sequence. Within a series of oligomers of a given type, the rates of recombination decrease with increasing chain length. Oligomers with the sequence A_nGCU_n recombine six to eight times slower than the other oligomers of corresponding chain length. The experimental enthalpies of activation of 6 to 9 kcal/mole suggest a nucleation length of one or two GC base pairs. The helix dissociation process has rate constants between 0.5 and 500 sec^?1 and enthalpies of activation of 25 to 50 kcal/mole. An increase of chain length within a given nucleotide series leads to decreased rates of dissociation and increased enthalpies of activation. An investigation of the effect of ionic strength on A_nGCU_n helix formation showed that the rates of recombination increase considerably with increased ionic strength.     

Kinetics of Thermal Inactivation and Molecular Asymmetry of Urease from Dehusked Pigeongea (Cajanus cajan L.) Seeds

The purified urease from pigeonpea was moderately stable at ?10°C. The shelf-life of the enzyme on storage in 0.1 M Tris-acetate buffer, pH 6.5, at ?10°C showed a single exponential decay with a t_1/2 of approx. 30 days. In the presence of additives like 5mM dithiothreitol the t_1/2 increased to 223 days at the same temperature, in a single exponential decay. The Arrhenius plot of the kinetics of the pigeonpea urease catalysed urea hydrolysis over the temperature range 27 to 77°C, was linear. The Q₁₀ value was found to be 1.46. The energy of activation calculated from the Arrhenius equation was found to be 5.1 kcal/mole active site. The thermal denaturation of pigeopea urease at 65 and 70°C was found to obey biphasic kinetics in which half of the activity is destroyed faster than the remaining half. The time course of thermal inactivation can be described by the following equation, consisting of two first order terms: A_t = A_fast.e^-k _fast + A_slow.e ^-kslow^.t where, A_t is the residual activity at time t, A_fast and A_slow, k_fast and k_slow are the amplitudes and the first-order rate constants of the fast and the slow phases, respectively. The data suggests the existence of site-site heterogeneity in oligomeric urease molecule from pigeonpea.     

Molecular-Dynamics Simulations for the Density Autocorrelation Function in a Supercooled Fluid Phase

Abstract

We have re-calculated the self part of the density autocorrelation function F_s(k, t) (incoherent scattering function) for the binary soft-sphere fluid with a much longer molecular-dynamics (MD) simulation than our previous MD calculations, and with a larger system size (N = 4000) to a longer time window as well as to study a system-size dependence, if it exists. The full density autocorrelation function F(k, t) was also computed. It is found that all F(k, t)'s that we have computed in this work can be fitted over a wide range of time steps (at least over three figures of the decay) by a Williams-Watts stretched exponential function F_s(k, t) = A exp [— (t/t ₀)^β], where A, β and t ₀ are adjustable parameters. Other significant dynamical behaviours were also presented in mean square displacements and non-Gaussian parameters for highly supercooled fluids with N = 4000. The present results are compatible to our previous computations with N = 500, but a significant size dependence is suggested.     

The role of lipid and antioxidant exchanges in cell division synchronization (mathematical model)

Cell-cycle synchronization of two diffusecoupled cells has been studied in the framework of the membrane model for the cell division cycle, proposed by Chernavskii et al. (1977). It has been shown semianalytically (using the averaging principle) and by computer stimulation that a) if the duration of theG1-phase (T _G1 ) for two identical cells is comparable with the duration of the remaining cycle (T _S+G2+M ), the lipid (L)-exchange results in a synchronization with phase difference =0. The antioxidant (A)-exchange leads to a phase-locking with =T ₀/2 (whereT ₀ is the cell cycle period; b) ifT _G1 T _S+G2+M (orT _G1 T _S+G2+M ) theL-exchange makes synchronization possible both with =0 and =T ₀/2 while theA-exchange results in phase-locking with confined to the region 0 toT ₀/2; c) for non-identical cells differing in the values of kinetic parameters, the locking band narrows as the population density increases (when some model parameters are close to the bifurcation thresholds). We expect that the cells selected artificially at a definite phase of cycle might maintain the synchronous division for a long time if the lipid exchange between cells were stimulated.     

Kinetics of protein aggregation. Quantitative estimation of the chaperone-like activity in test-systems based on suppression of protein aggregation

The experimental data on the kinetics of irreversible aggregation of proteins caused by exposure to elevated temperatures or the action of denaturing agents (guanidine hydrochloride, urea) have been analyzed. It was shown that the terminal phase of aggregation followed, as a rule, first order kinetics. For the kinetic curves registered by an increase in the apparent absorbance (A) in time (t) the methods of estimation of the corresponding kinetic parameters A _lim and k _I (A _lim is the limiting value of A at t and k _I is the rate constant of the first order) have been proposed. Cases are revealed when the reaction rate constant k _I calculated from the kinetic curve of aggregation of the enzymes coincides with the rate constant for enzyme inactivation. Such a situation is interpreted as a case when the rate of aggregation is limited by the stage of denaturation of the enzyme. A conclusion has been made that, in order to establish the mechanism of protein aggregation, the kinetic investigations of aggregation should be carried out over a wide range of protein concentrations. The refolding experiments after denaturation of proteins by guanidine hydrochloride or urea have been also analyzed. It was shown that aggregation accompanying refolding follows first order kinetics at the final phase of the process. The model of protein refolding explaining such a kinetic regularity has been proposed. When aggregation of protein substrate follows first order kinetics, parameters A _lim and k _I may be used for the quantitative characterization of the chaperone-like activity in the test-systems based on suppression of protein aggregation.     

Kinetics of thermal aggregation of tobacco mosaic virus coat protein

The kinetics of thermal aggregation of coat protein (CP) of tobacco mosaic virus (TMV) have been studied at 42 and 52°C in a wide range of protein concentrations, [P]₀. The kinetics of aggregation were followed by monitoring the increase in the apparent absorbance (A) at 320 nm. At 52°C the kinetic curves may be approximated by the exponential law in the range of TMV CP concentrations from 0.02 to 0.30 mg/ml, the first order rate constant being linearly proportional to [P]₀ (50 mM phosphate buffer, pH 8.0). The analogous picture was observed at 42°C in the range of TMV CP concentrations from 0.01 to 0.04 mg/ml (100 mM phosphate buffer, pH 8.0). At higher TMV CP concentrations the time of half-conversion approaches a limiting value with increasing [P]₀ and at sufficiently high protein concentrations the kinetic curves fall on a common curve in the coordinates {A/A _lim; t} (t is time and A _lim is the limiting value of A at t ). According to a mechanism of aggregation of TMV CP proposed by the authors at rather low protein concentrations the rate of aggregation is limited by the stage of growth of aggregate, which proceeds as a reaction of the pseudo-first order, whereas at rather high protein concentrations the rate-limiting stage is the stage of protein molecule unfolding.     

Effect of agitation on the peptide fibrillization: Alzheimer's amyloid‐β peptide 1‐42 but not amylin and insulin fibrils can grow under quiescent conditions

Many peptides and proteins can form fibrillar aggregates in vitro, but only a limited number of them are forming pathological amyloid structures in vivo. We studied the fibrillization of four peptides – Alzheimer's amyloid‐β (Aβ) 1‐40 and 1‐42, amylin and insulin. In all cases, intensive mechanical agitation of the solution initiated fast fibrillization. However, when the mixing was stopped during the fibril growth phase, the fibrillization of amylin and insulin was practically stopped, and the rate for Aβ₄₀ substantially decreased, whereas the fibrillization of Aβ₄₂ peptide continued to proceed with almost the same rate as in the agitated conditions. The reason for the different sensitivity of the in vitro fibrillization of these peptides towards agitation in the fibril growth phase remains elusive. Copyright © 2013 European Peptide Society and John Wiley & Sons, Ltd.     

Studies on the Critical Micellar Concentration and Phase Transitions of Stearoylcarnitine

The critical micellar concentration (CMC) of stearoylcarnitine was determined at different pH values at room temperature by fluorescence spectroscopy, monitoring the spectral changes of 8-anilinonaphthalene-1-sulfonate (ANS). The CMC was found to vary with pH, increasing from about 10 μM at pH 3.0 to ca. 25 μM at pH 7.0, but decreasing slightly with further increase in pH to approximately 19 μM at pH 10.0. Differential scanning calorimetry (DSC) shows that stearoylcarnitine dispersed in water at low concentration undergoes a broad thermotropic phase transition at 44.5°C, with a transition enthalpy of 15.0 kcal/mol. The transition temperature (T _t) shifts to ca. 50.5°C in the presence of 1 mM EDTA or when the concentration is increased significantly. The turbidity of aqueous dispersions of stearoylcarnitine was found to be considerably high at low temperatures, which decreases quite abruptly over a short temperature range, indicating that a transition occurs from a phase of large aggregates to one of much smaller aggregates, most likely micelles. The phase transition temperature was determined as 29.1°C at pH 3.0, which increased with increasing pH up to a value of 55.3°C at pH 8.6 and remains nearly constant thereafter up to pH 11.2. The pH dependence of CMC and T _t suggest that the pK _a of the carboxyl group of long chain acylcarnitines shifts to higher temperatures upon aggregation (micelles or bilayer membranes).     

Comparison of different assays for the aggregation of oral bacteria by human whole saliva

For comparison, human whole saliva-induced aggregation was studied by phase-contrast microscopy, spectrophotometry combined with macroscopic observations, and in microtiterplate assay under identical experimental conditions for Actinomyces viscosus HG 85 (T14-V) and HG 380 (T14-AV), Bacteroides gingivalis HG 66 (W 83), Streptococcus rattus HG 59 (BHT), and Streptococcus sanguis I HG 169. The entire process of formation, extension, and sedimentation of aggregates could merely be observed by the combination of these assays. The very first stages of aggregation could only be detected and quantitated by phase-contrast microscopy. Within 2 1/2 min. 50% of the A. viscosus, S. rattus, and S. sanguis cells were aggregated, denoted as T₅₀. In microtiterplates, however, aggregates were observed in general only after sedimentation at 30–45 min of incubation, expressed as T_A. For interpretation of the spectrophotometric curves, additional microscopic and macroscopic data were a prerequisite. The small decline in absorbance during the first 30–45 min (phase 1) corresponded to the formation and extension of non-sedimenting aggregates, whereas the subsequent pronounced fall in absorbance (phase 2) was caused by the massive sedimentation of aggregates. The moment of inflexion between both phases, T_I, marked the onset of sedimentation of aggregates and corresponded very well with T_A, at which time already 92–98% of the cells were aggregated as quantitated by microscopy. In conclusion, only by microscopy the formation and extension of aggregates could be observed within a few minutes and quantitated in terms of aggregation rate. From 30–45 min, merely the sedimentation of aggregates was visualized in microtiterplates, whereas the time course of the overall process was recorded indirectly by spectrophotometry.     

Temperature and humidity effects on branchlet gas-exchange in white spruce: an explanation for the increase in transpiration with branchlet temperature

In situ gas-exchange data, for branchlets of white spruce [Picea glauca (Moench) Voss.] in a mature mixed-wood boreal forest in central Canada (53°44′N 105°14′W), were subjected to a multiple regression analysis. Vapor pressure deficit (VPD) and branchlet temperature (t_leaf) were both significant predictors (P<0.0001) of stomatal conductance to water vapor (g_sw) and net photosynthesis (A_n), together explaining 67 and 64% of the variation in g_sw and A_n, respectively. Since VPD and t_leaf were autocorrelated in these field data, but also to further explore the nature of independent effects of temperature and humidity on water and CO₂ exchange in white spruce, steady-state gas-exchange was performed on well-watered greenhouse-grown seedlings of white spruce. Results from laboratory experiments supported the following conclusions: (1) Transpiration (E) increases with VPD to an inflection point that increases linearly with t_leaf. This t_leaf effect on E could not be explained by trends in VPD, RH, A_n or PFD. Rather, our data support a model in which E and g_sw are influenced by the balance between ’supply’ and ’loss’ of water to and from leaf tissue, respectively. The supply of water appears to be in accordance with Darcy’s law, where supply of water is proportional to the driving gradient in pressure/ tension, specific permeability (k), and inverse of water viscosity (n ^–1). Approximately half of the increase in E could be explained by the linear increase in n ^–1 with increasing t_leaf. We propose that increases in k explain the remainder of the increase in E with t_leaf. (2) VPD and t_leaf appear to have independent effects on g_sw. In contrast, RH effects on g_sw or E were subtle and could be explained by a combination of effects of t_leaf and VPD. (3) A_n was affected primarily by t_leaf, being reduced at low (10°C) and high (40°C) temperatures, and only indirectly by humidity parameters via stomatal conductance, viz. intercellular CO₂ concentrations. Our results have implications for the prediction of water fluxes from plants and canopies in areas where plant temperatures vary diurnally or seasonally. Received: 24 September 1998 / Accepted: 20 July 1999     

Effects of selective destruction of iron-sulfur center B on electron transfer and charge recombination in Photosystem I

Incubation of spinach thylakoids with HgCl₂ selectively destroys Fe–S center B (F_B). The function of electron acceptors in F_B-less PS I particles was studied by following the decay kinetics of P700⁺ at room temperature after multiple flash excitation in the absence of a terminal electron acceptor. In untreated particles, the decay kinetics of the signal after the first and the second flashes were very similar (t _1/22.5 ms), and were principally determined by the concentration of the artificial electron donor added. The decay after the third flash was fast (t _1/20.25 ms). In F_B-less particles, although the decay after the first flash was slow, fast decay was observed already after the second flash. We conclude that in F_B-less particles, electron transfer can proceed normally at room temperature from F_X to F_A and that the charge recombination between P700⁺ and F_X ^-/A₁ ^- predominated after the second excitation. The rate of this recombination process is not significantly affected by the destruction of F_B. Even in the presence of 60% glycerol, F_B-less particles can transfer electrons to F_A at room temperature as efficiently as untreated particles.Abbreviations DCIP 2, 6-dichlorophenol indophenol - F_A, F_B, F_X iron-sulfur center A, B and X, respectively - PMS phenazine methosulfate     

Selectively 13C-enriched DNA: Evidence from 13C1′ relaxation rate measurements of an internal dynamics sequence effect in the lac operator

Summary In order to study some internal dynamic processes of the lac operator sequence, the ¹³C-labeled duplex 5d(C₀G₁C₂T₃C₄A₅C₆A₇A₈T₉T₁₀) · d(A₁₀A₉T₈T₇G₆T₅G₄A₃G₂C₁G₀)3 was used. The spreading of both the H1 and C1 resonances brought about an excellent dispersion of the ¹H1-¹³C1 correlations. The spinlattice relaxation parameters R(C_z), R(C_x,y) and R(H_zC_z) were measured for each residue of the two complementary strands, except for the 3-terminal residues which were not labeled. Variation of the relaxation rates was found along the sequence. These data were analyzed in the context of the model-free formalism proposed by Lipari and Szabo [(1982) J. Am. Chem. Soc., 104, 4546–4570] and extended to three parameters by Clore et al. [(1990) Biochemistry, 29, 7387–7401; and (1990) J. Am. Chem. Soc., 112, 4989–4991]. A careful analysis using a least-squares program showed that our data must be interpreted in terms of a three-parameter spectral density function. With this approach, the global correlation time was found to be the same for each residue. All the C1-H1 fragments exhibited both slow (_s = 1.5) and fast (_f = 20 ps) restricted libration motions (S _inf2 ^sups =0.74 to 1.0 and S _inf2 ^supf =0.52 to 0.96). Relaxation processes were described as governed by the motion of the sugar relative to the base and in terms of bending of the whole duplex. The possible role played by the special structure of the AATT sequence is discussed. No evident correlation was found between the amplitude motions of the complementary residues. The 5-terminal residues showed large internal motions (S²=0.5), which describe the fraying of the double helix. Global examination of the microdynamical parameters S _inf2 ^supf and S _inf2 ^sups along the nucleotide sequence showed that the adenine residues exhibit more restricted fast internal motions (S _inf2 ^supf =0.88 to 0.96) than the others, whereas the measured relaxation rates of the four nucleosides in solution were mainly of dipolar origin. Moreover, the fit of both R(C_z) and R(HzC_z) experimental relaxation rates using an only global correlation time for all the residues, gave evidence of a supplementary relaxation pathway affecting R(C_x,y) for the purine residues in the (53) G₄A₃ and A₁₀A₉T₈T₇ sequences. This relaxation process was analyzed in terms of exchange stemming from motions of the sugar around the glycosidic bond on the millisecond time scale. It should be pointed out that these residues gave evidence of close contacts with the protein in the complex with the lac operator [Boelens et al. (1987) J. Mol. Biol., 193, 213–216] and that these motions could be implied in the lac-operator-lac-repressor recognition process.     

Molecular Asymmetry in Urease of Water Melon (Citrullus vulgaris)

Kinetics of urease-catalysed urea hydrolysis follows Arrhenius equation in the temperature range 10-50°C and shows an energy of activation equal to 7.14 kcal/mol. The kinetics of thermal inactivation of the enzyme is biphasic, In that half of the initial activity is destroyed more rapidly than the remaining half. The data are consistent with the rate equation: A_t = A_fast·e^-k _fast ^-t + A_slow ·e^-K _slow ^-t where A_t is the residual activity at time t, A_fast and A_slow, k_fast and k_slow are the amplitudes and the first-order rate constants of the fast and the slow phases, respectively. A similar activity decay (namely blphaslc) is also observed on storing the enzyme at +4 and ?4OC. The data suggest the existence of half-and-half distribution of sites which is a manifestation of a pre-exlstent site heterogeneity in the oligomeric protein molecule.     

Changes in the photosynthetic light response curve during leaf development of field grown maize with implications for modelling canopy photosynthesis

Changes in the photosynthetic light-response curve during leaf development were determined for the fourth leaf of maize crops sown on 23 April and 10 June. Temperatures were unusually mild during late spring/early summer and neither crop experienced chilling damage. The concept of thermal time was used to take into account the effects of different temperature regimes on developmental stage, thereby enabling photosynthetic light-response data to be combined for both crops to describe the general response. Large variations in the upper asymptote (A_sat) and convexity () of the light-response curve occurred during leaf development, but the maximum quantum yield of CO₂ assimilation remained relatively constant throughout. Dark respiration rates showed a small but significant decrease with leaf age and generally ranged between 5 and 10% of A_sat. A simple mathematical model was developed to assess the sensitivity of daily leaf photosynthesis (A_L) to reductions in the A_sat, and the initial slope () of the light-response curve at different stages of leaf development. On bright sunny days, and at all developmental stages, A_L was ca. twice as sensitive to reductions in A_sat than to reductions in and . In overcast conditions, however, all three parameters contributed significantly to reductions in leaf photosynthesis, although the contribution of was greatest during early leaf growth, while older leaves were most sensitive to depressions in A_sat. The implications of these results for modelling the sensitivity of canopy photosynthesis to chill-induced photoinhibition of the light-response curve are discussed.     

Thermodynamic and kinetic parameters of an oligonucleotide hairpin helix

The thermodynamics of the hairpin helix-single strand transition of A₆C₆U₆ has been analyzed by a staggering zipper model with consideration of single strand stacking. This analysis yields an enthalpy change of +11 kcal/mole for the formation of a first, isolated base pair. The stability constant of a first (intramolecular) base pair in A₆C₆U₆ is around 2 × 1O^?5 at 25°C, whereas a first (intermoleciilar) base pair in an A₆ · U₆ helix is characterised by a stability constant of about 4 × 10^?3M^?1 (25°C, extrapolated from A_n · V_n oligomer measurements). These data indicate a destabilizing effect of the C₆ loop.The rate constant of hairpin helix formation is 2 to 3 × 10⁴ sec^?1 associated with an activation enthalpy of +2.5 kcal/mote. The rate of helix dissociation of the A₆C₆U₆ hairpin is in the range of 10³ to lO⁵ sec^?1 with an activation enthalpy of 21 $\text{kcal}\text{mole}$. A comparison with the kinetic parameters obtained for A · U oligomer helices shows a specific influence of the C₆ loop due to the stacking tendency of the cytosine residues. This intluence is preferentially reflected in the relatively low value of the rate constant of helix formation.     

Phenomenological analysis of the clotting curve

A model-independent (phenomenological) characterization of the clotting curve is proposed. Three parameters are used to encapsulate the main features of the increase in absorbance observed at 350 nm due to the reaction of thrombin with fibrinogen that leads to clot formation: (1) the maximum increase in absorbance per unit time, A _m , at the inflection point of the clotting curve; (2) the time needed to reach the maximum increase in absorbance,t _m ; and (3) the clotting time,t _c , obtained from extrapolation of the slope att _m to the zero absorbance baseline. Clotting curves at low fibrinogen concentrations (0.125 ÷ 0.250 µM), well below the K_m, where thrombin amidase activity is rate-limiting with respect to the subsequent aggregation process, have been measured under a wide variety of experimental conditions, (i.e., as a function of thrombin concentration,pH and temperature) in order to explore the basic response of each parameter to changes in solution conditions. Under all conditions examined in this study we have observed thatt _m andt _c are linked through a linear relationship that appears to be an important invariant property of the clotting curve, regardless of experimental conditions. No such clear relationship exists between A _m andt _c , witht _c being associated with several possible values of A _m and vice versa, depending upon solution conditions. It is proposed thatt _c is strictly dependent on thrombin amidase activity, while A _m reflects properties of the aggregation process leading to clot formation. The clotting time shows apH and temperature dependence that closely resembles that of K_m/V_m for synthetic amide substrates. Futhermore,t _c changes linearly with either the inverse thrombin concentration and the concentration of competitive inhibitors of fibrinogen binding to thrombin, as expected for the ratio K_m/V_m. We show how the analysis of clotting curves obtained at different thrombin and inhibitor concentrations yields a quantitative measure of K_I that is in excellent agreement with the value determined independently from steady-state measurements of thrombin amidase activity.     

Kinetics of the work of the ion channels controlled by nicotinic cholinoreceptors in the membrane of mollusk neurons

Currents entering through single channels with conductivity 10 pS were produced on the membrane of an isolated neuron of the fresh-water molluskPlanorbarius corneus in the presence of suberyldicholine (5 µM) by the patch-clamp technique (cell-attached configuration). The times of stay of the channels in the open and closed states, as well as the durations of pulse bursts and clusters, were measured. The distributions of the time intervals obtained experimentally were approximated for open states by one exponential function: t_o=27±3 msec (n=21), and for closed states by a sum of three exponentials: t_c1=9.5±1.0 msec (n=21); t_c2=171±33 msec (n=19); t_c3=5.2±1.0 sec (n=21). The burst durations are characterized by the presence of two exponential functions in the distribution: t_b2=20±14 msec (n=10), t_b2=203±23 msec (n=10), and the clusters by three exponential functions: t_k1=33±11 msec (n=8), t_k2=274±84 msec (n=8), and t_k3=1.5±0.5 sec (n=9). Thus, for work of a chemoactivated channel associated with nicotinic-type cholinoreceptors in a mollusk neuron we can suggest a kinetic scheme with one open and three nonconducting states: C O D₁A₂ D₂A₂. The two "long-lived" closed states of the channel may be associated with desensitization of the integral response of the neurons to the application of suberyldicholine. Values were obtained for the rate constants of these proposed reactions. It is suggested that this model may be useful in analyzing the action of cholinomimetics and blockers on the molluskan neuronal membrane.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 23, No. 5, pp. 588–595, September–October, 1991.     

Growth and reproduction of the Nile perch,Lates niloticus,an introduced predator,in the Nyanza Gulf,Lake Victoria,East Africa

Synopsis Length-frequency data suggest Nile perch, Lates niloticus, from the Nyanza Gulf grew to a total length of 9 cm by age 118 days and 23 cm by age 287 days. A modified von Bertalanffy growth curve _t = 1.35·L(1-e^–K(t-t _o)) with the parameters L = 93.1, K = 0.272 and t_o = 0.046, is suggested to describe growth up to 5 years of age and the relationship _t = 1.35·(31.96 + 7.681t) for fish aged 6 years and above. Length-weight relationships were = 0.0234·-gt^2.74 for fish between 7 and 15.9 cm total length, = 0.0151·^2.94 for fish between 16 and 45.9 cm total length, and = 0.0023·^3.44 for fish between 46 and 120 cm total length. Male Nile perch first matured between 50 and 55 cm total length when they were probably 2 years old; female Nile perch first matured between 80 and 85 cm total length when they were probably 4 years old. Small males were common, large males were rare, with the reverse holding for females. Sex change, from male to female, is a possible explanation for this size dimorphism.     

Turbulent breakage of protein precipitates in mechanically stirred bioreactors

Experimental data relating to the breakage of isoelectric Soya protein precipitates in a mechanically agitated bioreactor are provided and examined in the light of a proposed mechanistic model which relates the size of the maximum attainable aggregate diameter to the energy dissipation rate in the vessel. The analysis suggests that protein precipitation results in the formation of scale-invariant fractal aggregates with a dimensionality of 2.2. Comparing the fractal dimensionality of the protein precipitates with reported values based on computer simulation studies suggests that the aggregates undergo considerable restructuring during agitation.List of Symbols A Hamaker constant (J) - D impeller diameter (m) - d _p primary particle diameter (m) - d _f maximum aggregate diameter (m) - G shear rate (s^–1) - H ₀ separation distance between two primary particles (m) - k constant in Eq. (5) - K constant in Eq. (6) - N impeller speed (rpm or rps) - r radial position in an aggregate, measured from the centre (m) - t time of exposure to shear (mins) - T _e eddy period (s^–1) - v _f aggregate volume (m³) Greek Symbols aggregate dimensionality constant - energy dissipation rate (W/kg) - dynamic viscosity of particle-free liquid (kg/ms) - kinematic viscosity of particle-free liquid (m²/s) - collision probability (–) - _p aggregate density (kg/m³) - _p continuous phase density (kg/m³) - aggregate mechanical strength (N/m²) - shear stress (N/m²) - particle concentration in an aggregate (m³/m³) - (r) porosity at radial position, r