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Low Substrate Concentrations (low + substrate_concentration)
Selected AbstractsMETABOLIC AND ECOLOGICAL CONSTRAINTS IMPOSED BY SIMILAR RATES OF AMMONIUM AND NITRATE UPTAKE PER UNIT SURFACE AREA AT LOW SUBSTRATE CONCENTRATIONS IN MARINE PHYTOPLANKTON AND MACROALGAE,JOURNAL OF PHYCOLOGY, Issue 2 2007T. Alwyn Marine phytoplankton and macroalgae acquire important resources, such as inorganic nitrogen, from the surrounding seawater by uptake across their entire surface area. Rates of ammonium and nitrate uptake per unit surface area were remarkably similar for both marine phytoplankton and macroalgae at low external concentrations. At an external concentration of 1 ,M, the mean rate of nitrogen uptake was 10±2 nmol·cm,2·h,1 (n=36). There was a strong negative relationship between log surface area:volume (SA:V) quotient and log nitrogen content per cm2 of surface (slope=,0.77), but a positive relationship between log SA:V and log maximum specific growth rate (,max; slope=0.46). There was a strong negative relationship between log SA:V and log measured rate of ammonium assimilation per cm2 of surface, but the slope (,0.49) was steeper than that required to sustain ,max (,0.31). Calculated rates of ammonium assimilation required to sustain growth rates measured in natural populations were similar for both marine phytoplankton and macroalgae with an overall mean of 6.2±1.4 nmol·cm,2·h,1 (n=15). These values were similar to maximum rates of ammonium assimilation in phytoplankton with high SA:V, but the values for algae with low SA:V were substantially less than the maximum rate of ammonium assimilation. This suggests that the growth rates of both marine phytoplankton and macroalgae in nature are often constrained by rates of uptake and assimilation of nutrients per cm2 surface area. [source] New insights into the P-glycoprotein-mediated effluxes of rhodaminesFEBS JOURNAL, Issue 3 2003Chatchanok Loetchutinat Multidrug resistance (MDR) in tumour cells is often caused by the overexpression of the plasma drug transporter P-glycoprotein (P-gp). This protein is an active efflux pump for chemotherapeutic drugs, natural products and hydrophobic peptides. Despite the advances of recent years, we still have an unclear view of the molecular mechanism by which P-gp transports such a wide diversity of compounds across the membrane. Measurement of the kinetic characteristics of substrate transport is a powerful approach to enhancing our understanding of their function and mechanism. The aim of the present study was to further characterize the transport of several rhodamine analogues, either positively charged or zwitterionic. We took advantage of the intrinsic fluorescence of rhodamines and performed a flow-cytometric analysis of dye accumulation in the wild-type drug sensitive K562 that do not express P-gp and its MDR subline that display high levels of MDR. The measurements were made in real time using intact cells. The kinetic parameter, ka = VM/km, which is a measure of the efficiency of the P-gp-mediated efflux of a substrate was similar for almost all the rhodamine analogues tested. In addition these values were compared with those determined previously for the P-gp-mediated efflux of anthracycline. Our conclusion is that the compounds of these two classes of molecules, anthracyclines and rhodamines, are substrates of P-gp and that their pumping rates at limiting low substrate concentration are similar. The findings presented here are the first to show quantitative information about the kinetic parameters for P-gp-mediated efflux of rhodamine analogues in intact cells. [source] Development of a mathematical model for Bacillus circulans growth and alkaline protease production kineticsJOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 2 2009Chaganti Subba Rao Abstract BACKGROUND: An unstructured mathematical model was developed to understand information on the relationship between Bacillus circulans growth and metabolism-related protease production (using logistic and Luedeking,Piret equations respectively) in a batch reactor with respect to glucose consumption and fermentation time. The objective was to develop an indispensable tool for the optimisation, control, design and analysis of alkaline protease production. RESULTS: Biomass growth and enzyme production titres changed with a change in substrate concentration. Modelling analysis of biomass and enzyme production titres at different substrate concentrations revealed significant accuracy in terms of statistical consistency and robustness with respect to fermentation kinetic profiles. CONCLUSION: With the B. circulans strain used, an economic protease yield (2837 × 103 U g,1) with respect to biomass and glucose ratio was achieved at low substrate concentration (10 g L,1). The developed model could be effectively utilised for designing, controlling and up-scaling the protease production process in high-density fermentation in selected bioreactors with statistical consistency. Copyright © 2008 Society of Chemical Industry [source] Effect of colloidal self-assemblies on the basic hydrolysis of 2-(4-bromophenoxy)quinoxalineJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 6 2003Angela Cuenca Abstract In the presence of cationic surfactants (C16H33NR3Cl; R,=,Me, n-Pr, n-Bu), the shape of rate versus surfactant concentration profiles for the basic hydrolysis of 2-(4-bromophenoxy)quinoxaline depends on substrate concentration. At low substrate concentration there is a single rate maximum and with a 10-fold substrate concentration increase a double rate maximum is observed. The first rate maximum is ascribed to reaction occurring in premicellar aggregates and the second to reaction in micelles. At low substrate concentration the effect of surfactant head group size was examined. Second-order rate constants in the micellar pseudophase increase with increasing head group size. Copyright © 2003 John Wiley & Sons, Ltd. [source] Properties of wheat bran polyphenol oxidaseMOLECULAR NUTRITION & FOOD RESEARCH (FORMERLY NAHRUNG/FOOD), Issue 1 2004dem Soysal Abstract Polyphenol oxidase (PPO) obtained from wheat bran catalyzed the oxidation of 4-methyl catechol. Phenolic compounds found naturally in crude extract played role as an endogeneous substrate and activity of crude extract needed correction. Activity versus enzyme concentration gave a linear plot at high substrate concentration whereas a nonlinear plot was obtained at low substrate concentration which proved the presence of endogeneous substrate. Adsorption on celite and extraction with polyvinylpyrrolidone (PVPP) caused the removal of phenols. Adsorption of PPO on celite yielded a 4-fold increase in specific activity whereas extraction with PVPP yielded a 2.5-fold increase in specific activity compared to the crude extract. The kinetics of PPO catalyzed oxidation obeyed Michaelis-Menten model; Km and Vmax values were found as 218 mM and 99 ,M/min, respectively. The enzyme was inhibited by ethyl alcohol, dithiothreitol (DTT) and isoproterenol and exhibited heat stability up to a temperature of 90°C. The optimum pH of the enzyme was found to be 5.0. [source] Development and calibration of a nitrification PDE model based on experimental data issued from biofilter treating drinking waterBIOTECHNOLOGY & BIOENGINEERING, Issue 2 2006I. Queinnec Abstract To remove ammonia for production of drinking water, nitrification can be performed in a bio-filter. At least 1 month is necessary to capture from the groundwater and then grow a sufficient amount of nitrifying bacteria to reach the desired removal efficiency. Improving start-up of bio-filters at low substrate concentration is therefore a major challenge. In this connection, it is important to develop appropriate models for designing, monitoring or analysing biofilm systems during start-up or following disinfection events. This study discusses the development and calibration of a nitrification PDE model which reflects the compromise between the complexity associated with the description of the full physical and biochemical mechanisms and the search for a simplified model with identifiable parameters. This model takes only the relevant phenomena (considering the full operating range) into account. The validity of the calibrated model has been evaluated through experiments under very different operational conditions, at the laboratory and under real industrial conditions, involving the full upstream chain of water treatment (iron oxidation and sand filter). © 2006 Wiley Periodicals, Inc. [source] Dynamics of Benzene and Toluene Degradation in Pseudomonas putida F1 in the Presence of the Alternative Substrate SuccinateENGINEERING IN LIFE SCIENCES (ELECTRONIC), Issue 4 2007I. Rüegg Abstract In batch and continuous culture, the regulation of benzene and toluene degradation by Pseudomonas putida,F1 was investigated in the presence of the alternative carbon and energy source succinate. In batch culture, benzene and toluene were used simultaneously, whereas succinate suppressed benzene consumption under carbon excess conditions resulting in diauxic growth. In carbon-limited continuous culture mixed substrate growth was observed. Since in nature low substrate concentrations and ever changing conditions prevail, this paper focuses on the dynamics of benzene/toluene degradation, biomass synthesis, and the regulation of benzene/toluene-degrading enzymes in cultures growing continuously at a dilution rate of 0.1,h,1, when shifting the supply of the carbon and energy source from succinate to various mixtures of succinate and benzene/toluene, or to benzene only. When the succinate concentration was kept constant (1.25,mM) and the medium was supplemented with benzene (2,mM), growth with benzene began already two hours after the shift. In contrast, replacing succinate with benzene only led to a wash out of biomass for more then ten hours, before biomass production from benzene started. A striking and reproducible transition pattern was observed for all shifts where the succinate concentration was reduced or succinate was omitted. After an initial period of biomass production from benzene, the culture collapsed and a wash-out of biomass was observed. However, this wash-out was not accompanied by an increase in benzene in the cultivation liquid, indicating a benzene uptake without conversion into biomass. Another possibility is that in phases of low biomass concentrations, cells were only able to use the low amounts of benzene/toluene dissolved in the cultivation liquid yielding low biomass concentrations whereas in phases of high biomass concentrations, they were able to rapidly utilize the aromatic solvents so that additional benzene from the gas phase diffused into the cultivation liquid resulting in more biomass production. In most cases, growth resumed again after 10 to 80,h. Currently, the reasons for the decrease in biomass after the first rise are unknown. However, several indications rule out intoxication of the cells by either the solvents benzene or toluene themselves, or by toxic degradation intermediates, or by-products. [source] Simplified yet highly accurate enzyme kinetics for cases of low substrate concentrationsFEBS JOURNAL, Issue 19 2009Hanna M. Härdin Much of enzyme kinetics builds on simplifications enabled by the quasi-steady-state approximation and is highly useful when the concentration of the enzyme is much lower than that of its substrate. However, in vivo, this condition is often violated. In the present study, we show that, under conditions of realistic yet high enzyme concentrations, the quasi-steady-state approximation may readily be off by more than a factor of four when predicting concentrations. We then present a novel extension of the quasi-steady-state approximation based on the zero-derivative principle, which requires considerably less theoretical work than did previous such extensions. We show that the first-order zero-derivative principle, already describes much more accurately the true enzyme dynamics at enzyme concentrations close to the concentration of their substrates. This should be particularly relevant for enzyme kinetics where the substrate is an enzyme, such as in phosphorelay and mitogen-activated protein kinase pathways. We illustrate this for the important example of the phosphotransferase system involved in glucose uptake, metabolism and signaling. We find that this system, with a potential complexity of nine dimensions, can be understood accurately using the first-order zero-derivative principle in terms of the behavior of a single variable with all other concentrations constrained to follow that behavior. [source] Inhibition of human cytochrome p450 1b1 further clarifies its role in the activation of dibenzo[a,l]pyrene in cells in cultureJOURNAL OF BIOCHEMICAL AND MOLECULAR TOXICOLOGY, Issue 3 2007Brinda Mahadevan Abstract Metabolic activation and DNA adduct formation of the carcinogenic aromatic hydrocarbon dibenzo{a,l}pyrene (DBP) was investigated in human mammary carcinoma MCF-7 cells and human cytochrome P450 (CYP) 1B1-expressing Chinese hamster V79 cells in culture. It has been shown that DBP is metabolically activated to DNA-binding diol epoxides both in vitro and in vivo. To further establish the role of human CYP1B1 in the activation of DBP, both cell lines were cotreated with DBP and a selective chemical inhibitor of CYP1B1, 2,4,3, ,5,-tetramethoxy-stilbene (TMS). Results from DBP,DNA adduct analyses revealed the complete inhibition of DNA binding when cells were cotreated with DBP and TMS in comparison to DBP alone. Inactivation of CYP1B1 by TMS was also demonstrated through a decrease in the 7-ethoxyresorufin O -deethylase (EROD) activity in microsomes isolated from these cells. Emodin, 3-methyl-1,6,8-trihydroxyanthraquinone, an active ingredient of an herb, has been recently shown of being able to induce CYP1 gene expression. Examination of human CYP1B1 induction and EROD activity confirmed an increase in protein levels upon cotreatment with emodin and DBP. Despite increases in protein levels and enzyme activity, there was no significant change in DBP,DNA binding levels at very low substrate concentrations (17 nM). The data obtained in this study emphasize the central role of CYP1B1 in the activation of DBP in human cells in culture. © 2007 Wiley Periodicals, Inc. J Biochem Mol Toxicol 21:101,109, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/jbt.20168 [source] Biodegradation kinetics of benzene, methyl tert -butyl ether, and toluene as a substrate under various substrate concentrationsJOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 1 2007Chi-Wen Lin Abstract Owing to the complexity of conventional methods and shortcomings in determining kinetic parameters, a convenient approach using the nonlinear regression analysis of Monod or Haldane type nonlinear equations is presented. This method has been proven to provide accurate estimates of kinetic parameters. The major work in this study consisted of the testing of aromatic compound-degrading cultures in batch experiments for the biodegradation of benzene, methyl tert -butyl ether (MTBE), and toluene. Additionally, batch growth data of three pure cultures (i.e., Pseudomonas aeruginosa YAMT421, Ralstonia sp. YABE411 and Pseudomonas sp. YATO411) isolated from an industrial petrochemical wastewater treatment plant under aerobic conditions were assessed with the nonlinear regression technique and with a trial-and-error procedure to determine the kinetic parameters. The growth rates of MTBE-, benzene-, and toluene-degrading cultures on MTBE, benzene, and toluene were significant. Monod's model was a good fit for MTBE, benzene and toluene at low substrate concentrations. In contrast, Haldane's equation fitted well in substrate inhibition concentration. Monod and Haldane's expressions were found to describe the results of these experiments well, with fitting values higher than 98%. The kinetic parameters, including a maximum specific growth rate (µm), a half-saturation constant (Ks), and an inhibition constant (Ki), were given. Copyright © 2007 Society of Chemical Industry [source] Human cytochromes mediating gepirone biotransformation at low substrate concentrationsBIOPHARMACEUTICS AND DRUG DISPOSITION, Issue 2 2003David J. Greenblatt Abstract Biotransformation of gepirone to 1-(2-pyrimidinyl)-piperazine (1-PP) and 3'-OH-gepirone, as well as two other hydroxylated metabolites, was studied in vitro using a human liver microsomal preparation and heterologously expressed human CYP3A4 and CYP2D6. The focus was on a low range of gepirone concentrations (1000 nM and below). Liver microsomes formed 1-PP and 3'-OH-gepirone with similar reaction velocities. Two other hydroxylated metabolites (2-OH- and 5-OH-gepirone) were also formed, but pure reference standards were not available for purposes of quantitative analysis. The CYP3A inhibitor ketoconazole completely eliminated 1-PP formation, reduced 3'-OH-gepirone formation to less than 20% of control, and reduced 2-OH-gepirone formation to 7% of control. All metabolites were formed by expressed CYP3A4; however, CYP2D6 formed 3'-OH- and 5-OH-gepirone, but not 1-PP or 2-OH-gepirone. Based on estimated relative abundances of the two isoforms in human liver, CYP3A4 was predicted to account for more than 95% of net clearance of gepirone in vivo at low concentrations approaching the therapeutic range. CYP2D6 would account for less than 5% of net clearance. The findings are consistent with previous in vitro studies of gepirone using higher substrate concentrations. Copyright © 2003 John Wiley & Sons, Ltd. [source] Low-Concentration 1,2- trans ,-Selective Glycosylation Strategy and Its Applications in Oligosaccharide SynthesisCHEMISTRY - A EUROPEAN JOURNAL, Issue 41 2009Chin-Sheng Chao Abstract This study develops an operationally easy, efficient, and general 1,2- trans ,-selective glycosylation reaction that proceeds in the absence of a C2 acyl function. This process employs chemically stable thioglycosyl donors and low substrate concentrations to achieve excellent ,-selectivities in glycosylation reactions. This method is widely applicable to a range of glycosyl substrates irrespective of their structures and hydroxyl-protecting functions. This low-concentration 1,2- trans ,-selective glycosylation in carbohydrate chemistry removes the restriction of using highly reactive thioglycosides to construct 1,2- trans ,-glycosidic bonds. This is beneficial to the design of new strategies for oligosaccharide synthesis, as illustrated in the preparation of the biologically relevant ,-(1,6)-glucan trisaccharide, ,-linked Gb3 and isoGb3 derivatives. [source] | |||||||||||||