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PubMed1988

EZ-FIT: a practical curve-fitting microcomputer program for the analysis of enzyme kinetic data on IBM-PC compatible computers.

EZ-FIT, an interactive microcomputer software package, has been developed for the analysis of enzyme kinetic and equilibrium binding data. EZ-FIT was designed as a user-friendly menu-driven package that has the facility for data entry, editing, and filing. Data input permits the conversion of cpm, dpm, or optical density to molar per minute per milligram protein. Data can be fit to any of 14 model equations including Michaelis-Menten, Hill, isoenzyme, inhibition, dual substrate, agonist, antagonist, and modified integrated Michaelis-Menten. The program uses the Nelder-Mead simplex and Marquardt nonlinear regression algorithms sequentially. A report of the results includes the parameter estimates with standard errors, a Student t test to determine the accuracy of the parameter values, a Runs statistic test of the residuals, identification of outlying data, an Akaike information criterion test for goodness-of-fit, and, when the experimental variance is included, a chi 2 statistic test for goodness-of-fit. Several different graphs can be displayed: an X-Y, a Scatchard, an Eadie-Hofstee, a Lineweaver-Burk, a semilogarithmic, and a residual plot. A data analysis report and graphs are designed to evaluate the goodness-of-fit of the data to a particular model.

Enzymes↗
PubMed2006

Automated method, based on micro-sequential injection, for the study of enzyme kinetics and inhibition.

A micro-reactor system with continual spectrophotometric detection has been operated in Sequential Injection lab-on-valve (SI-LOV) mode and applied to enzyme kinetics and inhibition studies, using acetylcholinesterase (AChE) and angiotensin-converting enzyme (ACE) as model systems. With the advantages of automation, real-time kinetic measurement, and thorough mixing, the SI-LOV micro-reactor system allows for the monitoring of initial reaction rates and determination of reactant concentrations in the reaction mixture, both of which are essential for the determination of kinetic constants for enzymes and inhibitors. Enzyme, substrate, and inhibitor are precisely metered by the syringe pump and delivered to a stirred micro-reactor, followed by a reference scan that establishes the baseline for the following reaction rate measurement. Michaelis constants (Km) for AChE and ACE were determined to be 0.16 mM and 0.30 mM, respectively, which are consistent with literature values. The type of inhibition (competitive, uncompetitive, or mixed), the dissociation constants for the inhibitors, and the inhibitor dose-response curves were also determined.

Acetylcholinesterase↗
PubMed1998

Real-time enzyme kinetics monitored by dual-color fluorescence cross-correlation spectroscopy.

A method for sensitively monitoring enzyme kinetics and activities by using dual-color fluorescence cross-correlation spectroscopy is described. This universal method enables the development of highly sensitive and precise assays for real-time kinetic analyses of any catalyzed cleavage or addition reaction, where a chemical linkage is formed or cleaved through an enzyme's action between two fluorophores that can be discriminated spectrally. In this work, a homogeneous assay with restriction endonuclease EcoRI and a 66-bp double-stranded DNA containing the GAATTC recognition site and fluorophores at each 5' end is described. The enzyme activity can be quantified down to the low picomolar range (>1.6 pM) where the rate constants are linearly dependent on the enzyme concentrations over two orders of magnitude. Furthermore, the reactions were monitored on-line at various initial substrate concentrations in the nanomolar range, and the reaction rates were clearly represented by the Michaelis-Menten equation with a KM of 14 +/- 1 nM and a kcat of 4.6 +/- 0.2 min-1. In addition to kinetic studies and activity determinations, it is proposed that enzyme assays based on the dual-color fluorescence cross-correlation spectroscopy will be very useful for high-throughput screening and evolutionary biotechnology.

Biochemistry↗
PubMed1994

Enzyme kinetics of lipolysis revisited: the role of lipase interfacial binding.

A model for the enzyme kinetics of lipolysis was developed where the rate limiting step of the reaction is the interfacial binding step. Binding involves the association of the enzyme with a cluster of substrate molecules and a conformational change in the enzyme, resulting in an interfacially penetrated lipase bound to a cluster of substrate molecules. The resulting derived rate equation is identical to the HIll equation. Fits of the model to experimental velocity vs. substrate concentration data from the literature allowed for the determination of enzyme-substrate interface dissociation constants and reaction order with respect to substrate concentration.

Animals↗
PubMed1963

A histochemical enzyme kinetic system applied to the trypsin-like amidase and esterase activity in human mast cells.

A method for the determination of enzyme kinetic constants V(m), K(m), and K(i) in a histochemical system has been devised. As a substitute for the reciprocal of the reaction velocity, the times necessary to reach a fixed amount of end product (the initial visible color) in a tissue site at various substrate concentrations are plotted, according to the method of Lineweaver and Burk, against the reciprocal of the substrate concentrations. The technique as applied to trypsin-like esterase and amidase activities in human mast cells indicates that a single enzyme or closely related enzymes in this site are responsible for the hydrolysis of both the amide and ester substrates and that typical trypsin substrates act as competitive inhibitors of their hydrolysis. Parallel biochemical studies were performed to evaluate the effect of certain aspects of the experimental histochemical method on a purified homospecific enzyme. The relative kinetic constants derived by the histochemical method afford a further means of characterizing enzymic activity in a histochemical system.

Amidohydrolases↗
PubMed1983

Adaptation of acyl-enzyme kinetic theory and an experimental method for evaluating the kinetics of fast-acting, irreversible protease inhibitors.

The theory of acyl-enzyme kinetics (Bender, M.L., Kézdy, F.J. and Wedler, F.C. (1967) J. Chem. Educ. 44, 84-88) has been adapted for use in evaluating the kinetics of inhibition of serine proteases by both natural and synthetic irreversible inhibitors. The new theory is based upon formal analysis of the case of an irreversible, active-site-directed inhibitor competing with an irreversible, active-site-directed substrate for the active site of a serine protease. From this theory, an experimentally simple and accurate method is described to obtain a second-order rate constant that is characteristic of the efficiency with which an irreversible inhibitor reacts. The experimental method is particularly useful for characterizing fast-acting, irreversible inhibitors. The theory and method which are applicable to a wide variety of enzymes are verified by analysis of the inhibition of bovine trypsin by three model inhibitors, p-nitrophenyl p'-guanidinobenzoate, soybean trypsin inhibitor and alpha-1-proteinase inhibitor as well as by human antithrombin III in the presence of heparin and by bovine pancreatic trypsin inhibitor.

Kinetics↗
PubMed2004

The total quasi-steady-state approximation is valid for reversible enzyme kinetics.

The Briggs-Haldane approximation of the irreversible Michaelis-Menten scheme of enzyme kinetics is cited in virtually every biochemistry textbook and is widely considered the classic example of a quasi-steady-state approximation. Though of similar importance, the reversible Michaelis-Menten scheme is not as well characterized. This is a serious limitation since even enzymatic reactions that go to completion may be reversible. The current work derives a total quasi-steady-state approximation (tQSSA) for the reversible Michaelis-Menten and delineates its validity domain. The tQSSA allows the derivation of uniformly valid approximations for the limit of low enzyme concentrations, ET<<ST+KM, and under certain more restrictive conditions also for high enzyme concentrations such that ST<<ET+KM. Using these simple analytical approximations, a sequential experimental-theoretical method is suggested for unambiguously estimating all the kinetic parameters of the reversible Michaelis-Menten scheme.

Biochemical Phenomena↗
PubMedDate not supplied

In vitro enzyme kinetics of microencapsulated tyrosinase.

In this study, we prepared microencapsulated tyrosinase and studied its enzyme kinetics and its stability at different pH and temperature. The Km for both free and microencapsulated tyrosinase is 465 microM. The Vmax for the microencapsulated tyrosinase is 49.02 mg/dl x min, whereas that of tyrosinase in free solution is 114.95 mg/dl x min. At the extreme pH of pH 2 and pH 10, free tyrosinase lost all its enzyme activity, whereas microencapsulated tyrosinase retained 14% of its original activity at pH 2 and 17% of its original activity at pH 10. Further in vitro studies were carried out to study the pH optimum of the microencapsulated enzyme at different pH values corresponding to those along the gastrointestinal tract. In temperature stability studies at 4 degrees C and 37 degrees C, higher enzyme activity remained in microencapsulated tyrosinase solution than that of free enzyme solution. Therefore, our results show that microencapsulated tyrosinase is significantly more stable than free tyrosinase. In subsequent studies, we show that daily oral administration of microencapsulated tyrosinase significantly lowers systemic tyrosinase after 3 days and maintains this low level as long as oral administration continues. When combined with one intravenous injection of polyhemoglobin-tyrosinase, the systemic tyrosine level can be lowered within one hour.

Animals↗
PubMed1979

A new procedure to derive weighting factors for nonlinear regression analysis applied to enzyme kinetic data.

The experimental variance of enzymic steady-state kinetic experiments depends on velocity as approximated by a power function (Var(v) = K1 . valpha (Askelöf, P., Korsfeldt, M. and Mannervik, B. (1976) Eur. J. Biochem. 69, 61--67). The values of the constants (K1, alpha) can be estimated by making replicate measurements of velocity, and the inverse of the function can then be used as a weighting factor. In order to avoid measurement of a large number of replicates to establish the error structure of a kinetic data set, a different approach was tested. After a preliminary regression using a 'good model', which satisfies reasonable goodness-of-fit criteria, the residuals were taken to represent the experimental error. The neighbouring residuals were grouped together and the sum of their mean squared values was used as a measure of the variance in the neighbourhood of the corresponding measurements. The values of the constants obtained in this way agreed with those obtained by replicates.

Enzymes↗
PubMed2001

A strategy for the determination of enzyme kinetics using electrospray ionization with an ion trap mass spectrometer.

A simple and rapid means of enzyme kinetic analysis was achieved using electrospray ionization mass spectrometry and a one-point normalization factor. The model system used, glutathione S-transferase from porcine liver, is a two-substrate enzyme catalyzing the conjugation of glutathione with a variety of compounds containing an electrophilic center. An internal standard that is structurally similar to the product was added to the reaction quench solution, and a single-point normalization factor was used to determine the product concentration without the need of a calibration curve. Kinetic parameters, such as Km, Vmax and Ki (for thyroxine), obtained by electrospray mass spectrometry agreed with those obtained from traditional UV-vis spectroscopy, and competitive vs noncompetitive inhibition reactions could be delineated via mass spectrometry. These results suggest that our method can be applied to enzymatic processes in which spectrophotometric or spectrofluorometric assays are not feasible or when the relevant substrates do not incorporate chromophores or fluorophores. This new method is competitive with traditional UV assays in that it is facile and it involves very little analysis time.

Animals↗
PubMed2000

Analysis of enzyme kinetics in individual living cells utilizing fluorescence intensity and polarization measurements.

BACKGROUND: The Cellscan mark-S (CS-S) scanning cytometer was used for tracing enzymatic reactions in the same individual cells under various physiological conditions over periods of minutes. On-line reagent addition and changes in the experimental conditions (buffers, ions, substrates and inhibitors) were performed. METHODS: Kinetic events were monitored by fluorescence intensity (FI) and fluorescence polarization (FP) measurements of fluorescein diacetate (FDA) and chloromethyl fluorescein diacetate (CMFDA) intracellular hydrolysis. FP measurements have been used to assess the intracellular marker's mobility restrictions. RESULTS: Kinetic measurement along 1000 s of FDA labeled individual Jurkat T cells, indicated variation of 65% for FI(t) and approximately 10% for FP(t). While FI increased linearly with time, FP(t) decreased nonlinearly and asymptotically, reaching a constant value. The FP(t) of CMFDA-labeled cells was different from that of FDA-labeled cells. Average cellular Km of 3.9 microM was calculated from individual cell FDA hydrolysis curves. CONCLUSIONS: (1) Analysis of the reaction kinetics of intracellular enzymes can be refined by using FP measurements of the products of fluorogenic substrates in addition to the FI measurements. (2) Subpopulations or individual cells could be classified according to their reaction rates. (3) A specific dependence of FP(t) on type of enzyme substrate is suggested.

Binding, Competitive↗
PubMed1999

Incorporating qualitative knowledge in enzyme kinetic models using fuzzy logic.

Modeling of metabolic pathway dynamics requires detailed kinetic equations at the enzyme level. In particular, the kinetic equations must account for metabolite effectors that contribute significantly to the pathway regulation in vivo. Unfortunately, most kinetic rate laws available in the literature do not consider all the effectors simultaneously, and much kinetic information exists in a qualitative or semiquantitative form. In this article, we present a strategy to incorporate such information into the kinetic equation. This strategy uses fuzzy logic-based factors to modify algebraic rate laws that account for partial kinetic characteristics. The parameters introduced by the fuzzy factors are then optimized by use of a hybrid of simplex and genetic algorithms. The resulting model provides a flexible form that can simulate various kinetic behaviors. Such kinetic models are suitable for pathway modeling without complete enzyme mechanisms. Three enzymes in Escherichia coli central metabolism are used as examples: phosphoenolpyruvate carboxylase; phosphoenolpyruvate carboxykinase; and pyruvate kinase I. Results show that, with fuzzy logic-augmented models, the kinetic data can be much better described. In particular, complex behavior, such as allosteric inhibition, can be captured using fuzzy rules. The resulting models, even though they do not provide additional physical meaning in enzyme mechanisms, allow the model to incorporate semiquantitative information in metabolic pathway models.

Bacterial Proteins↗
PubMed2005

Development of a biosensor for measuring plasma fibrinogen based on an enzyme kinetic model.

A biosensor has been designed for measuring plasma fibrinogen concentrations based on an enzyme kinetic model. The values obtained correspond linearly to those of a clinical method but have a higher sensitivity within the normal range of fibrinogen. By using the disposable biosensor technology, this method is easy to follow, quick (within 2 min) and precise (with coefficients of variation to be as low as below 5%) and requires only a small amount of plasma sample (<50 microl). This biosensor technology also holds potential for use in point-of-care diagnostics.

Animals↗
PubMed1997

Diacylglycerol partitioning and mixing in detergent micelles: relevance to enzyme kinetics.

For many of the enzymes that utilize or produce diacylglycerols, detergent mixed micelles are often used in assay systems to solubilize the lipophilic substrates or products. The assumption is often made that the diacylglycerol (DAG) is solubilized and well mixed throughout the population of micelles during the time course of the assay. In the present work the partitioning and exchange dynamics of diacylglycerols (from dihexanoyl-DAG to didecanoyl-DAG) in a variety of detergent micelles have been studied by NMR and fluorescence methods. In all detergents, the longer the DAG chain lengths, the more detergent is required for solubilization. However, efficiency of solubilization varies tremendously with Triton X-100 the most efficient (i.e. the least detergent is required), and deoxycholate the least efficient in solubilizing DAG. The mixing and exchange dynamics of pyrene-labeled DAG molecules in these micelles (measured by stopped-flow fluorescence) were fastest for Triton X-100 and slowest with charged bile salt micelles. Of the detergent systems characterized, Triton X-100 appears to be the optimal detergent for use in assays of enzymes that interact with DAG (beta-octylglucoside and diheptanoylphosphatidylcholine have good exchange dynamics, but higher amounts of these detergents are needed to solubilize DAG). Bile salt micelles provide the least solubilization and the slowest exchange kinetics (so slow that this could be a significant problem in some enzyme assays). This information on DAG behavior in micelles is discussed with respect to assays of an enzyme that generates DAG as product (phospholipase C) and one that uses DAG as substrate (DAG kinase). Although slow exchange of DAG occurs in some micelle systems, this does not appear to be a rate-limiting step in the kinetics for either of these enzymes.

Bacillus cereus↗
PubMed1980

Use of modified double-reciprocal plot for enzyme kinetic parameters. A pitfall.

The modified double-reciprocal plot suggested by Lee and Wilson (Lee, H.J. and Wilson, I.B. (1971) Biochim. Biophys. Acta 242, 519--522) has until now been accepted as giving a simple and accurate method for the determination of enzyme kinetic parameters V and Km, which can be used with little error even when as much as 50% of the substrate is consumed. In this paper it has been shown with analytical proof that the errors in the application of their method can in fact be much higher and that the integrated Michaelis-Menten equation should be used in its complete form.

Enzymes↗
PubMed1998

A kinetic enzyme immunoassay for the quantitation of antibodies to a humanized monoclonal antibody in human serum.

A kinetic enzyme immunoassay was developed and validated to quantitate human antibodies to the humanized monoclonal antibody CAMPATH1-1H (C1H) in human serum. The assay was configured using C1H-coated 96-well plates which were blocked with bovine serum albumin, and incubated with dilutions of human serum containing anti-C1H antibody. Antibody was detected using biotinylated C1H followed by streptavidin-conjugated alkaline phosphatase and p-nitrophenyl phosphate. Absorbance data were collected for 10 min, and mOD min-1 data were exported to MultiCalc data analysis software. A 4-parameter logistic-log algorithm was shown to model the data through the range of the standard curve within 15% of nominal values. The overall assay performance coefficient of variation by ANOVA was 9.2%. The lower limit of detection was defined at 160 Units ml-1. The anti-idiotype antibody standard stock solution is stable at 4 degrees C and at -80 degrees C for at least 11 months in buffer. The anti-idiotype antibody controls are stable for at least seven freeze-thaw cycles and at least 6 months in human serum stored at -20 degrees C. A strategy was devised by which to establish the specific antibody potency for any given batch of anti-C1H antibody standard relative to the Reference Standard. This EIA has been used to quantify and characterize anti-C1H antibody in human serum in support of clinical safety and efficacy studies.

Alemtuzumab↗