Enzyme assays.

The past year or so has seen the development of new enzyme assays, as well as the improvement of existing ones. Assays are becoming more rapid and sensitive as a result of modifications such as amplification of the enzyme product(s). Recombinant DNA technology is now being recognized as a particularly useful tool in the search for improved assay systems.     

Enzyme kinetic assays with surface plasmon resonance (BIAcore) based on competition between enzyme and creatinine antibody

A procedure is described which allows the characterization of enzyme by a hybrid approach using an enzyme and an antibody. The presented method is related to the affinity determination of antibodies by the 'affinity in solution' procedure for BlAcore. The antibody is used as an indicator for the concentration of substrate, which is also the antigen. A mixture of enzyme, substrate and antibody is incubated, and an aliquot of this solution is injected periodically into a flowcell containing immobilized substrate, which is bound by the antibody, but not cleaved by the enzyme. The chosen initial concentration of substrate inhibits the binding of antibody to the immobilized substrate by 90%. During the enzymatic reaction, increased amounts of antibody bind to the surface, as the substrate concentration is decreased. With this method, the cleavage of creatinine with creatinine iminohydrolase (6 mU/ml) was monitored for up to 11 h. A recently developed monoclonal antibody against creatinine was used as the indicating protein. For the calculation of enzyme activity, the signals were compared with a calibration curve for inhibition of antibody binding to the chip by creatinine in solution.     

Rapid enzyme kinetic assays of individualDrosophila and comparisons of field-caughtD. melanogaster andD. simulans

Techniques for performing numerous enzyme kinetic assays with minimum time and effort would be valuable to studies of the evolutionary genetics of metabolic control and the quantitative genetics of determinants of kinetic parameters. Microtiter plate readers have been used for a variety of repetitious analytical techniques, and instruments are available that can take repetitive readings with sufficient speed to perform kinetic assays. The ability of these instruments to assay rapidly the kinetic properties of small samples makes them potentially useful for a number of problems in population genetics. While the ability to handle large numbers of samples is very attractive, the small sample volumes and optical imprecision of microtiter plates result in some sacrifice in accuracy. This paper presents methods for performing kinetic assays on individual field-caughtDrosophila, quantifies the precision of these methods, and characterizes differences amongDrosophila melanogaster andD. simulans from samples caught in California and Pennsylvania. Comparisons between field-caught and laboratory rearedD. melanogaster show that most of the characters are very similar, with the exception of GPDH, which has a threefold higher mean activity among field-caught flies. The phenotypic correlations are presented with a brief discussion of their relevance to assessing the evolution of metabolic control of these enzymes.This work was supported by Grant BSR 8717495 from the National Science Foundation and by Grants HD 18379 and HD 00743 from the U.S. Public Health Service.     

Enzyme assays using permeabilized cells of Neurospora.

A procedure is described for the preparation of homogeneous, steady-state cultures of germinated conidia of Neurospora crassa. Permeabilization of such cells has been accomplished by a combination of toluene-ethanol and freeze-thaw treatments. These permeabilized cells have been used for the determination of enzyme activities. The method has been shown to be rapid and rellable and is applicable to nuclear, mitochondrial, and cytosolic enzymes.     

Enzyme kinetic studies from progress curves.

A method is described for fitting the velocities obtained from progress curves to a steady-state rate equation. It is based on the method of Markus & Plesser [(1981) in Kinetic Data Analysis: Design and Analysis of Enzyme and Kinetic Data (Edrenyi, ed.), pp. 317-339, Plenum Press, New York]. The obstacle of needing good initial estimates of kinetic parameters is removed by using the parameters provided graphically by a minor modification of the method of Yun & Suelter [(1977) Biochim, Biophys. Acta 480, 1-13]. This progress-curved-based method allows the same discrimination among rival models as do the initial-velocity-based methods, with a great saving of experimental time. The BASIC and FORTRAN 77 programs are deposited as Supplementary Publication SUP 50132 (17 pages) at the British Library (Lending Division), Boston Spa, Wetherby, West Yorkshire LS23 7BQ, U.K., from whom copies can be obtained on the terms indicated in Biochem. J. (1986) 233, 5-6.     

Quantitative radiochemical enzyme assays in single cells: purine phosphoribosyl transferase activities in cultured fibroblasts.

An ultra-microchemical method using radioactive substrates has been developed for enzyme activity measurements at the single cell level. In order to demonstrate the possibilities of this radiochemical microassay, activity measurements of hypoxanthine-guanine phosphoribosyl transferase (HG-PRT) and of adenine phosphoribosyl transferase (A-PRT) in isolated human fibroblasts are described. There was a linear relationship between the number of cells incubated and the enzyme activities found. It was observed that the HG-PRT activity in single, skin derived, fibroblasts did not differ from that in amniotic fluid derived fibroblasts, thus providing a new, quantitative assay for rapid prenatal diagnosis in the Lesch-Nyhan syndrome.     

Robust regression of enzyme kinetic data.

A method described previously [Cornish-Bowden & Endrenyi (1981) Biochem. J. 193, 1005-1008] for fitting theoretical equations to enzyme kinetic data without prior knowledge of weights or error distribution has been tested by computer simulation. With the equations for various kinds of linear inhibition as an example, the method performed well under all of the conditions examined, giving results that were often much better than those given by widely used least-squares alternatives, and were never appreciably worse. Although equations for two-substrate kinetics were not explicitly tested, the results for inhibition equations can be generalized to include two-substrate equations because the two are formally equivalent for simulation purposes. As a check on the results with inhibition equations the method was also tested for fitting bell-shaped pH-activity profiles and gave correspondingly good results.     

Enzyme assays for high-throughput screening

Assaying enzyme-catalyzed transformations in high-throughput is crucial to enzyme discovery, enzyme engineering and the drug discovery process. In enzyme assays, catalytic activity is detected using labelled substrates or indirect sensor systems that produce a detectable spectroscopic signal upon reaction. Recent advances in the development of high-throughput enzyme assays have identified new labels and chromophores to detect a wide range of enzymes activities. Enzyme activity profiling and fingerprinting have also been used as tools for identification and classification, while microarray formats have been devised to increase throughput.     

Enzyme activities in human breast tumours.

The activities of six enzymes associated with carbohydrate metabolism were measured both in carcinomas and in normal breast tissues. The following differences were observed. 1. The carcinoma showed higher enzyme activities than the normal mammary tissue. 2. The ratios of glutamate dehydrogenase, hydroxybutyrate dehydrogenase, glutathione reductase and catalase to lactate dehydrogenase were lower in carcinomas than in normal tissues. Similarly, the ratios of glutamate dehydrogenase, hydroxybutyrate dehydrogenase, glutathione reductase and catalase to glucose-6-phosphate dehydrogenase were also significantly lower in carcinomas. 3. There were no significant differences in enzyme activities between I and II stage of the disease and the metastatic tissues, however, there were significant differences between I and III stage. The significance of these findings is discussed in terms of the alterations in the balance between the metabolic pathways.     

Beta-D-glycosidase activities of Humicola grisea: biochemical and kinetic characterization of a multifunctional enzyme

A beta-D-glycosidase activity was purified from mycelium of Humicola grisea var. thermoidea grown on avicel as the main carbon source. The purified enzyme was a glycoprotein and migrated as a single polypeptide band on polyacrylamide gel electrophoresis under native or denaturing conditions. The apparent molecular weight of the enzyme was estimated to be 55 kDa by gel filtration and SDS-PAGE. The enzyme was active against o-nitrophenyl beta-D-galactoside; p-nitrophenyl beta-D-glucoside, p-nitrophenyl beta-D-fucoside, lactose and cellobiose, PNP fucoside (synthetic substrate) and cellobiose (natural substrate) being the best utilized. A comparison of the properties of beta-D-galactosidase, beta-D-glucosidase and beta-D-fucosidase showed that three activities exhibited similar pH and temperature optima and the same thermostability. The hydrolysis rate of substrate mixtures suggests that the enzyme possesses a common catalytic site for all the substrates assayed.     

Biosensor system for continuous flow determination of enzyme activities. II. Simultaneous determination of plural enzyme activities.

A biosensor system for continuous flow determination of plural enzyme activities was prepared from the combination of two pyruvate sensors, a prereactor and a flow cell. This system was applied to the simultaneous determination of lactic dehydrogenase (LDH) and glutamic-pyruvic transaminase (GPT) activities in the same sample. These enzyme activities can be determined by measuring pyruvate produced by the enzyme reactions as follows. The amount of pyruvic acid can also be determined from the amount of oxygen consumed upon oxidation of pyruvic acid by pyruvate oxidase. (Formula: see text). Therefore, both of the detectors for the determination of lactic dehydrogenase and glutamic-pyruvic transaminase activities were prepared from the combination of a pyruvate oxidase membrane and an oxygen electrode. Pyruvate oxidase was covalently immobilized on a membrane prepared from cellulose triacetate. A linear relation was obtained between the output current and LDH or GPT activities in the range of 50 to 3,600 IU l-1 or 6 to 1,000 IU l-1, respectively. Each assay of these enzyme activities was completed within 15 min. The results obtained had a precision of ca. 4%. The sensor was stable for more than 25 days at 5 degrees C.     

The kinetic model of the shikimate pathway as a tool to optimize enzyme assays for high-throughput screening

Four-enzyme section of the shikimate pathway (Aro B, D, E, and K) of Streptococcus pneumoniae has been studied. Kinetic properties of the individual enzymes and three- and four-enzyme linked reactions have been characterized in vitro. On the basis of the data measured in spectrophotometric and LC-MS experiments, kinetic mechanisms of the enzymes have been suggested and all kinetic parameters have been identified. Kinetic models for these three- and four-enzyme sections of the shikimate pathway have been constructed and validated. The model of the four-enzyme section of shikimate pathway has been employed to design an inhibition-sensitive reconstituted pathway for a high-throughput screening effort on the shikimate pathway. It was demonstrated that using the model it was possible to optimize this reconstituted pathway in such a way to provide equal sensitivity of the enzymes to inhibition.     

Polarimetry as a general method for enzyme assays.

Countercurrent distribution (CCD) and Martin-Synge distribution (MSD) were compared on the basis of the theory previously presented in this series. The comparison included the numbers of partition units required to obtain the same resolution degree of two compounds as well as the elution volumes and the widths of the elution curves. The ratio of the numbers of partition units, φ, was found to be proportional to αk₁ where α is the phase ratio and k₁ is the partition coefficient of the more rapidly moving component. The limit of φ when β → 1 was found equal to αk₁ + 1. Accordingly, in the case αk₁ ? 0, the methods possess approximately the same separation power, and in case αk₁ ? 1, about double the number of partition units is required in MSD as compared to CCD. In the case that αk_1,CCD = 1 and αk_1,MSD ? 0, the situation becomes roughly inversal. The ratio of the elution volumes, χ, in the two methods was found to be equal to φ in the case that the stationary phase volumes (ν_s) are equal and that the q = 1/(αk + 1) values are equal in the methods. On the same conditions, the ratio of the standard deviations of the elution curves, ψ, was found to be equal to φ^1/2/q^1/2, and the limit of ψ when β → 1 equal to 1/q₁. If, in addition, the condition αk₁ ? 0 is satisfied, ψ = 1. A practical comparison of the methods was also included, wherein attention was focused upon the real separation powers, the reproducibilities, the suitabilities for analytical or preparative purposes, the suitabilities for nonideal situations, the possibilities for automation, and the main structural features of some most important CCD and MSD apparatuses.     

Calibration of bioaffinity assays using kinetic data

Bioaffinity assays are usually calibrated by using a set of standard measurements fitted to a simple empirical model. In this paper, a new calibration approach based on mechanistic model of reaction kinetics is presented. When the calibration assay is known in terms of reaction mechanism, incubation time, initial concentration, and rate constants, one can back-calculate concentrations of unknown samples measured in a nonequilibrium time point. This paper describes a calculation method of unknown sample concentrations based on kinetically measured single calibration assay point. The theoretical results are verified by two common in-vitro diagnostic assays.