Enzyme reactors in unsegmented flow injection analysis

Characterization of CIM monoliths as enzyme reactors

The immobilization of the enzymes citrate lyase, malate dehydrogenase, isocitrate dehydrogenase and lactate dehydrogenase to CIM monolithic supports was performed. The long-term stability, reproducibility, and linear response range of the immobilized enzyme reactors were investigated along with the determination of the kinetic behavior of the enzymes immobilized on the CIM monoliths. The Michaelis-Menten constant K(m) and the turnover number k(3) of the immobilized enzymes were found to be flow-unaffected. Furthermore, the K(m) values of the soluble and immobilized enzyme were found to be comparable. Both facts indicate the absence of a diffusional limitation in immobilized CIM enzyme reactors.

Calibrationless determination of creatinine and ammonia by coulometric flow titration

A precise and sensitive working microflow titration procedure was developed to determine creatinine and ammonia in urine samples. This procedure is based on enzymatic conversion of creatinine, gas diffusional membrane separation of the released ammonia into an acid acceptor stream, and coulometric titration of ammonia with hypobromite. The hypobromite is formed after the electrogeneration of bromine in an electrolyte containing 1.0 M NaBr and 0.1 M sodium borate adjusted to pH 8.5. The electrolysis current follows a triangle-programmed current-time course. An amperometric flow detector records the resulting mirror symmetrical titration curves, which show two equivalence points. The analyte concentration is calculated from the time difference between the equivalence points. For quantitative conversion of creatinine and quantitative separation of present and released ammonia no calibration is necessary to get accurate results. Both ammonia/ammonium and creatinine were determined in the range between 2 microM and 2 mM with relative standard deviations between 3.0 and 1.0% (n = 5). High recoveries were obtained for the analysis of diluted urine samples for both creatinine and ammonia.

Oligosaccharide dehydrogenase-catalyzed assay for the determination of polysaccharides

Oligosaccharide dehydrogenase (ODH), an enzyme known to have a broad selectivity for reducing sugars of low molecular weight, was investigated to determine its catalytic properties with larger polysaccharides. Six substrates were studied: pullulan standards with molecular weights of between 5,400 and 90,900, debranched starch, and dextran. In addition, maltotriose, isomaltotriose, maltose, and glucose were used as substrates for comparison. ODH catalyzed the oxidation of the large pullulans with a degree of polymerization of at least 560. Isomaltotriose and dextran were not oxidized. ODH activity for the pullulans, expressed as the rate constant Kps, was only three times lower than that for maltose. When the oxidation of sugars with ODH was coupled to a color-forming reaction, quantitative spectrophotometric determination of sugars was possible using either Meldola's blue or N-methylphenazinium as electron acceptors in combination with nitrotetrazolium blue. Linear calibration curves for maltose, maltotriose, and debranched starch were obtained using this ODH method and compared with curves from the conventional spectrophotometric copper sulfate method. This work demonstrates that ODH can be advantageously used for the determination of polysaccharides.

Automation of selective assays for on-line bioprocess monitoring by flow-injection analysis

On-line analysis of one component in a complex media used for bioprocesses requires the application of selective tests such as enzymes assays. Because these assays are susceptible to interference by other medium components and have a limited detection range, automatic sample pretreatment is a prerequisite. The progress made with automatic sample pretreatment in flow-injection analysis makes this technique particularly suitable for on-line monitoring of bioprocesses. Moreover, newly developed software control systems may improve the necessary robustness of flow-infection analysis systems.

Discovering flow injection: journey from sample to a live cell and from solution to suspension

Twenty years after its inception, flow injection is seen as an ever-expanding method, as new modifications are discovered such as flow injection cytoanalysis and the flow injection on renewable surfaces technique. In this review, a personal view of the future rather than the history of flow injection is given, with comment on how research is actually being conducted.

Optimization of enzyme ratios in a coimmobilized enzyme reactor for the analysis of D-xylose and D-xylulose in a flow system

A coupled enzyme system for the detection of D-xylose and D-xylulose is presented. The system is based on three consecutive enzymatic steps. The enzymes xylose isomerase (XI), mutarotase (MT), and glucose dehydrogenase (GDH) are coimmobilized on controlled pore glass and packed in a bed reactor. The relative amount of enzymes, i.e., enzyme ratio, plays a critical role in driving the overall reaction, resulting in a system with linear response characteristics and an operational range of several orders of magnitude. Three different enzyme ratios are assayed to achieve maximum conversion efficiencies for xylose and xylulose. The highest enzyme unit ratio assayed, 13.4 of GDH to XI, gave the highest apparent pseudo-first-order rate constant showing the importance of the last enzymatic reaction in the coupled system to make the overall reaction thermodynamically favorable. A pH of 7.0 was found to be an optimum compromise for the multienzyme system. Sensitivity was dependent on NAD+ concentration. The study was carried out in a flow injection system. The optimized reactor has been applied for the catalytic detection of pentoses in flow injection analysis (FIA) and liquid chromatography (LC).

The characterisation of immobilised lignin peroxidase by flow injection analysis

Immobilised lignin peroxidase has been investigated using a flow system in the steady state and by flow injection analysis (FIA). In the steady state, the extreme sensitivity of the enzyme towards inactivation by H2O2 resulted in a stable response only in the presence of saturating levels of organic substrate and at very low (10 microM) peroxide concentrations. By contrast, the low contact time during FIA led to a stable response to injections of 100 microM H2O2. At higher peroxide concentrations a reproducible inactivation was observed, allowing a study of factors affecting both activity and stability. Lignin peroxidase substrates that undergo at least semi-reversible oxidation/reduction, including high-molecular-weight lignin fractions, could be detected by electrochemical reduction of the oxidation products. With this detection system it was possible to demonstrate the role of veratryl alcohol as mediator. This mediated oxidation of lignin functioned only when all components were present simultaneously, and was not observed when lignin was separated from the site of veratryl alcohol oxidation.

Flow injection analysis of serum urea using urease covalently immobilized on 2-fluoro-1-methylpyridinium salt-activated fractogel and fluorescence detection

Serum samples were analyzed for their urea content using fluorescence flow injection analysis incorporating an immobilized urease bioreactor and a gas permeable separator. The urease was immobilized under mild and facile conditions to a hydrophilic 2-fluoro-1-methylpyridinium-activated support. The ammonia released as a result of urease-catalyzed urea hydrolysis diffused through a gas permeable membrane into a constant stream of o-phthaldehyde solution to form a highly fluorescent product with lambda ex at 340 nm and lambda em at 455 nm. Up to 25 serum samples can be analyzed per hour. The within-day coefficient of variation (CV) was 1.12% and the day-to-day CV was 1.25% for serum containing 10.50 mg urea nitrogen dl-1. The bioreactor shows excellent storage (at 4 degrees C) and operational stabilities (at 37 degrees C).

Determination of penicillin in pharmaceutical formulations by flow injection analysis using an optimised immobilised penicillinase reactor and iodometric detection

An automated assay for the determination of penicillin in formulations suitable for use in pharmaceutical quality control is presented. The method is based on the classical iodometric penicillin assay which is incorporated in a flow injection analysis (FIA) system. The required hydrolysis is performed on-line by using an immobilised penicillinase reactor. Packed-bed and single-bead-string enzyme reactors are compared. It turns out that a packed-bed penicillinase reactor (10 cm x 1.5 mm i.d.) provides complete hydrolysis within short residence time, while only little back-pressure is generated. This enzyme reactor is stable for at least 9 months. Enzymatic hydrolysis of the beta-lactam ring results in the formation of the corresponding penicilloic acid, which consumes iodine. The iodine consumption is determined colorimetrically by measuring the decrease of the absorbance of the blue coloured iodine/starch complex. The optimum reactor length and flow rate for the colourimetrical detection reaction are determined. The optimised method is applied to the assay of penicillin in formulations and the results are compared with the "true" results obtained with a reference method: a mercurimetric titration. The reliability of the flow injection method is evaluated quantitatively by determining the maximum total error (MTE). The reliability is shown to be highest when measuring at a 0.3-mM level. Eight formulations including capsules, tablets and injectables containing penicillin G, amoxicillin or flucloxacillin are assayed. The MTE does not exceed the 6% level and the most probable MTE is between 1.5 and 3.5%.

Immobilization of glucose oxidase and peroxidase and their application in flow-injection analysis for glucose in serum

Glucose oxidase (GOD) and Horseradish peroxidase (HRP) were covalently coupled to alkylamine controlled pore glass by means of glutaraldehyde. About 700-800 U/g of immobilized GOD and 300-400 U/g of immobilized HRP were obtained. Some factors of affecting enzyme immobilization were discussed. The immobilized enzymes were packed into a plastic tube and used in flow-injection analysis (FIA) for glucose in serum. A good linearity range was observed for this immobilized enzyme system at 20 mg/mL to 1000 mg/dL D-glucose, the recovery was 95.4-103.5%, the within-batch imprecision was 0.8-2.2%, and the between-batch imprecision was 2.2-4.2%. More than 100 samples were measured within an hour. One enzyme column with five units of immobilized GOD and HRP, applied for 50 assays/d, has been used for more than 2 mo.

Liquid chromatographic separation and stereoselective detection of L- and D-amino acids with catalytic reaction detection using immobilized enzymes

A post-column LC detection system is described for the stereoselective detection of L- and D-amino acids. The effluent of the LC column passes an immobilized enzyme reactor (IMER) containing either L- or D-amino acid oxidase. The hydrogen peroxide formed in this reactor by the oxidation of the amino acids is then transported to a second IMER containing horse-radish peroxidase. With the addition of 4-aminophenazone and dichlorophenosulphonyl chloride to the carrier, the hydrogen peroxide is reacted to form a red-coloured complex which is detected in a flow-through photometric cell at 514 nm. Applications to the analysis of amino acids in bovine and human sera are described.

Immobilization of linamarase and its use in the determination of bound cyanide in cassava using flow injection analysis

Extracts from the tubers (cortex and parenchyma) and leaves of Manihot esculenta Crantz (cassava) were analyzed for their releasable cyanide content using flow injection analysis incorporating an immobilized linamarase bioreactor. Linamarase was immobilized under very mild conditions to an activated 2-fluoro-N-methylpyridinium Fractogel support. The released cyanide, which was monitored spectrophotometrically at 525 nm using an alkaline picrate reagent, was found to be highest in the cortex and lowest in the parenchyma.

Determination of acetylcholine and choline in microdialysates from spinal cord of rat using liquid chromatography with electrochemical detection

A method for the determination of acetylcholine and choline in microdialysates from rat spinal cord has been developed using liquid chromatography, enzyme-catalysed post-column derivatization and electrochemical detection. The sensitivity required was achieved by a certain degree of miniaturization, careful temperature control and a slight modification of the amperometric detector. Determinations were performed on 5-microliter samples.

High-performance liquid chromatographic separation of some mono- and disaccharides with detection by a post-column enzyme reactor and a chemically modified electrode

The effluent from a chromatographic column was mixed with nicotinamide adenine dinucleotide coenzyme (NAD+) buffer and passed through a packed-bed reactor containing immobilized glucose dehydrogenase. Oxidation of the carbohydrates emerging from the column produced an equivalent amount of reduced coenzyme (NADH), which was detected electrochemically using an electrode modified with 7-dimethylamino-1,2-benzophenoxazine (Meldola Blue). Separation was effected in three different chromatographic systems containing a protonated ion exchanger, a calcium(II)-saturated or a lead(II)-saturated ligand exchange column. Separation, detection and k' values are reported for glucose, 2-deoxyglucose, xylose, mannose, cellobiose, lactose, ribose and glucosamine. The detection limit was 2 ng for a 20-microliter injection of glucose and the response was linear up to 6300 ng. Samples from fermentation of penicillin were analysed for lactose and glucose with the described detector. A comparison with the recordings from a refractive index detector showed that the selectivity of the enzymes and the modified electrodes are necessary for the determination of glucose and lactose.