Enzyme Thermistor Assay of Cholesterol, Glucose, Lactose and Uric Acid in Standard Solutions as Well as In Biological Samples

An enzyme thermistor-based assay for total and free cholesterol

A method to evaluate the free (FC) and total cholesterol (TC) in human serum, bile and gallstone extract using an enzyme thermistor (ET)-based flow injection analysis (FIA) is presented. The cholesterol in high-density (HDL-C) and low density lipoprotein (LDL-C) have also been evaluated. A heparin functionalized Sepharose column was employed for the isolation of HDL and LDL fractions from serum. The estimation of cholesterol and its esters was based on their reaction with cholesterol oxidase (CO), cholesterol esterase (CE) and catalase (CAT). Three different enzyme columns, i.e. co-immobilized CO/CAT (column A), only CE (column B) and co-immobilized CO/CE/CAT (column C) were prepared by cross-linking the enzymes on glass beads using glutaraldehyde. Column A was used for estimating FC and column C was used for estimating total cholesterol (cholesterol plus esterified cholesterol). Column B was used as a pre-column which could be switched 'in' or 'out' in conjunction with column A for the estimation of TC or FC, respectively. A calibration between 1.0 and 8.0 mmol/l for FC and 0. 25 and 4.0 mmol/l for TC was obtained. For more than 2000 assays with the ET device a C.V. of less than 4% was obtained. The assay time was approximately 4 min per assay. The cholesterol estimations on the ET correlated well with similar estimations using a commercially available cholesterol diagnostic kit.

Continuous beds. Their applicability for immobilization of proteins

An epoxy-activated continuous bed can be prepared for immobilization of proteins in a simple, rapid, and cost-effective way. The concentration of epoxy groups on the continuous bed was as high as 600 mumol/mL compressed bed (compression of the bed decreases the peak broadening). Human transferrin, human serum albumin and particularly urease were employed as model proteins. The immobilization of urease was virtually completed within 1 h in 1 M potassium phosphate, pH 7.4. The binding capacity was 97 mg of urease/mL compressed bed. This bed is of clinical interest, since it is inexpensive to prepare and permits reproducible enzymatic determination of urea in serum and urine (the chromatographic step is finished within 1-2 min).

Amperometric assays of total and free cholesterols in serum by the combined use of immobilized cholesterol esterase and cholesterol oxidase reactors and peroxidase electrode in a flow injection system

A flow injection system for assays of total cholesterol and free cholesterol was described. The total cholesterol assay system included an amperometric peroxidase electrode to measure hexacyanoferrate(III) converted from hydrogen peroxide, which was generated by injecting a 2-microliter sample into the packed-bed reactors of immobilized cholesterol esterase and cholesterol oxidase covalently bound to silica. The free cholesterol was assayed with the same system without the cholesterol esterase reactor. The peak current was linearly related to cholesterol in the range 2-160 mg/dl and to total cholesterol in the range 3-300 mg/dl; the assay speed was about 80 samples/h for free cholesterol and 40 samples/h for total cholesterol. Reliable results were obtained in the assays of free cholesterol and total cholesterol in human sera. Both the reactors and the peroxidase electrode retained over 90% of their original activities, even after repetitive use for 4 and 2 months, respectively.

Simple routine assay for serum urea using immobilized urease

The method described is a simple routine assay suited for a short series of serum samples. The time needed for one assay is 2 to 3 min from a stand-by arrangement. The urease is immobilized on controlled pore glass. The beads are placed in the column of an enzyme thermistor unit that is part of a continuous flow system. The heat of reaction when urea is degraded to ammonia and carbon dioxide by immobilized urease is measured and recorded continuously. The technique was investigated as regards to flow dependence, linearity, recovery, precision and some possible interfering substances. The within day precision was 0.8% (C.V.) and the day to day precision, during 56 days, was 3.0% (C.V.). Furthermore, the coefficient of correlation between results obtained with the enzyme thermistor unit and a conventional spectrophotometric method was 0.991.

Use of an enzyme thermistor in continuous measurements and enzyme reactor control

The enzyme thermistor measures the heat produced by the action of an immobilized enzyme on a substrate present in the sample. Its application in analysis of discrete samples, e.g., in clinical chemistry, is well documented, but it has not been used so far for continuous measurements. We decribe here the application of the enzyme thermistor for continuous monitoring and control of enzyme reactors. An enzyme thermistor filled with coimmobilized glucose oxidase and catalase was used to measure the amount of glucose in the outflow from a column reactor containing immobilized lactase acting on a lactose solution pumped through the reactor. The lactose conversion was kept on a constant level, irrespective of the actual enzymatic activity in the reactor, by regulating the flow through the reactor. The experiments were carried out with aqueous solutions of lactose as well as with whey from cow's milk.

Thermometric enzyme linked immunosorbent assay in continuous flow system: optimization and evaluation using human serum albumin as a model system

Thermometric enzyme-linked immunosorbent assay (TELISA) is described. After the procedure of optimization, human serum albumin was assayed using anti-human serum albumin bound to Sepharose CL 4-B in the enzyme thermistor unit and catalase as label on the free antigen. The model system was used for assays down to 10(-13)M and the preparation of immobilized antibodies was used repeatedly up to 100 times. Comparative studies of the TELISA technique with bromocresol green, immunoturbidimetric and rocket immunoelectrophoretic methods were carried out and showed that TELISA could be used as an alternative method.

Enzyme thermistor determination of glucose in serum using immobilized glucose oxidase

An enzyme thermistor assay for serum glucose is described. The glucose present in the sample is reacted in a small column containing glucose oxidase immobilized to controlled pore glass (single thermistor device). The heat produced in the primary reaction is measured directly in the column without any need for coupling reactions. The useful linear range is 0.01-0.45 mM glucose, permitting 50-fold dilution of serum samples. Advantages are low enzyme cost, due to the immobilization, insensitivity for the color or any turbidity of the sample, and no requirement for coenzyme or any ancillary reaction. Improved sensitivity and extended linear range (0.01--0.9 mM) can be attained through a secondary reaction using catalase. The application to glucose analysis of a split-flow enzyme thermistor equipped with a reference column to eliminate unspecific heat effects is also described. The enzyme thermistor determinations were also compared with a spectrophotometric continuous flow technique using a small column with immobilized glucose oxidase and 4-aminoantipyrine and phenol as color reagents.

Application of cyanide-metabolizing enzymes to environmental control; enzyme thermistor assay of cyanide using immobilized rhodanese and injectase

The application of the enzyme thermistor in the analysis of cyanide in standard solutions as well as in blast furnace waste water is described. The heat signal is generated in the conversion of cyanide, catalyzed by the immobilized enzymes rhodanese (E.C. 2.8.1.1) and injectase (E.C. 4.4.19). Using the combination of cyanide-metabolizing enzymes and the enzyme thermistor unit, assays down to 20 microM cyanide can be carried out. Linear relationships were obtained at 20-600 microM cyanide for injectase and 20-1000 microM for rhodanese. The stability at 27 degrees C of the heat response was initially decreased, but soon stabilized at about 80% of the initial value and remained so for at least 200 hr. The technique was easily adapted to continuous analysis, applicable to environmental control (e.g., a "cyanide guard") with a response time at present within 2-3 min after a sudden change in cyanide concentration has appeared.

Thermometric enzyme linked immunosorbent assay: TELISA

A new method, thermometric enzyme linked immunosorbent assay (TELISA), for the assay of endogenous and exogenous compounds in biological fluids is described. It is based on the previously described enzyme linked immunosorbent assay technique, ELISA, but utilizes enzymic heat formation which is measured in an enzyme thermistor unit. In the model system studied determination of human serum albumin down to a concentration of 10(-10) M (5 ng/ml) was achieved, with both normal and catalase labelled human serum albumin competing for the binding sites on the immunosorbent, which was rabbit antihuman serum albumin immobilized onto Sepharose CL-4B.

Experiments and calculations concerning a thermal enzyme probe

A simple device capable of measuring almost any reactant in an enzyme-catalyzed reaction is created when an enzyme is immobilized onto one thermal sensor of a differential thermometer. Experiments are described in which two thermistors, one bare and one coated with immobilized enzyme, are immersed in a well-stirred solution. The response of this device to increases in glucose-ATP concentration was observed using hexokinase (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1), and to increases in glucose concentration using glucose oxidase (beta-D-glucose:oxygen 1-oxidoreductase, EC 1.1.3.4). A simple model is presented whose predictions are in reasonable agreement with the experimental results.