ENZYME SENSOR

Abstract: An enzyme sensor comprising an enzyme acting specifical­ ly on substrate and a transducer for converting the quantitative change of a substance or heat which is produced or consumed during an enzyme reaction to an electrical signal, wherein the enzyme is glucokinase is disclosed. By the enzyme sensor of the present invention, an accurate determination of an amount of glucose in a sample is possible as well as an accurate determina­ tion of adenosine-5′-triphosphate (ATP). A response time of the sensor is almost constant after a long-term use and a decrease of the detecting ratio is very small.

Description: The present invention relates to an enzyme sensor used for determining the concentration of glucose or adenosine-5'-triphosphate (ATP) in a sample. An enzyme sensor, which can quickly and easily determine the concentration of a component to be determined, generally comprises an enzyme for selectively identifying a specific chemical substance (such as a molecule and an ion), and a transducer for measuring a quantitative change in a substance or heat which is produced or consumed during the enzyme reaction. A transducer which employs an electrochemical device, such as a gas electrode or an ion selective electrode, is mostly used in the fields of clinical chemical examination and the food industry. According to the requirements of miniaturization and integration of enzyme sensors in various fields, a transducer using a semiconductor device, especially an ion sensitive field effect transistor (ISFET), becomes noteworthy. Protein, Nucleic Acid and Enzyme, Vol 30, No. 4, (p.245-p.298) at p.246 and p.285 proposes an enzyme sensor employing (as a transducer) a thermistor for measuring heat. Most studies in the field of enzyme sensors have related to the development of an enzyme sensor for determining glucose. One proposal has been to use glucose oxidase as the enzyme in a sensor which employs an electrochemical device as the transducer. This type of sensor utilizes a reaction system in which glucose is converted to hydrogen peroxide and gluconic lactone and consumes oxygen due to the action of glucose oxidase. Enzyme sensors using this reaction can be classified into two types. One is an enzyme sensor which detects the amount of consumed oxygen (which is in proportion to the concentration of glucose in a sample) with an oxygen electrode. The other is an enzyme sensor which detects the amount of produced hydrogen peroxide (which is also in proportion to the concentration of glucose in a sample) with a hydrogen peroxide electrode. It is also known that the amount of oxygen or hydrogen peroxide can be determined using an ISFET. In addition to the abovementioned reaction, it is also possible to use a reaction system in which glucose is reacted with adenosine-5'-triphosphate (ATP) in the presence of hexokinase to produce glucose-6-phosphate and adenosine-5'-diphosphate (ADP). The enzyme sensor utilizing this reaction system employs as a transducer a heat-measurement device determining the amounts of heat produced during the reaction (see Analytical Chemistry, Vol. 47, No. 6, p.786 to p.790). One enzyme sensor for determining adenosine-5'-triphosphate (ATP) employs glucose oxidase and hexokinase as enzymes (EP-A-125136 corresponding to Japanese Patent Publication (unexamined) No. 17347/1985). According to this sensor, the reaction of glucose oxidase and glucose co-exists with the reaction of hexokinase and glucose. The ratio of the two reactions is dependent on the amount of adenosine-5'-triphosphate (ATP) present. Another enzyme sensor for determining adenosine-5'-triphosphate (ATP) employs adenosine-5'-triphosphate hydrolase (AT Pase) with which adenosine-5'-triphosphate (ATP) is hydrolysed to produce hydrogen ions (see the Summary of Lectures, p.604 (1985), the 50th Annual Meeting in spring of the Chemical Society of Japan; the Summary of Lectures F-4, p.7, the 52nd Annual Meeting of Electrochemical Society of Japan). In this sensor, the pH change is measured by an ISFET. Of the abovementioned sensors, those which employ glucose oxidase have a long response time because of the lack of dissolved oxygen in solution. In the sensor employing an oxygen electrode, hydrogen peroxide decomposes during the measurement to oxygen and hydrogen oxide. The decomposed oxygen is also detected by the oxygen electrode resulting in a measurement error. In the sensor using a hydrogen peroxide electrode, reductive materials in the sample introduce errors into the measurement. In the case of the sensor employing hexokinase, the hexokinase can be effective not only on glucose but also on mannose and fructose which causes measurement errors. The sensors mentioned above have a long response time if they are kept without being used for a long time, and become poor at detecting the ratio. Further, if they are repeatedly used, their accuracy is reduced. In the sensor using adenosine-5'-triphosphate hydrolase (AT Pase), the activity of adenosine-5'-triphosphate hydrolase (AT Pase) falls in a short time and the detecting range is very small. An object of the present invention is to provide an enzyme sensor which selectively determines the amount of glucose or adenosine-5'-triphosphate (ATP) and endures long-term use without loss of accuracy. It has now been found that an enzyme sensor, in which glucokinase is used as an enzyme, does not increase in response time with long-term use and does not lower a detecting ratio. According to the present invention there is provided an enzyme sensor comprising an enzyme acting on a specific substrate and a transducer for converting the quantitative change of a substance or heat which is produced or consumed during an enzyme reaction to an electrical signal, wherein the enzyme is glucokinase. According to the enzyme sensor of the present invention, an accurate determination of the amount of ATP in a sample is possible as well as an accurate determination of glucose. The response time of the sensor is almost constant even after long-term use and the decrease in the detecting ratio is very small. The invention will be further described by way of example only with reference to the accompanying drawings, in which: Fig. 1 shows a circuit drawing comprising an ion sensitive field effect transistor (ISFET) 1 on which glucokinase is immobilized, an ion sensitive field effect transistor (ISFET) 2 on which glucokinase is not immobilized and an Ag/AgCl electrode 3. Fig. 2 is a drawing showing a relation between a concentration of glucose and a differential output. Fig. 3 is a drawing showing a relation between a concentration of adenosine-5'-triphosphate (ATP) and a differential output. The transducer employed in the present invention may be an electrochemical device, a semiconductor device, or a heat-measurement device. Preferably the transducer is a semiconductor device such as an ion sensitive field effect transistor (ISFET), which can be miniaturized and easily integrated. The glucokinase which is employed in the present invention may be obtained from various sources, e.g. it may be derived from microorganisms or from animals. The preferred glucokinase for use in the invention is produced from microorganisms cultivated at a temperature in the range of 50 to 85°C. Examples of the microorganisms are Bacillus sp. such as Bacillus stearothermophilus, Bacillus thermoproteolyticus, Bacillus acidocaldarius; Thermoactinomyces sp.; Thermus sp.; and Thermomicrobium sp. Typical examples of the microorganisms are Bacillus stearothermophilus, of which specific examples are ATCC 7933 strain (ATCC; The American Type Culture Collection, Maryland, U.S.A.), ATCC 7954 strain, ATCC 10194 strain, ATCC 12980 strain, NCA 1503 strain (NCA; National Canners' Association, Washington, D.C., U.S.A.), and UK 563 strain (PERM P-7275 strain, deposited at Fermentation Research Institute, Agency of Industrial Science and Technology, Ibaragi, Japan, on September 29, 1983). The glucokinase used in the present invention is generally immobilized on a water-insoluble carrier such as a membrane, a particle, a fiber, a hollow fiber, or a tube. The water-insoluble carrier on which glucokinase is immobilized may be placed in a reaction vessel, e.g. stirred vessel, a packed vessel, a flow type vessel or a tubular vessel, and incorporated into a flow system having an output flow which is led to a sensitive surface of the transducer. In a preferred embodiment, the water-insoluble carrier is in the form of a membrane on which the glucokinase is immobilized. The membrane may directly cover the sensitive surface of an electrode which provides the transducer. This latter process (in which the sensitive surface of the electrode is covered by a glucokinase membrane) is preferred from the point of view of miniaturization and integration of the enzyme sensor. The thickness of the glucokinase memebrane is, for example, 1 to 100 micron, preferably 10 to 50 micron. The immobilization of glucokinase to the water-insoluble carrier can be carried out by a covalent bond method or an absorbed method, which are conventionally known as described in "Immobilized Enzyme" by Ichiro, CHIBATA, Kodan-sha (1975). It can also be carried out by a crosslinking method or an entrapped method. The covalent bond method includes: a peptide bond method, in which CNBr-activated agarose or dextran is bonded with an amino group of glucokinase; a diazo method, in which a water-insoluble carrier having an aromatic amino group is changed by a nitrite to a diazonium salt to which tyrosine residues of glucokinase are coupled; a Schiff base method, in which a water-insoluble carrier having an amino group is bonded with glutaraldehyde and then to which an amino group of glucokinase is bonded; and a method in which a porous glass, silica, or metal oxide is aminosilanized by gamma-aminopropyltriethoxysilane and treated with glutaraldehyde and then bonded with an amino group of glucokinase. The absorbed method includes a method in which glucokinase is immobilized on a water-insoluble carrier, such as DEAE-cellulose or phenoxyacetylcellulose, with an ion bond or a physical force. The crosslinking method includes a method in which glucokinase and an amino group of albumin are crosslinked by glutaraldehyde to immobilize glucokinase on albumin. The entrapped method includes a method, in which a solution containing acrylamide monomers, a crosslinking agent (e.g. N,N'-methylenebisacrylamide), an initiator (e.g. riboflavin and peroxodisulfate), and a polymerization promoter (e.g. N,N,N',N'-tetramethylethylenediamine) is added to a glucokinase solution and polymerized with light under a nitrogen blanket; a method in which glucokinase is added to a collagenfibril suspension and dried after pouring it on a Teflon board; and a method in which two platinum electrodes are inserted in a suspension containing collagen and glucokinase and a direct current voltage is applied to form a collagen membrane containing glucokinase on the cathode. For determining an amount of glucose using the enzyme sensor of the present invention, a buffer solution containing a magnesium salt and adenosine-5'-triphosphate (ATP) may be passed through a reaction vessel containing the immobilized glucokinase. An output solution from the vessel may then be detected by the transducer. In this case, a change in the substance being detected (which is produced by adding a glucose solution) is determined by the enzyme sensor. Alternatively, the enzyme sensor may comprise a transducer having a sensitive surface covered by immobilized glucokinase. Such a sensor may be immersed in a buffer solution containing a magnesium salt and adenosine-5'-triphosphate (ATP) and a change in the substance being detected (which is produced by adding a glucose solution) may be determined by the enzyme sensor. In this case, a pH change produced by the following reaction was detected by a pH electrode or an ion sensitive field effect transistor (ISFET); Also, the change of heat may be detected by a thermistor. It is preferred that the pH change (due to H⁺) is detected by a pH electrode or an ion sensitive field effect transistor (ISFET). The preferred buffer solution for the determination contains 0.1 to 20 mM, preferably 0.3 to 5 mM of adenosine-5'-triphosphate (ATP), and 0.5 to 50 mM, preferably 2 to 30 mM of a magnesium salt. The buffer solution is prepared by dissolving adenosine-5'-triphosphate (ATP) and the magnesium salt in a buffer solution (pH 4 to 10, preferably pH 5.5 to 9.5), such as Tris-hydrochloric acid or imidazole acetic acid. Determination of adenosine-5'-triphosphate (ATP) is carried out by employing glucose instead of the ATP in the buffer solution. In this case, the amount of glucose is 0.1 to 30 mM, preferably 0.8 to 15 mM in the buffer solution. The temperature for the determination is preferably within a range of 5 to 75°C, preferably 15 to 55 °C. EXAMPLES The present invention is illustrated by the following non-limiting Examples. Example 1 and Comparative Examples 1 and 2 20 µℓ (23 units) of glucokinase derived from Bacillus stearothermophilus were mixed with 2 µℓ of 25 W/V % bovin serum albumin. 2 µℓ of the mixture obtained was added dropwise on a gate of an ion sensitive field effect transistor (ISFET) and air-dried at room temperature for 20 mimutes. 2 µℓ of 1 W/V-% glutaraldehyde was added dropwise on the gate and reacted at 4 °C for a whole day and night to form an immobilized membrane. This was immersed in a 0.1 M glycine-sodium hydroxide buffer solution at pH 8.5 for 15 minutes and rinsed with distilled water to obtain an ion sensitive field effect transistor (ISFET) on which glucokinase was immobilized. The glucokinase immobilized ion sensitive field effect transistor (ISFET) 1 and a non-immobilized ion sensitive field effect transistor (ISFET) 2 for control were immersed in a 25 ml of a reaction solution composed of 20 mM of tris-hydrochloric acid buffer solution (pH9.0), 20 mM of magnesium chloride and 4 mM of adenosine-5'-triphosphate (ATP). An Ag/AgCl electrode was immersed in the reaction solution for fixing a voltage of the solution. The ion sensitive field effect transistors (ISFET) had the circuit shown in Fig. 1, wherein a voltage between source drains of the two ISFETs set at 3.0 and an applied voltage of the Ag/AgCl electrode was 6.5 V. When a glucose solution was added to the reaction solution at a boundary surface of the ISFET, the pH partially changed. The change was detected as a differential output between the two ISFETs. The temperature for the reaction was 30 °C and stirring of the reaction solution was conducted at 200 rpm. The concentration of the glucose solution was adjusted to 2, 5, 10, 30, 100 and 200 mg/dl and the relation between the glucose concentration and the differential output is shown in Fig. 2. As is apparent from Fig. 2, the enzyme sensor exhibits good response properties in a concentration range of 2 to 200 mg/dl. It has been found that the enzyme sensor of the present invention can effectively determine an amount of glucose. Next, for testing stability of the enzyme sensor of the present invention in the long term the determination was carried out using a glucose solution of 10 mg/dl after leaving for one month, 3 months and 6 months at 37°C. The response time and glucose detecting ratio are shown in Table 1. For a comparison, the same test was made by employing hexokinase derived from yeast instead of glucokinase derived from Bacillus stearothermophillus . The result is shown in Table 1 as Comparative Example 1. Another comparison was made by using a commercially available enzyme sensor in which glucose oxidase was immobilized on a hydrogen peroxide electrode. The same test was carried out. The result is shown in Table 1 as Comparative Example 2. Example Comparative Example 1 2 Response time (min) Detecting ratio (%) Response time (min) Detecting Ratio (%) Response time (min) Detecting ratio (%) Immediate after preparation or purchase 2.0 100 3.0 100 2.5 100 After one month 2.0 100 25 30 12 50 After 3 months 2.3 98 60 5 30 10 After 6 months 2.4 97 - - - - It is clear from Table 1 that the enzyme sensor of the present invention is very stable in comparison with other sensors. For testing the stability of the sensor of the present invention after repeated and continuous use, the same test was carried out by using a serum sample containing a glucose concentration of 50 mg/dl after using the sensor 50 times/day for 10, 20 and 30 days. The result is shown in Table 2. For a comparison, the sensors used for the Comparative Examples were used and the same test was conducted. The result is shown in Table 2. Example Comparative Example 1 2 Response time (min) Detecting ratio (%) Response time (min) Detecting Ratio (%) Response time (min) Detecting ratio (%) Immediate after preparation or purchase 2.0 100 3.0 100 2.5 100 After 10 days 2.2 99 10 70 7 80 After 20 days 2.4 98 30 30 19 30 After 30 days 2.5 97 60 5 32 8 It is clear from Table 2 that the enzyme sensor of the present invention does not deteriorate in quality in comparison with the other sensors. Example 2 A relation between differential outputs and concentrations was obtained as generally described in Example 1 with the exception that 12 mM of glucose was employed instead of 4 mM of adenosine-5'-triphosphate (ATP) and the concentration of the ATP solution adjusted to 50, 100, 200, 300 and 500 mg/dl was employed instead of the glucose solution. The result is shown in Fig. 3. As is apparent from Fig. 3, the sensor of the present invention has good response properties in a range of 50 to 500 mg/dl and it has been found that the sensor of the present invention can determine adenosine-5'-triphosphate (ATP).

Claims: An enzyme sensor comprising an enzyme acting on a specific substrate and a transducer for converting either the quantitative change of a substance, or heat which is produced or consumed during an enzyme reaction, to an electrical signal, wherein said enzyme is glucokinase. The enzyme sensor according to Claim 1 wherein said substrate is glucose. The enzyme sensor according to Claim 1 wherein said substrate is adenosine-5'-triphosphate (ATP). The enzyme sensor according to Claim 1 wherein said substance produced during the enzyme reaction is a hydrogen ion. The enzyme sensor according to Claim 1 wherein glucokinase is derived from microorganisms cultured in a temperature range of 50 to 85° C. The enzyme sensor according to Claim 1 wherein the transducer is an electrode and the glucokinase is immobilised on a membrane which covers a sensitive surface of the electrode.

Inventors: IIDA, TAKEAKI, KAWABE, TAKESHI
Applicants: UNITIKA LTD.
Application year: 1987
Main classification: C12M1
IPC codes: G01N27, G01N25, C12Q1