Fluorescence immunoassay method utilizing pseudo-antigens combined with fluorescent quenchers

Abstract: A fluorescence immunoassay method is described in which the presence or absence of antigen is measured within one minute by measuring a variation in fluorescence emitted from an antibody. The method comprises reaction between a pseudo-antigen obtained by chemical combination of an antigen and a fluorescent quencher and an antibody by which fluorescence to be emitted from the antibody is quenched by the action of the quencher combined. When an analyte is added to a solution of the combination, the pseudo-antigen is replaced by an antigen if the analyte contains the antigen, so that the fluorescence increases in intesity. This intensity is measured and compared with the initially measured intensity to detect the presence of the antigen. The method does not make use of the action of antigen on antibody with respect to the fluorescence of the antibody and is applicable to all antigens.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to the art of immunological detection and more particularly, a method for the immunological detection wherein there are utilized antibodies capable of binding with analytes or substances to be detected. Description of the Prior Art Immunological detection utilizing naturally occurring or artificially prepared antibodies has merits in high selectivity and high sensitivity to analytes or objects to be detected and thus been currently employed for the purpose of detecting analytes present in very small amounts. This detection technique has been applied, for example, to clinical examinations such as of so-called disease markers which are specifically secreted when one suffers diseases accompanies by tumors, cardiac infraction, cerebral thrombosis and the like, or also to detection of substances contained in air in very small amounts. For the detection of these markers or substances, it is usual that the sensitivity for determination of the presence or absence of bound antibody is improved by the use of radioactive isotopes or enzyme reactions. However, the use of radioactive isotope is limited to a laboratory scale test because of the danger involved therein. A number of the detection techniques utilizing the enzyme reactions, i.e. enzyme immunoassays (hereinafter sometimes referred to as EIA), have been proposed using various combinations of the reactions (R.M. Nakamura, A. Voller and D.E. Bidwell, Enzyme Immunoassays: Heterogeneous and Homogeneous Systems in Handbook of Experimental Immunology 4th Edition, Vol. 1 and Immunochemistry, Chapter 27, edited by D. M. Weir, Blackwell Scientific Publications, Oxford, 1986). These EIA methods can be broadly divided into two classes, one being directed to high molecular weight substances or proteins as analyte and the other dealing with low molecular weight analytes, e.g. haptens. Analytes to which the present invention is directed are mainly haptens. Accordingly, an enzyme linked immunosolvent assay (ELISA) which is a typical technique for haptens is illustrated with respect to experimental procedures. The ELISA technique is generally carried out in the following procedure. (A) A conjugate of a carrier protein such as, for example, bovine serum alubmin (BSA) analyte or its derivative introduced with a functional group is dissolved in a buffer to form an antigen solution. The antigen solution is added to a microplate (e.g. a polyvinyl chloride or polyethylene 96 well plate) in an amount of 1⊘⊘ µ L/well and incubated at 2⊘°C overnight. (B) Blocking A phosphate buffered solution of BSA having a pH of 7.5 is added in an amount of 25⊘ µ L/well, followed by allowing to stand at room temperature for ⊘.5 to 2 hours and washing with a buffer or pure water 3 to 5 times. (C) Antibody Reaction A solution of an analyte is added to the well plate, to which an antibody solution is further added while shaking. After keeping it at normal temperatures for 3 to 5 hours, the antibody solution is removed by means of an aspirator, followed by washing with a buffer or pure water three to five times. (D) Second Antibody Reaction A solution of an anti-mouse IgG antibody against an antibody labelled with an enzyme such as, for example, peroxidase is added to the washed mixture and allowed to stand at normal temperatures for ⊘.5 to 2 hours, followed by washing with a buffer or pure water three to five times. (E) Reaction of Substrate and Its Termination A color developer such as, for example, o-phenylenediamine is dissolved in a buffer to obtain a solution. Immediately before its application, the solution added with 3⊘% hydrogen peroxide is further added as a substrate solution, followed by color developing reaction at room temperature. Five to twenty minutes after the addition, the reaction is terminated with sulfuric acid. (F) Measurement An absorbance at 492 nm is measured by the use of a spectrophotometer for microplate. The absorbance becomes weaker for a larger amount of the analyte, from which the analyte can be quantitatively determined. As will be apparent from the above, the ELISA technique has the advantage in treating a number of specimens at the same time, but requires the complicated procedure, which in turn requires the skill on the part of a tester. In addition, this technique is difficult to automate, coupled with the further problem that it takes a very long examination time of not shorter than 5 hours. These problems are true of the other EIA techniques set out in the literature indicated above. The reason why these EIA techniques take a long time and much labor is considered as follows. In most cases, these techniques make use of a procedure of separating antigen-bound antibody and free antibody from each other (B-F separation). This procedure involves either a heterogeneous reaction where solid and liquid phases coexist or a homogeneous reaction in solution where proteins including antibodies react with each other. In either case, the reaction speed is very slow and thus entails an increase of the detection time. However, assays have been developed which avoid the requirement of separating antigen-bound antibody and free antibody for antigen detection. The US-A-4 277 437 discloses a competitive protein binding method for the determination of an analyte, which method is based on the principle of using a chemiluminescence source quencher system, one part of which system being linked to the antigen and the other part being linked to the antibody or visa versa. The chemiluminescence output of the system is changed in the competitive presence of the analyte in the sample. An analoguous system, wherein however a fluorescer is used instead of a chemiluminescer as a luminescence source in the mixture of the competitive binding assay, is disclosed in the US-A-4 261 968. However, these known competitive binding immunoassays are still not satisfactory with respect to desired ease and speed of the immunoassay. SUMMARY OF THE INVENTION It is accordingly an object of the invention to provide an immunological method which ensures safe, simple and high speed detection of an antigen to be detected. It is another object of the invention to provide a fluorescence immunological method which is simple in procedure and is easy to automate. It is a further object of the invention to provide a fluorescence immunological method whereby an antigen can be detected within a very short time of, for example, not longer than 1 minute since any B-F separation as will be required in prior art procedures is not necessary. The above objects can be achieved, according to the invention, by a fluorescent immunoassay method which comprises: providing a solution of a pseudo-antigen which is obtained by chemical combination between a fluorescence quencher having the action or ability of quenching fluorescence emitted from an antibody and an antigen; adding the solution of the pseudo-antigen to a solution of an antibody capable of emitting fluorescence by irradiation of light, thereby permitting the antigen of the pseudo-antigen and the antibody to be combined; irradiating the solution with the light and measuring an intensity of the fluorescence emitted from the antibody by the action of amino acids of the antibody including tryptophane ; and adding an analyte to the solution and irradiating the solution with the light to measure a variation in the intensity of the fluorescence, from which the presence or absence of an antigen in the analyte is detected. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a graph showing the relationship between the time and the variation in fluorescence which is determined by the fluorescence quench inhibition immunoassay; and Fig. 2 is a graph showing the relationship between the concentration of methamphetamine used as an antigen and the rate of fluorescence quench inhibition which is determined by the immunoassay as used in Fig. 1. DETAILED DESCRIPTION AND EMBODIMENTS OF THE INVENTION The present invention is based on the principle that a system is provided wherein when an antibody is bound with an antigen, the fluorescence intensity of the antibody is varied, and the presence or absence of the antigen is detected based on the variation of the fluorescence intensity. In this system, the separation between bound antibody and free antibody is not necessary, with a success in remarkable shortage of the detection time. In the method of the invention, a pseudo-antigen is first provided as a solution in a buffer such as a phosphate buffer, a tris buffer, a citrate buffer and the like. This pseudo-antigen is prepared by chemical linking or binding of an antigen and a fluorescence quencher. The solution of the pseudo-antigen is added to a solution of antibody whereupon the pseudo-antigen binds with the antibody. In general, when an antibody is excited at approximately 28⊘ nm, fluorescence in the vicinity of 34⊘ nm is emitted therefrom. Accordingly, the irradiation of the solution of the antibody bound with the pseudo-antigen with light having a wavelength of about 28⊘ nm results in emission of the fluorescence. Since the pseudo-antigen has the quencher moiety, the fluorescence emitted from the bound antibody is more suppressed than that of the free antibody. In this state, an analyte is added to the solution. If the analyte contains an antigen, this antigen preferentially combines with the antibody by substitution with the pseudo-antigen. Accordingly, once decreased fluorescence increases to an extent corresponding to the content of the antigen in the analyte since the quencher moiety is dissociated. When the increment of the fluorescence determined by the irradiation of the light is measured, the quantitative detection of the antigen becomes possible. As stated above, when an antibody is irradiated and excited with a UV ray having a wavelength of approximately 28⊘ nm, fluorescence is emitted at about 34⊘ nm by the action of amino acids including tryptophane. Although the wavelengths of the excitation light and the emitted light may be increased or decreased by about 3⊘ nm around the above values, respectively. The Raman light of water appearing in the vicinity of the emission light of the antibody becomes a factor of causing noises. In order to minimize the influence of the Raman light, the wavelength used is conveniently about 28⊘ nm for the excitation light and about 34⊘ nm for the emission light. The pseudo-antigen used in the first step of the method according to the invention is obtained by chemical combination between a fluorescence quencher having the action or ability of quenching fluorescence emitted from an antibody and an antigen. Studies have been heretofore made on the influences of aromatic nitro compounds such as nitrobenzene, dinitrobenzene and trinitrobenzene on monoclonal antibodies. These aromatic nitro compounds have the capability of quenching fluorescence of antibodies to an extent. This capability has been utilized for measurement of affinity between antibodies and aromatic nitro compounds or the study of antibody-hapten interactions, see "Handbook of Experimental Immunology, Vol. 1, Blackwell Scientific Publications, 1979, p. 18.3 - 18.6. Accordingly, if these substances having the capability of quenching fluorescence are an object to be detected, it is possible to directly detect them by measurement of a variation in the intensity of the fluorescence. However, most of organic low molecular weight substances such as haptens exhibit no capability of the fluorescence quenching. Therefore, we have developed a fluorescence quenching immunoassay which is effective, irrespective of the kind of antigen, by use of pseudo-antigens which are made by chemically linking an antigen and a quencher substance. When bound to antibodies, these compounds are able to lower the fluorescence emitted from antibody upon irradiation of the UV light as discussed above. Although typical compounds which can be a quencher of such pseudo-antigens are aromatic nitro compounds, the presence invention should not be limited to these compounds only. All compounds may be used in the practice of the invention so far as they have the fluorescence quenching ability. For convenience' sake, use of the pseudo-antigen derived from aromatic nitro compounds is described herein. When the aromatic nitro compounds including nitrobenzene, dinitrobenzene and trinitrobenzene are introduced with functional groups such as a sulfonic acid group, a halogen, an amino group, a carboxyl group, a hydroxyl group, a thiol group, the resultant derivatives allow chemical linking with any antigens. Thus, pseudo-antigens can be conveniently prepared. In practical applications, a possible antigen which is considered to be contained in an analyte may be conveniently used as a counterpart for the aromatic nitro derivatives. Of the above derivatives of the aromatic nitro compounds, 2,4-dinitrofluorobenzene (DNFB) is preferred because it is readily available as a reagent for determination of the arrangement of amino acid and is readily combined with amino groups irrespective of haptens or proteins. If haptens have no functional groups such as amino groups, the fluorescence quencher may be chemically combined with haptens after introduction of functional groups into the hapten. The antigens to be combined with an organic low molecular weight compound having the quenching capability may be any antigens including haptens and proteins. Specific examples useful in immunological treatment according to the method of the invention include amphetamine, methamphetamine, ephedrine and the like. The pseudo-antigens obtained from the organic low molecular weight compounds, particularly aromatic nitro derivatives, and antigens have the function as an antigen which is to be bound with an antibody and the function to quench fluorescence of the antibody. The pseudo-antigen is applied in a subsequent step in the form of a solution in a buffer as set out before. Specific and, in fact, preferable pseudo-antigens are derived from methamphetamine or its derivative and 2,4-dinitrofluorobenzene and represented by the following formula wherein X represents hydrogen or -OCH₃, R1 represents hydrogen or -OH, R2 represents hydrogen or -CH₃, and n is ⊘ or a positive number. The preparation of this pseudo-antigen is particularly described in examples appearing hereinafter. More specific examples of the pseudo-antigen are shown below. Next, the pseudo-antigen is added to a solution of an antibody capable of emitting fluorescence by irradiation of light having a wavelength which is discussed before. By the addition, the pseudo-antigen binds to the antibody. In order to effectively cause a fluorescence quenching inhibition reaction in a subsequent step, the solution should preferably have a concentration of the antibody as small as possible provided that a variation of fluorescence is detectable. The concentration may differ depending upon the type of antibody and is generally in the range of 1⊘⁻⁶ to 1⊘⁻⁹ M. The reaction is completed, for example, within about 3⊘ seconds. After the binding, the light having such a wavelength as defined above is irradiated to measure an intensity of the fluorescence emitted from the antibody. When compared with an intensity of the fluorescence from the antibody solution, it decreases corresponding to a degree of the binding of the pseudo-antigen and the antibody owing to the action of the fluorescence quencher. In case where amphetamine, methamphetamine or ephedrine is used as an antigen for the pseudo-antigen, the antibody used in this step should preferably be a monoclonal antibody capable of binding with amphetamine, methamphetamine or ephedrine. Finally, an analyte or a substance to be detected is added to the solution in amounts not impeding the optical detection. If an antigen is contained in the analyte, this antigen preferentially combines with the antibody by substitution with the combined pseudo-antigen. In other words, the antigen contained in the analyte causes the pseudo-antigen and the antibody to dissociate and reacts with the dissociated antibody. As a result, the fluorescence emitted from the substituted antibody increases when irradiated with the light having a wavelength of about 28⊘ nm. This is because the quencher is removed from the antibody so that the antibody can emit fluorescence at a higher intensity. The measurement of the intensity of fluorescence can reveal the presence or absence of the antigen in the analyte by comparison with the intensity of the fluorescence after the combination between the pseudo-antigen and the antibody. This fluorescence quenching inhibition immunoassay according to the invention does not make use of a fluorescence variation characteristic of antigen relative to antibody and ensures a high speed fluorescence immunological measurement of all antigens. When aromatic nitro derivatives are used, the degree of the fluorescence quenching generally reaches about 2⊘% or over, leading to high sensitivity of the detection. Accordingly, for the preparation of monoclonal antibodies, choice of cell lines for the purpose of directing attention to fluorescence quenching properties is not necessary. This makes it possible to prepare such antibodies with a view to enhancing affinity for the pseudo-antigen. The present invention is described in more detail by way of examples. Example 1 Synthesis of a pseudo-antigen is briefly described. According to a known procedure (Tamaki, Fukuda and Takahashi, Jpn, J., Legal Med., 37(4), 417, 1983), N-(4-aminobutyl)methamphetamine of the following structural formula (hereinafter abbreviated as ABMA) was prepared. The ABMA and 2,4-dinitrofluorobenzene were mixed in equimolar amounts in acetone and agitated for 1 hour, followed by purification by the use of a thin layer chromatography for collection. As a result, a pseudo-antigen, N-(2,4-dinitrophenyl)-N′-(4′-aminobutyl)methamphetamine (DNPABMA), having the following structural formula was obtained. The antibody used in this example was anti-methamphetamine monoclonal antibody (hereinafter abbreviated as MAB1) prepared by us in the following manner. 1) Mice were each immunized with conjugate of N-(4-aminobutyl)amphetamine and key hole limplet hemocyanine (ABAPKLH). 2) Four month after the immunization, spleen cells were extracted from the mice and fused with myeloma cells from another mouse to make hybridoma cells. 3) Hybridoma cells which were able to produce antibody in high affinity for MA were selected and cloned by a known procedure. 4) The selected hydridoma were cultured, and monoclonal antibody was isolated from the culture through protein-A affinity chromatography column. This antibody was developed to impart affinity for methamphetamine (MA) and had an affinity of about 1⊘⁸ but did not exhibit little fluorescence reinforcing action. In other words, the antibody was nothing other than those prepared by the most common procedure. The fluorescence quenching inhibition immunoassay according to the invention using the antibody is described. (A) MAB1 was dissolved in a buffer obtained by passing a phosphate buffer having a pH of 7 through a filter having a mesh size of ⊘.45 µm to make a concentration of 1.9 x 1⊘⁻⁷ M. 36⊘ µ L of this solution was placed in a microcell for fluorescence measurement and subjected to measurement of an intensity of fluorescence using excitation light having a wavelength of 28⊘ nm (bandpass 5 nm), whereupon there was emitted fluorescence having a wavelength of 34⊘ nm (bandpass 1⊘ nm) at an intensity of about 61.5 (FL⊘). (B) 2⊘ µ L of each of buffer solutions of DNPABMA having different concentrations was added to the solution in (A). As a result, quenching took place with a reduction of the fluorescence intensity (FL1). As the concentration of DNPABMA increased, the value of FL1/FL⊘ increased. At the concentration of DNPABMA of 1⊘ -6,5 (final concentration), the value of FL1/FL⊘ reached a saturation of 6⊘%. In order to efficiently conduct the fluorescence quenching inhibition reaction in a subsequent step, the concentration of a pseudo-antigen e.g. DNPABMA is preferably as low as possible within a range where the variation in fluorescence can be detected. In this example, the concentration of DNPABMA used was 1⊘⁻¹¹ M (final concentration). The quenching reaction reached an equilibrium in about 15 seconds. (C) 2⊘ µ L of each of MA buffer solutions having different concentrations was added to the solution in (B), whereupon the antibody and MA bound together while dissociating the pseudo-antigen, DNPABMA, thereby impeding the quenching. As a result, the fluorescence intensity increased as FLx. This inhibition reaction reached an equilibrium in about 3⊘ seconds. The variation of the fluorescence intensity in the above procedure is schematically shown in Fig. 1. As will become apparent from the above, when MA is added to the solution of (B), the fluorescence intensity increases, from which the presence of the MA is detectable. In order to confirm the detection sensitivity under the above test conditions, a variation in the fluorescence intensity which was determined using different concentrations of MA is shown in Fig. 2. In Fig. 2, the concentration of MA is a final concentration. The quenching inhibition (QI, %) in the ordinate of Fig. 2 is defined as follows: QI(%) = FLx - FL1 FL⊘ - Fl1 x 1⊘⊘ As will be seen from Fig. 2, MA having a concentration of about 1⊘ -7.5 M was detected by the fluorescence quenching inhibition immunoassay. Example 2 The general procedure of Example 1 was repeated except that there was used instead of DNPABMA a compound of the following structural formula as a pseudo-antigen. As a result, similar results were obtained but the sensitivity was down to 1⊘ -6.5 M. Example 3 The general procedure of Example 1 was repeated except that there was used instead of DNPABMA a compound of the following structural formula as a pseudo-antigen. As a result, similar results were obtained but the sensitivity was down to 1⊘ -6.5 M as in Example 2. In the foregoing examples, the method of the invention is illustrated for detection of MA, but may be applicable to detection of any other substances. In addition, the dinitrobenzene derivatives were used in the examples since they are readily available, but similar effects can be obtained using other quenchers including nitrobenzene and trinitrobenzene derivatives. A fluorescence immunoassay method is described in which the presence or absence of antigen is measured within one minute by measuring a variation in fluorescence emitted from an antibody. The method comprises reaction between a pseudo-antigen obtained by chemical combination of an antigen and a fluorescent quencher and an antibody by which fluorescence to be emitted from the antibody is quenched by the action of the quencher combined. When an analyte is added to a solution of the combination, the pseudo-antigen is replaced by an antigen if the analyte contains the antigen, so that the fluorescence increases in intensity. This intensity is measured and compared with the initially measured intensity to detect the presence of the antigen. The method does not make use of the action of antigen on antibody with respect to the fluorescence of the antibody and is applicable to all antigens.

Claims: A fluorescence immunoassay method which comprises: providing a solution of a pseudo-antigen which is obtained by chemical combination between a fluorescence quencher having the action of quenching fluorescence emitted from an antibody, and an antigen; adding the pseudo-antigen to a solution of an antibody capable of emitting fluorescence by irradiation of light, thereby permitting the pseudo-antigen and the antibody to be bound; irradiating the solution with the light and measuring an intensity of the fluorescence emitted from the antibody by the action of amino acids of the antibody including tryptophane; and adding an analyte to the solution and irradiating the solution with the light to measure a variation in the intensity of the fluorescence from which the presence or absence of an antigen in the analyte is detected. A fluorescence immunoassay method according to claim 1, wherein said quencher is a member selected from the group consisting of derivatives of nitrobenzene, dinitrobenzene and trinitrobenzene having at least one functional group allowing reaction with the antigen. A fluorescence immunoassay method according to claim 2, wherein said quencher is a derivative of dinitrobenzene. A fluorescence immunoassay method according to claim 1, wherein said antibody is a monoclonal antibody capable of binding to amphetamine, methamphetamine or ephedrine. A fluorescence immunoassay method according to claim 1, wherein the first-mentioned antigen is amphetamine, methamphetamine or ephedrine. A fluorescence immunoassay method according to claim 1, wherein said pseudo-antigen is a compound of the following formula wherein X represents hydrogen or -OCH₃. R1 represents hydrogen or -OH, R2 represents hydrogen or -CH₃, and n is 0 or a positive number. A fluorescence immunoassay method according to claim 6, wherein said pseudo-antigen has the following structural formula A fluorescence immunoassay method according to claim 6, wherein said pseudo-antigen has the following structural formula A fluorescence immunoassay method according to claim 6, wherein said pseudo-antigen has the following structural formula A fluorescence immunoassay method according to claim 1, wherein the light for the irradiation has a wavelength of about 280 nm. A fluorescence immunoassay method according to claim 1, wherein said solution of the pseudo-antigen is within a range where the variation of the fluorescence is detectable. A fluorescence immunoassay method according to claim 1, wherein the second-mentioned antigen in the analyte is quantitatively detected. A fluorescence immunoassay method which comprises: providing a solution of a pseudo-antigen of the following formula which is obtained by chemical combination between a methamphetamine derivative and a 2,4-dinitrobenzene derivative having the function of quenching fluorescence from an antibody and having at least a functional group capable of reaction with the methamphetamine derivative: wherein X represents hydrogen or -OCH₃. R1 represents hydrogen or -OH, R2 represents hydrogen or -CH₃, and n is 0 or a positive number; adding the solution of the pseudo-antigen to a solution of an antibody capable of emitting fluorescence by the action of amino acids of the antibody including tryptophane when irradiated with light having a wavelength of about 280 nm, thereby permitting the antigen of the pseudo-antigen and the antibody to be combined; irradiating the solution with the light and measuring an intensity of the fluorescence having a wavelength of about 340 nm emitted from the antibody whose fluorescence has been quenched by the action of 2,4-dinitrobenzene; and adding an analyte to the solution and irradiating the solution with the light to measure a variation in the intensity of the fluorescence at about 340 nm, from which the presence or absence of an antigen in the analyte is detected by comparison between the intensities measured. A fluorescence immunoassay method according to claim 13, wherein said pseudo-antigen is a compound of the following formula A fluorescence immunoassay method according to claim 13, wherein said pseudo-antigen has the following structural formula A fluorescence immunoassay method according to claim 13, wherein said pseudo-antigen has the following structural formula

Inventors: MITSUMATA TADAYASU, MIYAZAKI JINSEI, MOTOYAMA NOBORU, MITSUMATA, TADAYASU, MIYAZAKI, JINSEI, MOTOYAMA, NOBORU
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1989
Main classification: G01N33
IPC codes: G01N33