Abstract:
The present invention provides such a measuring method of the CK isoform and a reagent therefor. Accordingly, the present invention provides a reagent for measuring creatine kinase (CK) activity comprising an antibody, wherein said antibody inhibits CK-M T  subunit, but does not inhibit CK-M S  subunit. 
     The present invention also provides a method for measuring CK activity comprising determining an inhibition of CK of body fluids using a reagent for measuring CK activity, wherein said reagent comprises an antibody which inhibits CK-M T  subunit, but does not inhibit CK-M S  subunit.

Description:
This is a continuation application of Ser. No. 08/138,076, filed Oct. 20, 1993; now U.S. Pat. No. 5,804,394, which is a continuation-in-part of now abandoned Ser. No. 08/013,989, filed Feb. 5, 1993, which is a continuation of now abandoned Ser. No. 07/870,099, filed Apr. 16, 1992, which is a continuation of now abandoned Ser. No. 07/334,864, filed Apr. 6, 1989. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a reagent for measuring creatine kinase isoforms in body fluids and the method of measurement thereof. 
     BACKGROUND OF THE INVENTION 
     Creatine kinase (CK) catalyzes the following reaction: 
     
         creatine phosphate+ADP←→creatine+ATP 
    
     Measurement of CK activity in body fluids, especially in serum, is very important for the diagnosis of skeletal and myocardiac muscle disorder, especially, acute myocardial infarction (AMI). CK consists of two subunits, M (muscle type) and B (brain type), and three kinds of isoenzymes are present in cytoplasm, CK-MM, MB and BB. In particular, the separate measurement of CK-MB is recommended for specific diagnosis of myocardial infarction. 
     It has been elucidated that there are several post-translational isoforms of CK-MM and MB isoenzymes (George S., et al., J. Biol. Chem., 259, 2667-74 (1984). M subunit originally present in tissue, for example in cardiac muscle, is &#34;tissue type M subunit (designated M T ).&#34; Once M T  subunits in CK-MM or MB are released into blood stream, carboxypeptidase in plasma cleaves C-terminal lysine of the M T  subunits, generating &#34;serum type M subunit (designated M S ).&#34; Thus, CK-MM is converted from MM3 (tissue type, M T  M T ) via MM2 (hybrid type M T  M S ) to MM1 (serum type, M S  M S ). 
     It has been also clarified that the measurement of CK isoforms is a good index for earlier diagnosis of AMI and earlier detection of reperfusion after treatment of coronary thrombolysis. For example, it was reported that the ratio of MM3 to MM1 (MM3/MM1) is approximately 0.3 in healthy subjects, however, the ratio rises to 2.0, 3 to 6 hours after onset of AMI (Hashimoto H., et al., Circulation, 71, 366 (1985). Jaffe, et al. also reported (Circulation, 74, 105-9(1986) that 86% of the ratio in the first serum samples collected after onset from AMI patients exceeded its reference interval, on the other hand, the mean of total CK or CK-MB, those of which are conventional markers for diagnosis of AMI, still remained in their reference intervals. 
     Accordingly, the measurement of CK isoforms is very useful for the earlier diagnosis of AMI, compared with conventional AMI markers such as total CK activity and CK-MB. Earlier diagnosis of AMI makes it possible to treat AMI patients earlier with coronary thrombolysis or angioplasty. 
     The methods for the measurement of CK isoforms which have been reported are electrophoresis, isoelectric focusing, chromatofocusing, and liquid chromatography. Those methods are complicated, time-consuming, difficult to automate, and thus inadequate for emergency and routine assay. 
     Immunoassay is widely used in clinical chemistry field, and is a very excellent method because of its high sensitivity and specifity. Shah et al. showed immunoassay of CK isoforms using isoforms of specific antibodies (U.S. Pat. No. 4,900,662). This method, however, requires a long incubation time and separation and washing steps of solid phase from liquid phase, indicating that the method is complicated, laborious and unsuitable for emergency assay. 
     Immunoinhibition assay is also used in clinical laboratories. This method employs antibodies that inhibit part of enzyme activity such as particular isoenzymes. This method is being applied to the measurement of CK-MB isoenzyme using antibodies that inhibit CK-M (both M T  and M S ). In the assay, after all M subunit activity is inhibited by the antibodies, the remaining B subunit activity is then measured (Japanese Patent Publication 19239/1981 and 20274/1983). The immonoinhibition assay requires no separation procedures. The assay is completed in a short time and is easily automated, indicating that the assay is adequate for emergency and routine measurement. Immunoinhibition assay, however requires very specific antibodies. Antibodies used for immunoinhibition assay must have the ability not only to distinguish isoenzymes or isoforms from each other but also to inhibit particular enzyme activity. Generally, the antibodies against enzymes do not have either the distinguishing or inhibiting ability. The antibodies disclosed in the U.S. Pat. No. 4,900,662 cannot inhibit the enzymatic activity, meaning that they cannot be used in the immonoinhibition assay of CK isoform. The antibodies disclosed in the U.S. Pat. Nos. 19239 and 20274 cannot distinguish the CK-M T  subunit from CK-M S . They also cannot be used in the immunoinhibition assay. 
     Therefore, a faster and easier method for the measurement of CK isoforms is eagerly required. The present invention provides the method for the measurement of CK isoform by immunoinhibition assay. 
     SUMMARY OF THE INVENTION 
     The present invention provides the antibodies that can inhibit CK-M T  subunit but not MB, the method for measurement of CK isoforms and a reagent therefor. Accordingly, the present invention provides a reagent for measuring creatine kinase (CK) activity comprising an antibody, wherein said antibody inhibits the creatine kinase (CK) M T  subunit, but does not inhibit the creatine kinase (CK) M S  subunit. 
     The present invention also provides a method for measuring creatine kinase (CK) activity comprising determining an inhibition of creatine kinase (CK) of body fluids using a reagent for measuring creatine kinase (CK) activity, wherein said reagent comprises an antibody which inhibits the creatine kinase (CK) M T  subunit, but does not inhibit the creatine kinase (CK) M S  subunit. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The antibody of the present invention can be either polyclonal or monoclonal, but a monoclonal antibody is preferred. Physicochemical characteristics of the antibody are shown as follows; 
     (1) Action 
     The antibody acts on creatine kinase (CK) to raise an antigen-antibody reaction, then inhibits creatine kinase (CK) M T  subunit activity. 
     (2) Reaction specificity 
     The antibody inhibits only the M subunit of the creatine kinase (CK) M T  subunit, but does not inhibit the M S  subunit. 
     (3) Optimum pH 
      6 to 9 
     (4) Suitable pH range 
      3 to 11 
     (5) Measuring method of potency 
     Measured by a dilution rate which is detected from the measurement of inhibition. 
     (6) Suitable temperature range for action 
      0 to 40° C. 
     (7) Condition of inactivation 
     The antibody is inactivated at 100° C. for 30 minutes. 
     (8) Molecular weight 
      130,000 to 210,000 
     For obtaining the monoclonal antibody, a hybridoma cell strain which can produce a monoclonal antibody inhibiting creatine kinase (CK) M T  activity is necessary. Such a hybridoma cell strain has the following cytological characteristics; 
     (1) Derivation 
     Fused cells of lymphocytes producing anti-CK-M antibody and myeloma cells. 
     (2) Shape 
     The fused cells have similar shape to myeloma cells. They have a size of 10 to 20 micrometer. 
     (3) Function 
     They constantly produce monoclonal antibodies inhibiting CK-M T  activity which recognize one antigenic determinant. 
     (4) Growth ability 
     They show proliferation ability similar to myeloma cells. Thus, they proliferate about ten times in number for 72 hours. 
     (5) Storage stability 
     They are stored for a long period of time at a temperature of not more than -120° C. 
     (6) Optimum proliferation conditions 
      37° C., pH 7.2 
     (7) Proliferation range 
      32 to 42° C., pH 6.5 to 7.8 
     For obtaining such cell strain, hybridomas which produce a monoclonal antibody inhibiting CK-M T  activity are obtained by, for example, a method described in Japanese Patent Application Ser. No. 191899/1986, from which some hybridomas which do not inhibit M S  subunit are selected in a similar manner mentioned above. 
     As mentioned above, hybridomas producing antibodies which inhibit CK-MM 3 , but do not inhibit CK-MM 1  are obtained. One of hybridomas which is actually obtained by the above mentioned method is deposited on Jan. 26, 1988 at Fermentation Research Institute Agency of Industrial Science and Technology, Ibaraki-ken, Japan and assigned the number FERM P-9839, and was deposited under the Budapest Treaty as BP-2280 at the Fermentation Research Institute, Agency of Industrial Science and Technology, 1-3, Higashi 1-Chrome, Tsukaba-shi, Ibaraki-ken 305, Japan. This hybridoma is named CKH-5 and can be permanently stored at -120° C. 
     The hybridomas are cultured and antibodies are obtained therefrom. The method for obtaining the antibody is known to the art, especially a method described in Japanese Patent Application Ser. No. 191899/1986. The obtained antibodies may be used intact. They may be treated with proteolytic enzymes, such as pepsin, papain and the like to form Fab, Fab&#39;, F(ab&#39;) 2  fragments. 
     The reagent for measuring creatine kinase activity is prepared and used as generally described in Japanese Patent Publications (examined) 19239/1981 and 20274/1983 and Japanese Patent Application Ser. No. 191899/1986. For example, the reagent is generally composed of two packages. The first package generally contains 0.1 to 40 unit/ml of glucokinase (GlcK) or hexokinase (HK), 0.1 to 40 unit/ml of glucose-6 phosphate dehydrogenase (G6PDH), 0.001 to 5 mg/ml of M subunit inhibiting antibody (Ab), 0.1 to 20 mM of adenosine diphosphate (ADP), 0.05 to 20 mM of nicotinamide adenine dinucleotide (NAD + ) or nicotinamide adenine dinucleotide phosphate (NADP + ), 1 to 200 mM of glucose, 0.2 to 20 mM of adenosine monophosphate (AMP), 0.001 to 0.1 mM of diadenosine pentophosphate (Ap5A), 0.5 to 50 mM of N-acetylcysteine (NAC) or another thiol compound, 0.5 to 30 mM of magnesium salts (e.g. magnesium acetate), 0.5 to 50 mM of sodium azide, 0.1 to 20 mM of ethylenediamine tetraacetate (EDTA) and 5 to 50 mM of a buffer solution (pH 6.7). The second reagent contains 2 to 70 mM of creatine phosphate (CrP), 0.5 to 50 mM of sodium azide and 5 to 500 mM of a buffer solution. Preferably, in the first reagent, Glck or HK is present in an amount of 0.2 to 20 unit/ml, G6PDH is in an amount of 0.2 to 20 unit/ml, Ab is 0.005 to 2 mg/ml, ADP is 0.2 to 10 mM, NAD +  or NADP +  is 0.1 to 10 mM, glucose is 2 to 100 mM, AMP is 0.5 to 15 mM, Ap5A is 0.002 to 0.050 mM, NAC is 2 to 30 mM, magnesium salts are 1 to 15 mM, sodium azide is 1 to 30 mM, EDTA is 0.2 to 10 mM and the buffer solution (pH 6.7) is 10 250 mM. In the second reagent, preferably CrP is present in an mount of 5 to 40 mM, sodium azide is in an amount of 1 to 30 mM and the buffer solution is 10 to 250 mM. 
     As a measuring method, 0.5 ml of the first reagent is mixed with 0.001 to 0.02 ml of an enzyme solution (or serum) containing creatine kinase and kept at 25 to 37° C. for 5 to 10 minutes. Then, 0.125 ml of the second reagent is added to it and the residual creatine kinase activity is measured by an absorption change of 340 nm using a spectrometer. Thereafter, as a control, the first reagent which does not contain the antibody is added to the enzyme solution (or serum) and measured in a same manner as mentioned above. An inhibition rate is given by the following formula from the residual creatine kinase (CK) activity and the control creatine kinase (CK) activity; ##EQU1## 
     The antibody of the present invention does not inhibit the creatine kinase (CK)-M S  subunit, but inhibits the creatine kinase (CK)-M T  subunit. M T  subunit of creatine kinase which has been released from the heart by myocardial infarction is modified with carboxypeptidase of blood. Thus, CK-M T  M T  (M 3 ) is modified through M T  M S  (MM2) to M S  M S  (MM 1 ), and CK-MB is modified to M S  (MB 1 ) with the passage of time. A large amount of creatine kinase just after being released from the heart is modified very little so as to obtain a high inhibition. As the modification proceeds, the inhibition is lowered. Accordingly, by measuring inhibition with the reagent of the present invention, myocardial infarction is examined early and a time when a patients&#39; failure from the disease must be estimated. 
    
    
     EXAMPLES 
     The present invention will be illustrated by the following Examples which are not to be construed as limiting the present invention to their details. 
     Example 1 
     An emulsion mixture of 50 micrograms of pig CK-MM with complete Freund adjuvant (available from Nakarai Chemical Company) in a ratio of 1:1 was administered to 8 weeks mouse Balb/c (available from Nippon Clea Company). After three weeks, 50 microgram of pig ck-MM was intravenously injected. It was then fused with mouse myeloma X63.6.5.3 by a method by Oi, V. T., Selected Methods in Cellular Immunology, p. 351, W. H. Freeman and Co., 1980 and hybridoma grew in 54 wells/96 wells. The supernatant was added to a reagent for measuring creatine kinase (CK) activity of the following Reference Example 2 and an inhibition of CK-MM was measured with respect to CK-MM 3  and CK-MM 1  to select antibodies which do not inhibit Ck-MM 1  but inhibit CK-MM 3 . The selected strain was cloned by a limit dilution method to obtain a hybridoma CKH-5 of monoclone. The CKH-5 (BP-2280) shows the same cytological properties as mentioned before. 
     The hybridoma CKH-5 was cultured on RPM11640 culture medium containing 10% fetal bovine serum to a cell number of 2×10 6  /ml, 300 ml of which was then subjected to centrifugal separation to collect the supernatant. It was then fractionated with ammonium sulfate to obtain crude antibody fraction, and absorbed to Protein A Cephalose (pH 8.0) after dialyzing. The absorbed fraction was eluted with a citric acid buffer solution to obtain a monoclonal antibody (CKA-5) of 3.2 mg. 
     Reference Example 1 
     The antibody was added to reagents for measuring creatine kinase (CK) activity to determine inhibition of CK-MM 3 , MM 2  and MM 1  in serum separated by a chromatofocusing. 
     The first reagent was prepared by mixing 1.4 unit/ml of GlcK derived from Bacillus stearothermophilus (available from Seikagaku Kogyo Co., Ltd.), 1.2 unit/ml of G6PDH derived from Leuconostoc mesenteroides (available from Oriental Yeast Co., Ltd.), 1.2 mM of ADP, 0.75 mm of NADP, 25 mM of glucose, 6.25 mM of AMP, 125 micro M of Ap5A, 12.5 mM of N-acetylcystein, 12.5 mM of magnesium acetate, 10 mM of sodium azide, 2.5 mM of EDTA and 150 mM of an imidazoleacetate buffer solution (pH 6.7). The second reagent was prepared by mixing 100 mM of CP, 10 mM of sodium azide and 25 mM of a Tris-acetate buffer solution (pH 8.5). 
     The monoclonal antibody CKA-5 inhibiting CK-M activity was added to the first reagent and CK-MM 3 , then MM 2  or MM 1  was added to 0.5 ml of the mixture. It was then charged in a cell having a light length of 1 cm and incubated for 15 minutes. Next, 0.125 ml of the second reagent was added and the residual CK activity was measured by an absorption of 340 nm using a spectrometer which was kept at 30° C. As control, CK-MM 3 , MM 2  or MM 1  was added to the reagent which does not contain the antibody, and subjected to the same measurement. The result is shown in Table 1. 
     
                       TABLE 1______________________________________Inhibition activity of CKA-5        Activity with                     Activity with   no antibody antibody Inhibition  Isoform (u/l) (u/l) (%)______________________________________CK-MM.sub.3    221           1        100  CK-MM.sub.2 210 101 52  CK-MM.sub.1 197 195 1______________________________________ 
    
     Note: the values are an average of three samples. 
     As shown in Table 1, CKA-5 does not inhibit CK-MM 1  of serum type, but inhibits CK-MM 3  of tissue type. It also inhibits a half of MM 2  of hybrid type. Thus, CKA-5 does not inhibit the M T  subunit, but inhibits the M S  subunit, and is inside of the physicochemical characteristics. 
     Reference Example 2 and Comparative Example 1 
     CK-MM (800U/l) purified from cardiac muscles was added to blood plasma and incubated at 37° C. Inhibition of CK-MM by CKA-5 was determined with time as generally described in Reference Example 2. A diagnostic reagent for measurement of CK-MB (CK-MB kit) which contains CK-MM inhibiting antibody (Boehringer Mannheim Company) was employed as a comparative example and inhibition was measured. The result is shown in Table 2. 
     
                       TABLE 2______________________________________Change with time of inhibition by CKA-5  Incubation     Inhibition Inhibition  time (hours) by CKA-5 (%) by CK-MB kit (%)______________________________________0            97.3       100  1 57.6 100  2 38.9 100  3 23.5 100  4 16.1 97.0  5 8.2 96.7  6 0 100______________________________________ 
    
     As the purified CK-MM(MM 3 ) is modified by carboxy peptitase N in blood plasma to MM 2  and MM 1 , inhibition of CK-MM by CKA-5 reduces with time and 6 hours later there is no inhibition. This corresponds to the change of isoform. On the other hand, the inhibition of CK-MM by the CK-MB kit does not change after 6 hours and therefore this can not trace the change of isoform. 
     Example 2 and Comparative Example 2 
     As generally described in Reference Example 2, the CK activity and inhibition by CKA-5 of the serum from patient A who had onset of myocardial infarction was determined with time. For a comparison, the activity of CK-MB was determined using a reagent for CK-MB measurement kit which contains a CK-MM inhibiting antibody. The result is shown in FIG. 1 and FIG. 2. As shown in the figures, the inhibition by CKA-5 shows a peak after 6 hours from the onset, but CK-MB which has been used for a diagnosis of myocardial infarction shows a peak after 21 hours from the onset. Accordingly, it is very clear that the measurement of the inhibition by CKA-5 is very useful for an early diagnosis of myocardial infarction. 
     Example 3 and Comparative Example 3 
     As generally described in Example 1, the CK activity and inhibition by CKA-5 of the serum from patient B who had a fit of myocardial infarction was determined with time. For a comparison, the activity of CK-MB was determined as generally described in Comparative Example 2. The result is shown in FIG. 1 and FIG. 4. As shown in the figures, the inhibition by CKA-5 shows a peak after 6 hours from the onset, but that of CK-MB which has been used for a diagnosis of myocardial infarction shows a peak after 16 hours from the onset. Accordingly, it is very clear that the measurement of the inhibition by CKA-5 is very useful for an early diagnosis of myocardial infarction. 
     BRIEF EXPLANATION OF THE DRAWINGS 
     FIGS. 1 and 3 show a relation of an inhibition by CKA-5 with time after onset of myocardial infarction. 
     FIGS. 2 and 4 show a relation of a total CK activity (A) and CK-MB activity (B) with time after onset of myocardial infarction.