Immunochemical LDH1 assay
BACKGROUND OF THE INVENTION 
     U.S. Pat. No. 4,046,634 discloses an assay for isoenzymes, including LDH isoenzymes, which employs ion exchange column chromatography to isolate a mixture of LDH 1  and LDH 2  isoenzymes which are then detected by conventional techniques such as the Wacker LDH method. 
     German Auslegeschrift No. 21 28 670 discloses a method for assaying for isoenzymes, including LDH isoenzymes by treating the test sample with a large excess of a specific antibody to either LDH 1  or LDH 5  isoenzymes so as to effect quantitative precipitation of the antigen-antibody complex. After incubation and centrifugation of the precipitated complex, the enzyme activity of the supernatant was determined by the method reported in Z. Klin. Chemie und Klin. Biochemie 8, 658 (1970). 
     Sussman et al., Journal of Biological Chemistry 243, 160 (1968) have described an assay for individual organ specific isoenzymes of human alkaline phosphate using a two step procedure. In the first step the specific antibody to the desired isoenzyme was reacted with the test sample and then in a subsequent step the antibody-antigen complex is precipitated with a second antibody (anti-γ-globulin). After centrifugation the supernatant is tested for residual isoenzyme activity. 
     The use of a second antibody insolubilized on a solid support material in radioimmunoassay procedures is described in U.S. Pat. No. 4,048,298. The disclosure includes the use of second antibodies adsorbed to the surface of polymeric solid support materials. 
     U.S. Pat. No. 3,843,443 relates to a method of immobilizing proteins on a fluorocarbon polymer support. Included within the disclosed proteins are antibodies and the preferred polymer support material is polyvinylidene fluoride (Kynar-a trademark of the Pennwalt Corp.). 
     DESCRIPTION OF THE INVENTION 
     The present invention relates to an improved immunochemical assay for the isoenzyme LDH 1  which isoenzyme is a known marker for myocardial infarction. 
     In the improved method of the present invention a test sample such as a serum sample, is treated with a soluble antibody specific against the M subunit of the LDH isoenzymes, i.e., LDH 2 , LDH 3 , LDH 4  and LDH 5 . After mixing and incubating for a short time, a second antibody insolubilized on a solid phase support material is added and the mixture is mixed and then incubated for another short period. The insoluble antigen-antibody(1)-antibody(2)-solid support complex is centrifuged down and the supernatant tested for LDH enzyme activity. The activity observed will be essentially that of the LDH 1  isoenzyme component of the original sample. 
     The method of the present invention has substantial advantages over procedures utilized in the prior art for isoenzyme assay. It is very rapid, highly accurate and reproducible. The preparation of the LDH 1  containing supernatant can be accomplished in a matter of minutes instead of the substantial number of hours previously required for immunological techniques. Moreover, the present method provides a clean separation of LDH 1  from the other LDH isoenzymes which is not possible by ionexchange column procedures. 
     The specific antibody against the M subunit of LDH used in the present invention is known in the art. See for example the previously indicated German Auslegeschrift No. 21 28 670. Further disclosures relating to such antibody are to be found in J. S. Burd et al., Clinica Chimica Acta 46, 205-216 (1973) and J. S. Burd et al., Biochimica, Biophysica Acta, 310, 238-247 (1973). 
     The second antibody is prepared by immunization of a different animal than the one in which the specific first antibody is prepared with a gamma globulin from the blood of the host species used for the first antibody preparation. Thus the second antibody will be immunoreactive for the first antibody and will complex with it. 
     The second antibody is insolubilized by attaching said second antibody to an insoluble support material. Suitable support materials include water insoluble organic polymeric substances such as cellulose or other polysaccharide, a vinyl addition polymer or condensation polymer or a water insoluble inorganic substance of polymeric nature, such as glass or silicone resins or the second antibody may be adsorbed to the surface of a solid support such as polystyrene, polypropylene, polyfluoroethylene or polyvinylidene fluoride. The method of attachment of the second antibody to the solid support is not narrowly critical and may include (1) covalently coupling the soluble second antibody to any insoluble polymeric substance; (2) converting the soluble second antibody to an insoluble polymerized form, such as by reaction with an insolubilizing agent; (3) physical entrapment of particles of the second antibody in the pores of a gel polymer such as a cross-linked polyacrylamide; or (4) by physical adsorption on an insoluble polymeric substance. 
     In a preferred embodiment of the present invention the second antibody is supported by adsorption on activated Kynar utilizing the general procedures disclosed in U.S. Pat. No. 3,843,443. 
    
    
     The method of the present invention is further illustrated by reference to the following Example. 
     Example 
     1. Reagents 
     a. Antiserum to LDH 5  : goat anti LDH 5  serum is diluted to a concentration that binds 300-400 IU/l of purified LDH 5  isoenzyme employing Fisher Diagnostic or equivalent reagents for the determination of LDH enzymatic activity. The dilution of the antiserum is made in 0.02M Tris pH 7.5 with 0.1% NaN 3 . 
     b. Insoluble antiserum to goat gamma globulin. 
     The following specific steps are followed in preparing an insoluble antiserum to goat gamma globulin (second antibody). The starting material is unsintered Kynar (vinylidene fluoride) resin powder, grade 301 F, Pennwalt Corp. The powder is dispersed in isopropanol (2-propanol) in the proportions of 50 grams Kynar in 1000 ml of isopropanol. The suspension is then homogenized by a Brinkmann Polytron for 5 minutes at a pulse-frequency of 4000 c.p.s. The Kynar-isopropanol mixture is then transferred to a cylinder containing ten liters of saline and stirred until dispersed. The Kynar is then allowed to settle out and most of the supernatant is decanted. After two water washes, the Kynar is resuspended in phosphate buffered saline (pH 7.0) with merthiolate (0.01%) to yield a 2% Kynar concentration. The Kynar is now in the activated state and able to accept protein. While the isopropanol activated Kynar is stirring, 0.5 ml of donkey antigoat gamma globulin serum per gram of Kynar is added. The mixture is then homogenized again by the Polytron for 5 minutes at the same pulse-frequency as before. The suspension is then continuously stirred at room temperature for a minimum of 6 hours followed by stirring at 4° C. for a minimum of 12 hours. The suspension is now ready to be washed. This is accomplished by centrifugation at 1500×g for 10 minutes followed by resuspension in 0.02M Tris Hydroxymethylaminomethane (Tham) pH 7.5 with 0.1% NaN 3 . This process is repeated once more and the final material resuspended in 0.02M Tris pH 7.5 with 0.1% NaN 3  to 100 grams of Kynar per 1000 ml of buffer. The mixture is again stirred and 5 grams of bovine serum albumin (BSA) per 100 grams of Kynar added. Homogenization with the Polytron at 4000 c.p.s. for 5 minutes is the final step in this procedure. 
     2. Procedure 
     a. To 200 μl of patient&#39;s serum add 10 μl of soluble goat anti LDH 5  serum and vortex. Wait 5 minutes. 
     b. Add 200 μl of insoluble second antibody* and vortex. Wait 5 minutes. 
    
     c. Spin at approximately 1000 g for 5 minutes. 
     d. Withdraw from the supernatant whatever amount is needed for a conventional LDH activity assay. Use same assay procedure. 
     3. Calculations 
     LDH 1  activity=Activity in supernatant×2.05 
     4. Interpretation 
     To decide on a cut-off point for LDH 1  an arbitrary value is established. This value will differ for each individual laboratory depending on the normal total LDH range of the enzyme assay being used. To establish an upper limit of normal for LDH 1  (H 4 ) take 30% of the upper level of normal for the total LDH enzyme activity assay. For example, the Fisher Diagnostic LDH Assay provided an upper limit of normal of 149 I.U./l. (The normal range is 52-149 I.U./l.) Therefore, the cut-off point adopted turned out to be 30% of 149 or 45 I.U./l. Any serum showing an LDH 1  activity above 45 I.U./l will be considered positive for myocardial infarct. 
     Clinical Results 
     Sera from 106 patients from a Cardiac Care Unit were examined for LDH 1  elevation. In all 72 patients where a myocardial infarct was diagnosed, an LDH 1  elevation was observed. However, LDH 1  remained non-elevated for all 34 non-myocardial infarct patients. A cutoff of 45 IU/l was designated for biochemical diagnoses of infarct. The range for LDH 1  activity in MI patients was 46-470 IU/l. The range for the non-MI patients was 9-44 IU/l. Presently most laboratories determine the LDH 1  /LDH 2  &#34;flip&#34; by electrophoresis. This technique is tedious and time consuming. By contrast the present immunochemical procedure is simple and fast. In addition, determining LDH 1  elevation is more sensitive than the flip. In 59 of the 72 MI patients, the LDH flip occurred the same day as the LDH 1  evaluation. However, in 8 cases the LDH flip occurred one day after the LDH 1  elevation, and in 5 MI patients a flip was not obtained (see Tables I, II and III). 
     
                       TABLE I                                                     
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            LDH Flip                                                      
                  Same day  Day after                                     
Number of                                                                 
         LDH-1    as LDH-1  LDH-1                                         
MI patients                                                               
         Elevated elevation elevation                                     
                                    Not present                           
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72       72       59        8       5                                     
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                       TABLE II                                                    
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Number of                                                                 
Non-MI       LDH-1        LDH-Flip                                        
patients     Elevated     Present                                         
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34           None         None                                            
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                                           TABLE III                               
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Part A                                                                    
            LDH-1 Elevated    LDH Flip Present                            
Number of                                                                 
      CPK-MB                                                              
            Same Day                                                      
                  One Day     Same Day                                    
                                    One Day                               
MI patients                                                               
      Present                                                             
            as CPK                                                        
                  after CPK                                               
                        Not at All                                        
                              as CPK                                      
                                    after CPK                             
                                          Not at All                      
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58    55    43    12    None  31    19    5                               
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Part B                                                                    
                      LDH Flip Present                                    
                      Same Day                                            
                              One Day                                     
Number of                                                                 
        DPK-MB                                                            
             LDH-1    as LDH-1                                            
                              after LDH-1                                 
MI patients                                                               
        Absent                                                            
             Elevated Elevation                                           
                              Elevation                                   
                                     Not at All                           
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58       3    3        2       1     None                                 
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