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For the determination of creatine kinase isoenzyme catalytic concentrations in serum two methods based on immunological reactions are presented: One method uses inhibiting antibodies, which selectively block the activity of creatine kinase M subunits ("Inhibition Test"). This test is used for routine measurements of creatine kinase MB catalytic concentration; Another method uses precipitating antibodies, which allows a quantitative differentiation of creatine kinase isoenzymes MM, MB and BB ("Precipitation Test"). This test is used as a control for the Inhibition Test for the possible presence of creatine kinase BB activities in doubtful cases. Procedures, specificity, correlation and application of these methods are discussed.

This is a new method for the determination of creatine kinase isoenzyme MB activity in serum. The method uses direct activity measurement of creatine kinase B subunit activity after blocking of CK-M subunit activity by inhibiting antibodies. The test takes no longer than 15 min. The method yields an intra-serial C.V. of 2.0-12.9%, and a C.V. from day to day of 5.5%. The detection limit is 3.4 U/l creatine kinase MB. In the 95 cases with proven myocardial infarction several types of creatine kinase MB activity kinetics could be determined. The percentage of creatine kinase MB of peak CK-total is 6-25%, with a mean of 11.1%. The amount of creatine kinase MB with respect to total CK activity after reinfarction is higher than the amount after initial infarction.

Creatine kinase MB isoenzyme was measured (using antibody inhibition) in serum of patients with exogenous intoxication, acute pancreatitis, cerebrovascular accidents, meningitis, encephalitis, skeletal muscle disease, shock, postoperative states and after coronary arteriography, cardiac catherisation of cardioversion. CK-MB activity was revealed only in sera of patients with exogenous intoxication (severity III and IV), polymyositis, scleroderma, after operation or after coronary arteriography, cardiac catherisation or cardioversion. As it is not possible to differentiate between CK-MB and CK-BB using inhibiting antibodies against CK-M subunit, CK-isoenzyme activity was determined in parallel, using precipitating antibodies. No CK-BB was found in any case. The determination of CK-MB isoenzyme after blocking of the CK-MB subunit by means of inhibiting antibodies is suitable for clinical diagnosis. The method significantly increases the value of creatine kinase measurement.

We have established several mouse hybridoma cell lines which secrete monoclonal antibodies with creatine kinase M subunit inhibiting capacity. Some of these antibodies are useful tools for the determination of the creatine kinase-B subunit activity (CK-MB test). The monoclonal mouse anti-bodies and the conventional polyclonal goat antibodies were compared.