Abstract

A radioimmunoassay for the MB isoenzyme of creatine kinase (myocardial isoenzyme) has been developed based on an antibody to the B subunit. The antibody cross-reacts with MB but exhibits no cross-reactivity with MM C.K. even when present in 20 000 molar excess over MB C.K. unlabelled MB but not MM (up to 85 000 mI.U./ml) competitively displaces precipitable counts. This sensitive and specific radioimmunoassay accurately detects MB in a concentration of 0-01 mI.U./ml. Plasma MB C.K. in 100 healthy controls averaged 1 mI.U./ml+/-0-6 (S.D.), similar to results in 20 patients with chest pain without infarction. In contrast, in 50 patients with myocardial infarction MB C.K. values averaged 97+/-30 mI.U./ml. When hourly blood-samples from 10 of these patients admitted within an hour of chest pain were analysed by R.I.A., a 100% increase in MB was detectable within four hours of the onset of chest pain, generally before total C.K. exceeded the normal range. Development of an R.I.A. suitable for assay of an isoenzyme in plasma provides a specific and sensitivity assay of MB C.K., facilitating analysis of multiple samples and early detection of myocardial infarction. It may also serve as a prototype for radio-immunoassay of multiple forms of other enzymes of clinical importance.