CHARACTERIZATION OF CARDIAC TROPONIN SUBUNIT RELEASE INTO SERUM AFTERACUTE MYOCARDIAL-INFARCTION AND COMPARISON OF ASSAYS FOR TROPONIN-T AND TROPONIN-I

We examined the release of cardiac troponin T (cTnT) and I (cTnI) into the blood of patients after acute myocardial infarction (AMI). Three postAMI serum samples were applied in separate analytical runs onto a calibrated gel filtration column (Sephacryl S-200), and the proteins w ere separated by molecular weight. Using commercial cTnT and cTnI assa ys measured on collected fractions, we found that troponin was release d into blood as a ternary complex of cTnT-I-C, a binary complex of cTn I-C, and free cTnT, with no free cTnI within the limits of the analyti cal methodologies. The serum samples were also examined after incubati on with EDTA and heparin. EDTA broke up troponin complexes into indivi dual subunits, whereas heparin had no effect on the assays tested. We added free cTnC subunits to 24 AMI serum samples and found no marked i ncrease in the total cTnI concentrations, using an immunoassay that ga ve higher values for the cTnI-C complex than free cTnI. To characteriz e the cross-reactivity of cTnT and cTnI assays, purified troponin stan dards in nine different forms were prepared, added to serum and plasma pools, and tested in nine quantitative commercial and pre-market assa ys for cTnI and one approved assay for cTnT. All nine cTnI assays reco gnized each of the troponin I forms (complexed and free). In five of t hese assays, the relative responses for cTnI were nearly equimolar. Fo r the remainder, the response was substantially greater for complexed cTnI than for free cTnI. Moreover, there was a substantial difference in the absolute concentration of results between cTnI assays. The comm ercial cTnT assay recognized binary and ternary complexes of troponin on a near equimolar basis. We conclude that all assays are useful for detection of cardiac injury. However, there are differences in absolut e cTnI results due to a lack of mass standardization and heterogeneity in the cross-reactivities of antibodies to various troponin I forms.