Characterization of cardiac troponin subunit release into serum after acute myocardial infarction and comparison of assays for troponin T and I

Abstract

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 were separated by molecular weight. Using commercial cTnT and cTnI assays measured on collected fractions, we found that troponin was released into blood as a ternary complex of cTnT-I-C, a binary complex of cTnI-C, and free cTnT, with no free cTnI within the limits of the analytical methodologies. The serum samples were also examined after incubation with EDTA and heparin. EDTA broke up troponin complexes into individual subunits, whereas heparin had no effect on the assays tested. We added free cTnC subunits to 24 AMI serum samples and found no marked increase in the total cTnI concentrations, using an immunoassay that gave higher values for the cTnI-C complex than free cTnI. To characterize the cross-reactivity of cTnT and cTnI assays, purified troponin standards in nine different forms were prepared, added to serum and plasma pools, and tested in nine quantitative commercial and pre-market assays for cTnI and one approved assay for cTnT. All nine cTnI assays recognized each of the troponin I forms (complexed and free). In five of these assays, the relative responses for cTnI were nearly equimolar. For 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 commercial 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 absolute cTnI results due to a lack of mass standardization and heterogeneity in the cross-reactivities of antibodies to various troponin I forms.

The diagnostic value of two commercially available human cTnI assays in goat kids with myocarditis

“…However, to some degree this may not be realistic in the evaluation of pathologic conditions because cTnI could form complexes with other molecules which interfere with the validity . The cTnI molecules are released into blood as a ternary complex of cTnT‐I‐C, a binary complex of cTnI‐C, and free cTnI in people with myocardial injury, with the binary cTnI‐C complex being the predominant form . More extensive studies are necessary to determine circulating forms of cTnI after myocardial injury in goats.…”

Evaluation of a high‐sensitivity assay for measurement of canine and feline serum cardiac troponin I

“…It has been documented that rapid modifications of cTnI, depending to some degree on complex interactions with cardiac troponin C and T, occur due to degradation or phosphorylation, which may affect binding of antibodies to the targeted epitope . In contrast to several other assays, the ADVIA Centaur CP TnI‐ultra assay has been reported to recognize free cTnI, cTnI‐T‐C, and binary cTnI‐TnC forms equally well, the latter of which is the main form released into the blood in people with cardiac injury . The degree and type of modification, and the most frequent form of cTnI released in dogs and cats are not known, but the presence of variable modified forms may be a cause of varying antibody affinity.…”

Biochemical markers of myocardial injury

“…Free cTnT was released initially, followed by the ternary troponin T/I/C complex, with a small amount of free TnT. 128 Fractionation of cTnI from the plasma of patients with AMI was performed using combinations of 15 monoclonal antibodies directed against various domains of the cTnI molecule. 72 This showed that the dominant form of cTnI released was the cTnI/TnC complex.…”