Comparison of Immunoreactivity of Five Human Cardiac Troponin I Assays toward Free and Complexed Forms of the Antigen: Implications for Assay Discordance

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Abstract
Troponin, consisting of three components, troponin C (TnC), troponin I (TnI), and troponin T (TnT), is a major component of the structural proteins involved in striated and cardiac muscle contraction (1)(2). TnI and TnC bind tightly to each other in the presence of Ca2+ with an association constant, Ka, of ∼108-109 L/mol (1)(2)(3). TnT binds to both TnC and TnI, although less weakly than the binding between TnC and TnI. The cardiac isoforms of TnI (cTnI) and TnT are structurally different from the corresponding skeletal isoforms, and therefore they have recently established themselves as biochemical markers of myocardial damage (4)(5)(6)(7).