The troponin complex of cardiac and skeletal muscle is a group of contractile proteins composed of three non-identical subunits. Troponin C (TnC) is the calcium sensitive subunit and contains four Ca2+ binding sites. Troponin I (TnI), the inhibitory subunit, binds actin in the relaxed state, thereby preventing muscle contraction by inhibiting the ATPase activity of actomyosin. Troponin T (TnT) is involved in the attachment of the troponin complex to the thin filament, binding tropomyosin and actin. The binding of intracellular Ca2+ by TnC induces a conformational change in the troponin complex, which causes TnI to release actin, subsequently allowing actin to interact with myosin, resulting in muscle contraction. Each subunit of the troponin complex exists in various isoforms depending on its tissue origin.
TnC exists in cardiac, fast-twitch skeletal muscle, and slow-twitch skeletal muscle isoforms, but the slow-twitch and cardiac isoforms are thought to be identical. TnI exists in distinct cardiac, fast-twitch skeletal muscle, and slow-twitch skeletal muscle isoforms. The cardiac isoform is approximately 40% dissimilar from the skeletal muscle isoforms and, in addition, contains 31 N-terminal amino acids not found in the skeletal muscle isoforms.
Elevated serum levels of the cardiac isoforms of the troponin subunits are well-documented in myocardial infarction (MI). Evidence suggests that the subunits exist as the binary Troponin IC complex and as the complete Troponin ICT complex in the serum of MI patients. As such, immunoassays specific for cardiac isoforms must detect these complexes.
Several clinical studies indicate that immunoassays for TnI and TnT are more specific for MI than those for creatine kinase MB isoenzyme. In addition, immunoassays for TnI and TnT are proving useful in the risk stratification of suspected MI patients and in detecting perioperative MI associated with various surgical procedures.
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