Troponin in Acute Coronary Syndrome

The sarcomere is the basic unit of myocardial contraction. Each sarcomere is composed of two longitudinal muscle groups: thick filaments and thin filaments. The thick filaments, which are located in the middle of the sarcomeres, 1.5-1.6 mm in length and 10 nm in diameter, are composed of myosin molecules. Myosin (with a molecular weight of 500000 kDa) is a complex, asymmetrical molecule, with a globular subunit on one end, which is the activity site for ATPase as well as the bridge between actin and myosin. The cardiac myosins come from the family of myosins whose function is in molecular level movement. There are two additional proteins in the thick filaments, which are titin and myosin-binding protein C. The thin filaments are composed of actin, tropomyosin (Tm) and troponin (Tn). The thin filaments are shaped as double helix chains, with actins twisted around each other and around tropomyosin, which acts as the backbone of the filament. A group of regulator proteins, troponin C (TnC), troponin I (TnI), and troponin T (TnT), are found at regular intervals on this filament. 2 The troponin complex consists of three different proteins: troponin C (calcium-binding protein), troponin T (tropomyosin binding protein), and troponin I (inhibitory protein). TnC acts as a calcium binding site, TnI is bound to Tm and TnC, whereas TnT connects Tn to Tm. The Tm-Tn complex is responsible for the binding of calcium ions to TnC and also has a role as the cross bridge formation regulator. The protein regulates calcium dependent interactions between actins and myosins of the muscle. Cardiac troponin C is identical to skeletal muscle’s troponin C; therefore it is of limited clinical use. On the other hand, cardiac troponin T (TnT) and troponin I (TnI) are derived from specific genes for the heart and are not produced by skeletal muscles. 12, 16, 18 Any damage to the myocardium, whether it is injury, infarction, or necrosis will increase troponin T and troponin I levels. 1,2,6,8,9 The presence of increased levels of troponin T and I in non-cardiac cases, such as in cirrhotic liver, and chronic renal failure, had raised questions on the specificity of these markers in diagnosing myocardial damage, particularly in acute myocardial infarction. Troponin T and troponin I have similar diagnostic power. The difference is on the laboratory analysis used. Troponin T is examined using one test only, whereas troponin I are assessed with several tests. Therefore, to compare both is very difficult. Some researchers have shown the superiority of troponin T assessment compared with troponin I. 6,15