A 52-year-old male presents 8 hours after onset of severe chest pain. His serum troponin I is markedly elevated. Which property of cardiac troponin I makes it MOST specific for myocardial injury compared to troponin T?
- A Cardiac troponin I has an N-terminal extension unique to the cardiac isoform, absent in skeletal muscle isoforms ✓
- B Troponin I has a longer half-life in serum than troponin T
- C Troponin I is released only from necrotic myocardium, not from reversibly injured cells
- D Troponin I is present exclusively in the thin filaments of cardiac myocytes and not in smooth muscle
Correct answer: A. Cardiac troponin I has an N-terminal extension unique to the cardiac isoform, absent in skeletal muscle isoforms
Explanation
Cardiac troponin I possesses a unique N-terminal extension of 31 amino acids that is absent in both fast and slow skeletal muscle isoforms; antibodies targeting this region give cTnI its high cardiac specificity. Troponin T also has a cardiac-specific isoform, but skeletal muscle disease (e.g., myositis, renal failure) can still elevate cTnT via cross-reactivity with some assays. Half-life and release mechanism are not the basis of isoform specificity.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.