Time Course of Degradation of Cardiac Troponin I in Patients With Acute ST-Elevation Myocardial Infarction

Abstract

Abstract-Although measurement of troponin is widely used for diagnosing acute myocardial infarction (AMI), its diagnostic potential may be increased by a more complete characterization of its molecular appearance and degradation in the blood. The aim of this study was to define the time course of cardiac troponin I (cTnI) degradation in patients with acute ST-elevation myocardial infarction (STEMI). In the ASSENT-2 substudy, 26 males hospitalized with STEMI were randomized to 2 different thrombolytic drugs within 6 hours after onset of symptoms. Blood samples were obtained just before initiation of thrombolysis and at 30 minutes intervals (7 samples per patient). Western blot analysis was performed using anti-cTnI antibodies and compared with serum concentrations of cTnI. All patients exceeded the cTnI cutoff for AMI during the sampling period; at initiation of therapy, 23 had elevated cTnI values. All patients demonstrated 2 bands on immunoblot: intact cTnI and a single degradation product as early as 90 minutes after onset of symptoms. On subsequent samples, 15 of 26 patients showed multiple degradation products with up to 7 degradation bands. The appearance of fragments was correlated with higher levels of cTnI (PϽ0.001) and time to initiation of treatment (Pϭ0.058). This study defines for the first time the initial time course of cTnI degradation in STEMI. Intact cTnI and a single degradation product were detectable on immunoblot as early as 90 minutes after onset of symptoms with further degradation after 165 minutes. Infarct size and time to initiation of treatment was the major determinant for degradation. Key Words: myocardial infarction Ⅲ troponin Ⅲ troponin degradation Ⅲ diagnostics T he criteria for diagnosing acute coronary syndrome and myocardial infarction (AMI) changed in the year 2000 with the endorsement of the American College of Cardiology/ European Society of Cardiology guidelines, which designated cardiac troponin (cTn) as the biochemical marker of choice. 1 Troponin is a regulatory protein of the thin filament of striated muscle and consists of 3 tightly interacting subunits: T (37 kDa), I (24 kDa), and C (18 kDa). In addition, there are generally thought to be free cytoplasmic components 2,3 : for cardiac troponin T (cTnT), amounting to approximately 6% to 8% of the total pool; and for cardiac troponin I (cTnI), 3% to 4%. Troponin is released into the bloodstream 4 to 6 hours after AMI, peaks after approximately 18 to 24 hours, and can stay elevated for up to 14 days. Assessment of cTnI or cTnT by automated assay is today the most sensitive and specific method for diagnosing AMI. However, cTn is not released only in response to ischemic insults but by any condition that is associated with and/or causes cardiac injury, eg, decompensated heart failure, pulmonary embolism, end-stage renal disease, and stroke. 4 -11 In addition, there is significant variability in the sensitivity, selectivity, and specificity among various diagnostic cTnI immunoassays. 12 Some of these assay differences have ...

Development of a Qualitative Sequential Immunoassay for Characterizing the Intrinsic Properties of Circulating Cardiac Troponin I

“…The purified cTn ITC used in this assay was obtained from heart tissue from individuals diagnosed with heart failure, suggesting a possible difference in the intrinsic pathological degradations between tissue from patients with different heart diseases (ischemia vs heart failure) and those present in serum before or during the purification process. This N-terminal degradation was also phosphorylation dependent (5E6 detected both bands whereas 22B11 detected only the upper band), consistent with previous findings (9,14 ). Ultimately, it is only through analysis of larger numbers of individual samples that this seeming ambiguity in terminal degradation will be sorted out.…”

“…Although the C-terminus of cTnI is proposed to be degraded first in myocardium from patients undergoing coronary bypass surgery (10 ) and in isolated rat hearts undergoing ischemia (9,24 ), and has been detected in the blood of MI patients (11,14 ), the N-terminal degradation product has also been observed (11,13,38 ). The purified cTn ITC used in this assay was obtained from heart tissue from individuals diagnosed with heart failure, suggesting a possible difference in the intrinsic pathological degradations between tissue from patients with different heart diseases (ischemia vs heart failure) and those present in serum before or during the purification process.…”

“…Modification of cTnI can also occur through selective proteolysis at the N-and/or C-terminus with increasing myocardial ischemia (5, 9 -11 ) or pressure overload (12 ). Some of these posttranslational modifications to the primary cTnI sequence have been detected, albeit in a limited number, in sera, including degradation (13)(14)(15), phosphorylation (5 ), and oxidization (15,16 ).…”

Myofilament proteins: From cardiac disorders to proteomic changes

“…The N-terminal degradation of cardiac TnI was reported in a tail suspension rat model that simulated microgravity [34]. More interestingly, several groups have reported degradation products of TnI in serum of patients after acute myocardial infarction (AMI) [35][36][37]. Degradation products of TnT were also found both in animal models of ischemia/reperfusion injury [38,39] and in serum of patients with AMI [40,41].…”

“…Protein degradation often involves significant molecular weight (MW) reduction, which can be readily detected by 1-D-gel separation followed by Western blot analysis [31,32,[34][35][36][37][38][39][40][41]. Comparative 2-DE was also used to study proteomic changes on a broader scale, and PMF can be employed to identify changed proteins [46].…”