Degradation of human cardiac troponin I after myocardial infarction

Cardiac troponin I (TnI) is the inhibitory subunit of the troponin complex and a specific biochemical marker for myocardial infarction (MI). It is released into the bloodstream within 4-6 h following MI, peaks after 18-24 h and remains elevated for up to 7 days. In this work, I have identified TnI forms present in MI-patient serum. By immobilizing anti-TnI antibodies, which recognize various epitopic sites on the TnI molecule, we were able to isolate TnI from MI-patient serum pools. Western-blot analysis following SDS/PAGE shows two major TnI fragments with apparent molecular masses of 18000 and 14000 Da. Either or both fragments are seen in serum obtained from individuals with MI. The fragments are generated as a result of proteolytic processing from the C-terminal region of TnI. Partial processing from the N-terminal of TnI is also seen and is associated with the generation of the 14000 Da fragment. Very little unprocessed intact TnI is detected in patient serum after MI. The degradation in vitro of cardiac TnI was studied by incubating either bovine or human recombinant TnI in serum. Western-blot analyses with TnI antibody showed that purified TnI spiked into normal human serum or MI-patient serum depleted of TnI degrades rapidly to lower molecular mass fragments. Degradation of TnI is associated with a loss in immunological activity. Serum TnI isolated by anti-TnI antibody, under non-dissociating conditions, is associated with at least troponin C (TnC) and troponin T (TnT). This complex is bound by anti-TnI, anti-TnC and anti-TnT antibodies.