ИСТИНА

BACKGROUND-AIM Troponin complex (ITC-complex) consists of troponins I, T and C (TnC) and plays an important role in muscle contraction. Specific cardiac isoforms of troponins I and T (cTnI and cTnT, respectively) are established biomarkers of cardiomyocyte damage, especially myocardial infarction (MI). Various studies have shown that after MI troponins appear in human blood as ternary ITC-complex, binary IC-complex (consists of cTnI and TnC) and cTnT fragments. However, it is still not clear if all these forms are generated in necrotic myocardium or dissociation can also occur in the bloodstream or in blood samples collected for analysis. In this work, we studied the in vitro stability of ITC-complex spiked in various types of blood samples. METHODS Native ITC-complex was incubated in different matrices at +4°C, +20°C and +37°C for up to 24 hours and studied by sandwich immunoassays, gel filtration and Western blotting. RESULTS ITC-complex dissociated at +37°C into a binary IC-complex and free cTnT in all tested matrices (serum, citrate, heparin and EDTA plasmas) with half-lives of 1.02, 3.98, 2.52 and 0.27 hours,respectively. Moreover, afterincubation in EDTA and heparin plasmas, IC-complex further broke down to free cTnI and TnC. ITC-complex was stable in citrate and heparin plasmas and in serum at +4°C and at +20°C but dissociated at these temperatures in EDTA plasma (half-lives are >24 and 6.03 hours, respectively). The dissociation of ITC-complex in citrate and heparin plasmas cannot be explained by protein degradation since we have shown by Western blotting that cTnI and cTnT were stable in these matrices. In EDTA plasma and serum, both dissociation and degradation of cTnI and cTnT took place during the incubation at +37°C. ITC-specific autoantibodies present in blood of some people impeded the dissociation of ITC-complex. CONCLUSIONS Based on the obtained data, we suggest that ITC-complex is not stable in serum and plasmas if they were incubated at +37°C. So, the appearance of IC-complex in blood samples of MI patients may be both due to its release from the damaged myocardium and due to the dissociation of ITC-complex in the circulation, especially in the presence of heparin, or during blood sample preparation.