Cardiac Troponin

Figures

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  Figure 1. Cardiac Troponin I Cardiac troponin I is shown in blue with different functional regions labeled accordingly. Identified protease cleavage sites are marked with red lightning bolts and phosphorylation sites (P) are shown in black. The ovals indicate regions with helical secondary structure. SR indicates the switch region of cTnI. Known interaction sites with other subunits can be seen throughout the protein. The green box within the C-terminal region is where the cTnI complex interacts with tropomyosin-actin in the absence of calcium. The blue pair of antibodies represents the reagents of the Abbott Architect Assay, and the green pair represent the Roche E170 assay.62 The antibodies demonstrate the large differences between two commercial assays for cTnI. The annotations “C” and “D” above the antibodies denote capture and detection, respectively. cTnI = cardiac troponin I.

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  Figure 2. Cardiac Troponin T Cardiac troponin T is shown in yellow with different functional regions labeled accordingly. Cleavage sites are marked with the red lightning bolts and phosphorylation sites (P) are shown in black. The ovals correspond to the helical secondary structures in cTnT, and the interaction points with other troponin subunits can be seen within the C-terminus. There are also two sites of interaction with actin-tropomyosin which are indicated by green boxes. The position of the antibody pair used in the 5th generation Roche hs-cTnT assay are indicted accordingly. The capture antibody (blue) is directed against an epitope corresponding to amino acid residues 125 to 131 and biotinylated (purple) for immobilization. The detection antibody (green) is a ruthenium-tagged mouse-human chimera is directed against amino acid residues 136 to 147. Residues 203 to 271 of TnT interact with TnI and TnC and contribute to the I-T arm motif of the whole complex. cTnT = cardiac troponin T.

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  Figure 3. Mixed Sandwich Immunoassays Targeting the Troponin Complex This figure, adapted from damen et al,47 illustrates the mixed sandwich immunoassays used in their study. Similar approaches have been adopted by other groups using different antibodies targeting various epitopes of the individual subunits. In this figure, cTnT and cTnI are truncated at their N- and C-terminal ends, respectively. (A) Uses capture antibody to cTnI (aa 27-40) and detection to cTnI (aa 41-49). It will detect all forms of cTnI including T-I-C and non-complexed cTnI due to the close proximity of the antibodies. (B) Uses capture antibody 20C6, and detection antibody cTnI (aa 27-40). It cannot detect noncomplexed cTnI as the antibodies target different subunits. (C) Uses capture 20C6 and detection TNT1C11 (cTnT aa 171-190). It detects only full complex T-I-C or partially truncated T-I-C due to the location of the antibodies. (D) Details the antibodies used each assay. Abbreviations as in Figures 1 and 2.

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  Central Illustration. Detecting the Fragments of cTnT in Different Forms of Myocardial Injury cTnT fragmentation varies across different forms of myocardial injury, and overall ctnt concentration depends on the assay used. The blue circle represents acute myocardial infarction (AMI), where T-I-C fragments into free TnT and I-C. The yellow circle represents chronic myocardial injury, and postmarathon AMI. Here, cTnT appears as truncated N/C terminal forms and cTnI fragmentation is unknown. cTnT assay 1: A commercial hs-cTnT assay with a capture antibody targeting aa 125 to 131 and detection targeting aa 136 to 147, identifying intact and truncated forms of cTnT. cTnT assay 2: A central capture antibody and a C-terminal detection antibody, detecting longer cTnT forms while missing N/C-terminal fragments. This figure highlights variable fragmentation among types of myocardial injury. Troponin concentration depends on assay choice, as different assays target different portions of the complex. Assays could be developed to exclude specific fragments or target T-I-C to improve specificity for the targeted form of injury. Abbreviations as in Figures 1 and 2.