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Clinical Chemistry 45: 18-20, 1999;
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(Clinical Chemistry. 1999;45:18-20.)
© 1999 American Association for Clinical Chemistry, Inc.

Opinion

Clinical and Analytical Standardization Issues Confronting Cardiac Troponin I

Fred S. Applea

Department of Laboratory Medicine, and Pathology, Hennepin County Medical Center and, University of Minnesota, School of Medicine, Minneapolis, MN
a Address for correspondence. Hennepin County Medical Center, Clinical Laboratories, MC 812, 701 Park Avenue, Minneapolis, MN 55415. Fax 612-904-4229; e-mail fred.apple@co.hennepin.mn.us.


   Introduction
 
Troponin I, a subunit of the ternary troponin complex, is a regulatory protein of the thin filament of striated muscle. Cardiac troponin I (cTnI) is uniquely different from the troponin I found in either slow or fast skeletal muscle. Possessing an additional 31-amino acid sequence at its N terminus, cTnI is found exclusively in the myocardium and, therefore, has been extensively studied as a sensitive and specific serum marker for myocardial injury (1). The forms of cTnI found in the circulation include complexes with troponin T and C (TIC and IC) as well as free subunits (2). In addition, cTnI has been identified in oxidized, reduced, and phosphorylated forms (3)(4). The signals generated by immunoassays developed with anti-cTnI monoclonal antibodies are dependent on the epitope region recognized by these antibodies (5). Thus, protein regions within the cTnI molecule that are susceptible to oxidation, reduction, or phosphorylation or to degradation by the action of proteases are likely to exhibit variable immunoreactivity and even loss of reactivity, leading to altered signal generation in cTnI immunoassays that use antibodies against such regions.

In the December 1998 issue of Clinical Chemistry, Katrukha et al. (6) presented Western blotting and immunoassay data demonstrating the in vivo and in vitro degradation of both the C and N termini of cTnI in human myocardium and serum after acute myocardial infarction (AMI). More importantly, they demonstrated that the signal detected by cTnI immunoassays is highly dependent on the epitope regions recognized by the anti-cTnI antibodies used in the (sandwich) immunoassays. Thus, whether cTnI is complexed with troponin T and or with troponin C as the three-subunit complex TIC or the two-subunit complex IC or as a free subunit, immunoassays designed with antibodies selected against the . . . [Full Text of this Article]


   Acknowledgments
 

   References
 



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