Synthetic Peptide Substrates in Hemostatic Testing

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Abstract
Since the introduction of synthetic chromogenic and fluorogenic peptide substrates from serine proteases, the testing of coagulation has undergone a dramatic conceptual and methodological change. The concept of coagulation profiling has emerged and automated methodologies are being introduced. Synthetic substrate methods for the evaluation of antithrombin-III; progressive antithrombin; plasminogen; antiplasmin; prekallikrein; antikallikrein ; alpha 1-antitrypsin; prothrombin; heparin; platelet factor IV; urokinase; tissue activator of plasminogen; factor assays; amidolytic equivalents of prothrombin time; and partial thromboplastin time have been developed. Studies on antithrombin-III indicate that immunological methods evaluate the total immunoreactive-antithrombin-III (antigenic) level and do not discriminate between functionally active forms and the AT-III serine protease complex. The clinical significance of AT-III measured by immunological methods is highly questionable. The coagulant assays for the measurement of AT-III require purified alpha-thrombin preparations. The noncoagulant forms of thrombin (beta- and gamma-) result in falsely low antithrombin-III quantitation. The molecular heterogeneity in a given thrombin preparation if standardized in terms of its amidolytic activity does not produce any errors in the quantitation of AT-III levels with synthetic peptide methods. None of the immunological methods provide clinically relevant information except in normal plasma where the immunological and functional activities are identical. Analysis of pathologic plasma samples using laser nephelometry, radial immunodiffusion and radioimmunoassay methods revealed that the functional activity of various serine protease inhibitors is greatly reduced but the reduction in the immunological quantities is minimal. Since coagulation proteins are functional, a ratio between their functional activity and absolute protein levels may be a useful parameter. Employing human and bovine thrombin; bovine and human Xa with their respective substrates, the absolute quantitation of heparin is satisfactorily carried out, however, these assays only measure heparin concentrations and do not reflect the overall anticoagulant effect of heparin. Using the synthetic substrates, the value of measuring absolute concentrations of heparin in a patient on heparin therapy is questionable. With the introduction of fluorogenic substrates, the presence of activated coagulation factors may be demonstrated in patients with thrombotic disorders.(ABSTRACT TRUNCATED AT 400 WORDS)