Immunoradiometric quantitation of tissue plasminogen activator-related antigen in human plasma: crypticity phenomenon and relationship to plasma fibrinolysis

A two-site immunoradiometric assay for tissue plasminogen activator (tPA) antigen has been developed using immunoaffinity purified antibody. Various treatments enhanced the detection of tPA antigen in the plasma samples. Maximum detection was obtained by acidification of plasma to pH 4.8 to 6.5 or addition of 0.5 mol/L of L-lysine or L- arginine. Acidification or addition of lysine to plasma is also required for maximum immunoadsorption of plasma tPA antigen on anti-tPA- Ig-sepharose. These results indicate that plasma tPA antigen is partially cryptic to antibody in untreated plasma. The plasma tPA antigen isolated by immunoadsorption of either untreated plasma or acidified plasma on anti-tPA-Ig-sepharose consists mainly of a 100-kd plasminogen activator species as determined by fibrin-agar zymography. The 100-kd activity is possibly a tPA:inhibitor complex. A standardized sample preparation method was conveniently adopted by mixing 3 vol of plasma and 1 vol of 2 mol/L of L-lysine for the assay. Reconstitution and recovery studies showed that the method is specific and permits full detection of both free tPA and tPA:inhibitor complex. The validity of the assay is further supported by the finding that the spontaneous plasma fibrinolysis previously demonstrated to be dependent on plasma tPA antigen is correlated with tPA antigen content. Using the standardized assay, we found that tPA antigen concentrations in 16 blood bank plasmas are equivalent to 3.7 to 20 ng of 60 kd tPA/mL. In all the plasma tested, more than half of the antigen is undetected unless the plasma is treated as described above.