SummaryPolyclonal antibodies have been raised against the inhibitor moiety in the purified complex between tissue plasminogen activator and its fast inhibitor (PA-inhibitor) in human plasma/ serum. A radioimmunoassay for quantitation of PA-inhibitor antigen was developed. The polyclonal antiserum and a previously described monoclonal antibody against the PA-inhibitor (14) have been used to study the immunological relationship between PA-inhibitors from plasma, serum, platelets, placenta extract and conditioned media from Hep G2 and HT 1080 cells. It was demonstrated that the ratio between PA-inhibitor activity and antigen varied considerably between the different sources. In the plasma samples studied, similar activity and antigen concentrations were found, suggesting that the PA-inhibitor in these samples mainly was in an active form. On the other hand the other sources seemed to contain variable amounts of inactive PA-inhibitor forms. Immunoadsorption experiments revealed that the PA-inhibitor (activity and antigen) from all the sources were specifically bound to the insolubilized antibodies (polyclonal and monoclonal). In no case, however, could active PA-inhibitor be eluted from the immunoadsorption columns. Also the competitive radioimmunoassays suggested that the PA-inhibitors from the different sources studied, were closely immunologically related.
