Tissue plasminogen activator in human plasma measured by radioimmunoassay

A radioimmunoassay (RIA) has been set up using purified human melanoma tissue plasminogen activator and a specific antiserum raised against it. Concentration of antigen in citrated plasma of resting human subjects was found to be 7.1 +/- 1.6 ng/ml (mean +/- S.D.) while the lower limit of sensitivity of the RIA was 2 ng/ml. After venous occlusion or DDAVP infusion the levels of antigen were invariably elevated and a significant increase in the proportion of antigen having fibrin binding properties was observed. Decay studies in vitro and gel filtration of post-stimulus plasma indicated that the RIA detects active and inactive forms of antigen and the possible nature of the inactive antigen is discussed.