Sensitive and Direct Assays for Functionally Active Plasminogen Activators (tissue-type and urokinase-type) in Plasma

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Abstract
A previously described bioimmunoassay for tissue-type plasminogen activator (t-PA) has been modified in terms of antibody concentration and conditions of incubation to achieve a sensitivity range of approximately 5–500 IU/L of t-PA. A similar assay has been developed for urinary-type plasminogen activator (UPA), also known as urokinase (UK), achieving a sensitivity range of approximately 5–500 × 10−1 IU/L. The direct sensitive t-PA assay has been demonstrated to be a quantitative alternative to the traditional semiquantitative euglobulin clot lysis time (ECLT) assay, with correlation coefficients of 0.967 and 0.914, depending on the laboratory where the ECLT was carried out. This assay has the added advantages of avoiding loss of t-PA or UK during the formation of the euglobulin fraction and of calibration in terms of International Standards for both t-PA and UK. With both these assays, it has been demonstrated that the enhanced fibrinolytic activity observed after exercise results from increased levels of t-PA while the level of UK in plasma remains unaltered. Free t-PA (10–60 IU/L) has been demonstrated in resting plasma, whereas the level of free UK activity was 50–100 IU/ L. Venous occlusion of matched groups of subjects with and without deep venous thrombosis (DVT) showed a wide variation of response in t-PA levels in each group and no change in UK levels. There was no difference between the DVT and non-DVT groups, and this suggests that the notion of the “nonresponder” being more prone to thrombosis needs to be reexamined.