Complexes between tissue-type plasminogen activator and proteinase inhibitors in human plasma, identified with an immunoradiometric assay.

Extrinsic (tissue-type) plasminogen activator antigen in human plasma, as measured by a two-site immunoradiometric assay, is composed of a fibrin-adsorbable and a nonadsorbable fraction. Gel filtration on Ultrogel AcA 44 in 1.6M KSCN of the fibrin-adsorbable fraction showed a peak with Mr congruent to 70,000, which contained plasminogen activator activity and was assumed to represent free extrinsic plasminogen activator. The nonadsorbable fraction showed a broad peak with Mr congruent to 140,000 without plasminogen activator activity. Overnight incubation at 37 degrees C of postexercise plasma revealed a shift of the Mr congruent to 70,000 peak to the Mr congruent to 140,000 position, suggesting that the Mr congruent to 140,000 peak consists of extrinsic plasminogen activator-protease inhibitor complex(es). alpha 2-Antiplasmin is the main inhibitor of extrinsic plasminogen activator in plasma 13 and is therefore most probably at least in part responsible for the generation of the Mr congruent to 140,000 component. A possible involvement of other plasma proteinase inhibitors was explored by incubation of 125I-labeled extrinsic plasminogen activator in alpha 2-antiplasmin-depleted plasma. A complex was formed with a t1/2 of about 1 hr, which was identified by immunoprecipitation as extrinsic plasminogen activator-alpha 1-antitrypsin complex. Additional evidence for the presence of extrinsic plasminogen activator complexes with alpha 2-antiplasmin and alpha 1-antitrypsin in plasma was obtained from two-site immunoradiometric assays, in which solid-phase anti-inhibitor antibody bound the corresponding complex, which was then detected with radiolabeled, affinospecific antibody against extrinsic plasminogen activator. It was concluded that plasma contains both free extrinsic plasminogen activator and plasminogen activator complexes with alpha 2-antiplasmin and alpha 2-antitrypsin. These complexes are also present in plasma collected on the active site inhibitor, D-Phe-Pro-Arg-CH 2Cl, at rest and after exercise and are therefore assumed to circulate in vivo.