COMPLEXES BETWEEN TISSUE-TYPE PLASMINOGEN-ACTIVATOR AND PROTEINASE-INHIBITORS IN HUMAN-PLASMA, IDENTIFIED WITH AN IMMUNORADIOMETRIC ASSAY

Abstract

Extrinsic (tissue-type) plasminogen activator antigen in human plasma, as measured by a 2-site immunoradiometric assay, is composed of a fibrin-adsorbable and a nonadsorbable fraction. Gel filtration on Ultrogel AcA 44 in 1.6 M KSCN of the fibrin-adsorbable fraction showed a peak with MW .simeq. 70,000, which contained plasminogen activator activity and was assumed to represent free extrinsic plasminogen activator. The nonadsorbable fraction showed a broad peak with MW .simeq. 140,000 without plasminogen activator activity. Overnight incubation at 37.degree. C of postexercise plasma revealed a shift of the MW .simeq. 70,000 peak to the MW .simeq. 140,000 position, suggesting that the MW .simeq. 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 and is therefore most probably, at least in part, responsible for the generation of the MW .simeq. 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 [half life] of .apprx. 1 h, 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 2-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. Plasma apparently contains both free extrinsic plasminogen activator and plasminogen activator complexes with .alpha.2-antiplasmin and .alpha.1-antitrypsin. These complexes are also present in plasma collected on the active site inhibitor, D-Phe-Pro-Arg-CH2Cl, at rest and after exercise and are therefore assumed to circulate in vivo.