Further studies of plasminogen proactivator

Abstract

A proactivator for human plasminogen was isolated from the euglobulin fraction of human plasma. Highly purified, the proactivator shows two peaks on gel filtration, through Sephadex G-200 (both peaks are in the macroglobulin fraction), or through Biogel A-50m. The proactivator forms precipitates at 4°C overnight in Tris-HCl solution. The supernatant shows one peak in a later fraction in Biogel. Concentration of proteins in this peak and its elution through Biogel A-1.5m results in one peak for protein and proactivator activity. The proactivator loses its activity in 5 M urea, suggesting that further decomposition of the molecule results in loss of its activity. The molecular weight was calculated to be 300 000 from the elution volume in Biogel A-1.5m fractionation. The proactivator migrates in the α 2-macroglobulin region in electrophoresis, but does not cross-react with anti-α 2-macroglobulin or anti-β-lipoprotein. Ultracentrifugal analyses indicate that the proactivator has a sedimentation rate of 11.7 S ( s 20, w 15.6), and is very homogeneous. Sedimentation equilibrium analyses indicated a molecular weight of 275 000 ± 25 000. The proactivator activity of the human euglobulin fraction was demonstrated, and the yield of the purified proactivator was calculated from the activity of the starting euglobulin fraction. The proactivator has no effect on the activation of human plasminogen by urokinase.