Summary
The levels of fibrinogen and of Profibrinolysin (plasminogen) in urokinase-treated
plasma as a function of time of incubation were measured. The Profibrinolysin concentration
was estimated through its complete conversion to fibrinolysin and the inhibition of
the enzyme by crystalline soybean trypsin inhibitor. The dissociation constant of
the FL-STX complex was determined to be 7 × 10–9 M. The average concentration of Profibrinolysin in normal human citrated plasma was
found to be 8 × 10–7 M. From the decrease of fibrinogen with time in the urokinase-treated plasma, the
free fibrinolysin was calculated. Free fibrinolysin in normal human blood in vivo was estimated from the half-life of fibrinogen and other data obtained in this study
to be present at a concentration of 1.7 × 10–10 M. The plasmakinase activity in vivo, expressed as urokinase molarity, is also about 2 × 10–10 M.
