The effect of HK25c on the activation of plasminogen. HUVEC were incubated with HK (70 nmol/L) for 60 minutes at 4°C and then the residual HK was removed. PK (2.5 nmol/L), HK25c, plasminogen (60 nmol/L), prourokinase (4.5 nmol/L), and H-D-Val-Leu-Lys-amino methyl coumarin (50 nmol/L) were added in the presence of HEPES buffer, pH 7.0, and Zn²⁺ (50 μmol/L) and incubated for 170 minutes at 23°C. The plasmin activity is represented as nanomoles of substrate hydrolyzed per minute (mean ± SEM of 3 separate experiments).