Effect of HK25c on the activation of PK to kallikrein in the presence of HK and HUVEC. HUVEC were incubated with HK (70 nmol/L) for 60 minutes at 4°C and the residual HK was removed. Prekallikrein (20 nmol/L), HK25c, and S2302 (0.37 mmol/L) were added and incubated at 23°C for 24 minutes. Kallikrein activity is expressed as nanomoles of substrate S2302 hydrolyzed per minute (mean ± SEM of 3 separate experiments).