Effect of plasma prekallikrein on the binding of ¹²⁵I-kallikrein to HUVEC-bound HK. After preincubation with HK, HUVEC were incubated with increasing concentrations of plasma prekallikrein in the presence of ¹²⁵I-kallikrein (20 nmol/L). The percentage of specific binding was calculated by obtaining the ratio of the specific binding in the presence of plasma PK to the specific binding in the absence of PK. Nonspecific binding is defined as the binding in the absence of HK and was subtracted from the total binding to yield the specific binding. The results are the mean ± SEM of three separate experiments.