Specificity of biotin-PK binding to HUVECs in the absence of added HK. (A) PK and biotin-PK compete for binding to HUVECs. HUVECs were incubated with 40 nmol/L biotin-PK and 10 to 2,000 nmol/L PK in the absence of added HK. Binding of biotin-PK in the presence of PK is expressed as a percentage of its binding in the absence of PK. The data shown are the mean ± SEM of three experiments. (B) Binding of 40 nmol/L biotin-PK to HUVECs was measured in the absence of added HK but in the presence of increasing concentrations of the peptide SDD31, which corresponds to the PK binding site on HK. The data shown are the mean ± SEM of three experiments. (C) The effect of MoAbs to HK and PK on the binding of biotin-PK to HUVECs in the absence of added HK. Binding of biotin-PK to HUVECs was measured in the presence of 1- to 10-fold molar excess of the MoAbs, HKL13 (□), HKL16 (⋄), and PK6 (○). Binding of biotin-PK in the presence of the antibodies is expressed as percent of the binding in their absence. The data presented are the mean ± SEM of three experiments.