Binding of PK to HUVEC. (A) The effect of HK and Zn⁺² on biotin-PK binding to HUVECs. HUVECs were incubated with 20 nmol/L HK (□) for 1 hour at 37°C in the presence of 50 μmol/L Zn⁺². Unbound HK was removed and biotin-PK (20 nmol/L) was added in HEPES-Tyrode's binding buffer containing Zn⁺² for variable times. In other wells, HUVECs were incubated with biotin-PK (20 nmol/L) for variable times at 37°C in the absence of HK, but in the presence (⋄) or absence (○) of Zn⁺². The specific binding of biotin-PK bound is shown. The data presented represent the mean ± SEM of three experiments. (B) Binding of ¹²⁵I-PK (20 nmol/L) to HUVECs in suspension in HEPES-Tyrode's binding buffer containing 50 μmol/L Zn⁺² in the absence (total binding; □) or presence (nonspecific binding; ⋄) of 50-fold molar excess PK. The points presented represent the mean ± SEM of three experiments. The absence of standard error bars at some points indicates that the variation was too low to indicate visually.