Competition for factor IXa–binding sites by lactadherin.

The active site histidine of factor IXa was derivatized with fluorescein-Glu-Gly-Arg chloromethyl ketone as described in “Materials and methods.” First, 4 nM fluorescein-labeled factor IXa was mixed with various concentrations of lactadherin (●) or unlabeled factor IXa (▴) in the presence of 5 mM Ca⁺⁺ prior to the addition of lipospheres and evaluation of bound fluorescein-labeled factor IXa as described for Figure 1. Lactadherin and unlabeled factor IXa both competed with labeled factor IXa for membrane-binding sites. Results are displayed with the abscissa on a log scale to enable direct comparison. Data displayed are representative of 2 experiments.