Role of y-Carboxyglutamic Acid Residues in the Binding of Factor IXa to Platelets and in Factor-X Activation

To study the requirements for factor-lXa binding to platelets and factor-)< activation, we examined the consequences of chemical modification (factor lX,,,) or enzymatic removal (factor lXDEs) of ycarboxyglutamic acid (Gla) residues. In the presence of factor Vllla and factor X, there were 344 (552) binding sites/ platelet for factor IXa, (apparent dissociation constant [kd,,,] = 4.5 f 0.9 nmol/L) and 275 (+35) sites/ platelet for factor lXaDEs (kd,pp = 5.0 f 0.8 nmol/L) compared with 580 (265) sites/platelet for normal factor IXa (factor IXa,) (kd,,, = 0.61 5 0.1 nmol/L) and 300 (k62) sites/ platelet for factor IX (kd.,, = 2.9 k 0.29 nmol/L). The concentrations of factor IXa,, factor lXaMoD and factor IXa,,, required for half-maximal rates of factor-Xa formation were 0.67 nmol/L, 3.5 nmol/L, and 6.7 nmol/L. Whereas maximal