Table 3.

Inhibition of factor X activation by intrinsic Xase by factor X variants

Kinetic constants were obtained from initial velocity studies conducted with varying concentrations of factor X with different fixed concentrations of the indicated variant. Studies were done using 14 different concentrations of factor X (0–0.3 μm), five different fixed concentrations of the indicated variant (0, 0.1, 0.3, 0.5, and 1 μm), and intrinsic Xase assembled using 0.1 nm IXa, 15 nm VIIIa, and 30 μm PCPS ([E] = 0.1 nm).

Inhibitor	Inhibition typea	V/Eb	Km	Ki
		s−1	nm	nm
XS195A	Competitive	1.95 ± 0.04	60.4 ± 3.8	118 ± 7
XS195A	Competitive	2.98 ± 0.06	96.3 ± 5.2	347 ± 20
XR15Q	Competitive	2.18 ± 0.04	60.4 ± 3	38.1 ± 1.6
XR15Q	Competitive	2.80 ± 0.05	109.4 ± 5.2	114.9 ± 4.4

^aOther forms of reversible inhibition were eliminated by analysis according to alternate rate expressions as described “under Data Analysis”.

^bKinetic constants are reported ± 95% confidence limits.