Transmembrane voltage regulates binding of annexin V and lactadherin to cells with exposed phosphatidylserine

BMC Biochemistry

Table 1 Transmembrane voltage regulates annexin V binding to phospholipid vesicles.

[K⁺] outside (mM)	[K⁺] inside (mM)	Valino-mycin	Number of assays	EC₅₀ (mM)	Slope	pK_d (experimental)	pK_{d, app} at 1.25 mM Ca²⁺ (theoretical)
100	0	-	10	0.113 ± 0.004	8.7 ± 0.2	41.5 ± 1.0	16.2
100	0	+	11	0.116 ± 0.005	10.1 ± 0.4	46.6 ± 1.5	17.2
					Difference:	+5.1*	+1.0
1	100	-	4	0.135 ± 0.003	9.2 ± 0.2	42.5 ± 0.7	15.8
1	100	+	3	0.132 ± 0.004	8.1± 0.1	38.5 ± 0.3	15.0
					Difference:	-4.0*	-0.8

Binding of fluorescein-annexin V-117 to phospholipid vesicles containing 25% PS was measured by calcium titration at low membrane occupancy [15] and binding parameters EC₅₀, slope, and pK_d (log of binding affinity) were determined as described under Methods. A positive change in pK_d indicates higher-affinity binding. Assays were performed with the indicated concentrations of KCl outside and inside the vesicles; a transmembrane diffusion potential was created by the addition of 1 μM valinomycin, a potassium-selective ionophore. Uncertainties on experimental parameters are given as SEM; asterisks indicate differences that are statistically significant (p < 0.02) by two-tailed t test. The theoretical apparent pK_d at 1.25 mM calcium chloride was calculated from Equation 5.

ISSN: 1471-2091