Iiheparin Binding To Platelets

Appreciation of the functional effects of heparin on platelets led to studies of heparin binding to these cells, which was found to be specific and saturable (Sobel and Adelman, 1988; Horne, 1988; Horne and Chao, 1989). The negative charge density of the ligand (heparin) largely determines its binding specificity (Horne, 1988; Horne and Chao, 1990). Polysaccharide molecules with various primary structures can displace heparin from platelets if they are sufficiently charged (Horne, 1988; Greinacher et al., 1993) (Table 1). The identity of the platelet-binding site(s), which provides a complementary positive charge, is uncertain. One report indicates that glycoprotein IIb/IIIa (integrin aupb3) contains a heparin-binding site (Sobel et al., 2001), but this is inconsistent with other studies (Horne, 1988, 1991).

Next to negative charge density, molecular size has the greatest effect on polysaccharide binding to platelets. Heparin molecular weight, for example, affects both its platelet-binding affinity and capacity (Horne and Chao, 1990). Since medicinal heparin is a mixture of molecules varying in mass from about 4000 to about 30,000 Da, the mass of a mole of heparin (i.e., approximately 6 X 1023 molecules) depends upon the mean size of the molecules in the sample. The maximum number of molecules bound per platelet is approximately the same for heparin species with molecular weights between about 5000 and 15,000 Da (Table 1). However, larger molecules bring more glycosaminoglycan (GAG) mass to the platelet surface than smaller molecules (Fig. 1). Therefore, when heparin-binding capacity is expressed in terms of mass rather than moles or molecules, the capacity of larger heparins is greater than that of smaller heparins.

TABLE 1 Platelet-Binding Parameters for Heparin Fractions of Different Molecular Mass

Heparin Mr range (Da)

Sulfate/carboxylate (mol/mol)

Dissociation constant

Binding capacity

(mg/L)

(nM)

(mg/1015 cells)

(molecules/cell)

14,000-16,000 9500-10,500 4500-5500 2700-3300

2.0 ± 0.29a 1.8 ± 0.26 1.9 ± 0.15 1.7 ± 0.25

4.6 ± 1.1 3.9 ± 2.1 3.2 ± 1.0 4.0 ± 2.0

310 ± 73 390 ± 210 640 ± 200 1300 ± 650

66 ± 2.5 56 ± 8.4 23 ± 5.7 10 ± 5.4

2600 ± 100 3400 ± 500 2800 ± 680 2000 ± 1100

aValues are means ±1 standard deviation. Source: Horne and Chao, 1990.

aValues are means ±1 standard deviation. Source: Horne and Chao, 1990.

Similar distinctions apply to the parameters of binding affinity. Longer heparin molecules contain more potential platelet-binding domains than shorter molecules. Therefore, a large heparin species can half-saturate platelets at a lower molar concentration (Kd) than a smaller heparin species, although the concentration of heparin platelet-binding domains in the suspension is the same for both species at half-saturation (Horne and Chao, 1990) (Fig. 1).

Heparin Mr

Platelet-binding domains per molecule

Bound Heparin Binding Capacity

0 0

Post a comment