Kinetic analysis of heparin and glucan sulfates binding to P-selectin and its impact on the general understanding of selectin inhibition

P-Selectin, expressed on activated endothelial cells and platelets, is a high kinetic adhesion receptor involved in leukocyte rolling of the inflammatory response, or in tumor cell binding in the course of metastasis. Thus, P-selectin inhibition is a promising therapeutic target. The anti-inflammatory and anti-metastatic activities of heparin have partly been related to the inhibition of P-selectin binding. Here we apply a quartz crystal microbalance (QCM) biosensor to determine the kinetic constants of heparin and other sulfated polysaccharides binding to immobilized P-selectin. Binding kinetics of the derivatives were correlated with their inhibitory capacity in a P-selectin cell rolling assay. Three commercial heparins differ in cell rolling inhibition and display slightly different affinities (K-D 1.21 x 10(-6) M to 5.86 x 10(-7) M). Inhibitory capacity appears to be mainly driven by a slow off-rate from the receptor (2.27 x 10(-3) s(-1) to 1.23 x 10(-3) s(-1)). To correlate the impact of binding kinetics on inhibitory capacity structurally, we analyzed six semisynthetic glucan sulfates. They display different degrees of sulfation (DS), which has a strong influence on inhibitory activity. Kinetic data illustrate that the inhibitory capacity correlates excellently with the off-rate of these polysaccharides (R = 0.99), while the association (on-rate) affects activity to a lesser extent. In general, the consideration of binding kinetics sheds new light on the mechanism of selectin inhibition. A much slower dissociation of the inhibitors from the receptor than the physiological ligands is key for inhibitory capacity. Structurally, highly charged compounds with a slow off-rate, such as heparin or glucan sulfates, appear as potent candidates for P-selectin inhibition.