Certain high molecular weight heparin chains have high affinity for vitronectin

Vitronectin is a 70-kDa protein that is found in both the extracellular matrix as well as serum. Vitronectin is one of the few proteins that regulates both the complement and the coagulation systems. Heparin is known to bind to vitronectin. Review of the literature reveals apparently conflicting outcomes of the interaction of heparin, vitronectin, and the complement system. Previous studies demonstrated that heparin diminishes vitronectin inhibition of complement activity. Numerous studies have also demonstrated that heparin exerts a net inhibitory effect on complement. We used two dimensional affinity resolution electrophoresis (2DARE) to examine this apparent paradox. 2DARE allowed simultaneous determination of binding affinity of heparin for vitronectin as well as the M_r of the heparin species. In the 2DARE experiment, the interaction of heparin with vitronectin caused retardation of the movement of the heparin through the tube gel in the first dimension. The degree of the retardation of movement was used to calculate the approximate K_d of that interaction. The heparin from the tube gel was then subjected to a second dimension electrophoresis to determine the M_r of the heparin. 2DARE analysis of the interaction of heparin with vitronectin clearly demonstrated that a sub-population of heparin chains with M_r > 8000 bound vitronectin with high affinity whereas most high M_r chains and all lower M_r chains showed little to no affinity for vitronectin. Our findings are consistent with the hypothesis that a unique binding domain exists in certain heparin chains for vitronectin.