Alteration of vitronectin. Characterization of changes induced by treatment with urea

Vitronectin circulates in blood as a 70-kDa monomer that interacts with complexes generated in the terminal steps of the coagulation and complement cascades. Vitronectin complexed to thrombin-antithrombin or C5b-9 is conformationally altered as evidenced by enhanced reactivity with monoclonal antibody 8E6 and binding to heparin; these same alterations also occur when vitronectin is treated with urea (Tomasini, B. R., and Mosher, D.F. (1988) Blood 72, 903-912; Høgåsenk, K., Molnes, T.E., and Harboe, M. (1992) J. Biol. Chem. 267, 23076-23082). We have modified the purifications of native and urea-treated vitronectin to better control conformational state and characterized the alterations induced by urea. After treatment with N-ethylmaleimide to prevent formation of disulfide-linked multimers, purification in 8 M urea, and dialysis against physiological saline, vitronectin was largely oligomeric (approximately 800 kDa) as assessed by gel filtration and polyacrylamide gel electrophoresis in the absence of sodium dodecyl sulfate. Oligomeric urea-treated vitronectin reacted more strongly with the 8E6 antibody, bound biotinylated heparin more strongly, and neutralized the anticoagulant activity of heparin better than monomeric altered vitronectin or native vitronectin. After incubation with urea at 25 degrees C, native vitronectin, treated during purification with dithionitrobenzoic acid to force free sulfhydryls to intramolecular disulfides, exhibited increased reactivity with antibody 8E6, increased binding to heparin, and oligomerization. In addition, incubation in urea caused rearrangement of disulfides as assessed by loss of the light chain of two-chain vitronectin. The transition for these effects occurred between 2 and 4 M urea. Thus, an irreversible conformational alteration occurs upon treatment of vitronectin with urea, resulting in oligomers that bind avidly to heparin.