Association of fibronectin with carboxy-group-modified proteins in vitro

Treatment of human immunoglobulin G, albumin and fibronectin with water-soluble carbodi-imide at pH4.75 in the presence of glycine ethyl ester resulted in an avid binding of ¹²⁵I-labelled native fibrinectin to the modified proteins. Succinoylation, reduction and alkylation or heat-denaturation had no such effect. In affinity chromatography under physiological conditions, serum was depleted of fibronectin when run through columns of the carbodi-imide-treated proteins coupled to agarose. Fractions eluted from such columns with urea were enriched in fibronectin. The binding of radiolabelled fibronectin to the carbodi-imide-treated proteins was inhibited by unlabelled fibronectin in relatively low concentrations, but also by albumin in higher concentrations. Heat-denatured albumin inhibited at concentrations approx. 10–30 times lower than native albumin. The binding reaction had a pH optimum of 6–8. It was inhibited at high ionic strength and in the presence of urea. Anionic detergents inhibited at millimolar concentrations, but non-ionic detergents did not inhibit the binding reaction. The results were interpreted as showing that: (1) fibronectin is capable of binding to itself, to immunoglobulin G and to albumin after a reduction of the negative surface charge of these proteins, and may have a general ability to bind such modified proteins; (2) this binding can take place under physiological conditions; (3) carboxy-group-modified proteins selectively bind fibronectin from serum. This novel binding phenomenon could be important in terms of the opsonin function of circulatory fibronectin. We propose that fibronectin may recognize modified (denatured) proteins and mediate their uptake by the reticuloendothelial system.