Inhibition of Fibronectin Binding to Matrix Components by Nonenzymatic Glycosylation

We examined the effect of nonenzymatic glycosylation on the ability of fibronectin, an extracellular glycoprotein that interacts with cell surfaces and matrix components, to bind to collagen and heparin. Nonenzymatic glycosylation was accomplished by incubation of the protein with glucose, both cold and [¹⁴C]-labeled, and documented by measurement of ketoamine-bound carbohydrate with the thiobarbituric acid assay. Effect on binding was assessed with affinity chromatography on heparin-Sepharose and gelatin-Sepharose, and with an in vitro assay that detects complexation of fibronectin with [³H]-heparin. Glycosylated fibronectin did not bind to these immobilized matrix components, and in vitro binding of the glycosylated protein was reduced compared with that of nonglycosylated fibronectin. Inhibition of heparin binding in the in vitro assay was observed even with levels of glycosylation about threefold those of control, which is comparable to the degree of glycosylation determined in fibronectin isolated from plasma of two patients with uncontrolled diabetes. The findings indicate that nonenzymatic glycosylation of fibronectin inhibits its binding to connective tissue components, and suggest that this process contributes to faulty integrity of extracellular matrices in diabetes.