One free sulfhydryl group of plasma fibronectin becomes titratable upon binding of the protein to solid substrates.

The accessibility in human plasma fibronectin of the two free sulfhydryl groups per chain to sulfhydryl reagents 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) and a maleimide derivative has been examined. For soluble fibronectin, the free sulfhydryl groups are not accessible to DTNB unless urea or guanidine hydrochloride is added [Smith et al. (1982) J. Biol. Chem. 257, 5831-5838]. Upon binding to polystyrene beads, 0.87 +/- 0.05 sulfhydryl group per chain becomes titratable to DTNB. Experiments using fibronectin fragments demonstrate that this newly exposed sulfhydryl group is located in a Type III homologous unit between the DNA-binding and the cell-binding domains. The results suggest that, upon adsorption to solid substrates, plasma fibronectin undergoes a conformational change, thereby exposing one buried sulfhydryl group. These findings have implications regarding the "surface activation" of adhesion activities of fibronectin.