Disulfide-bonded polymerization of plasma fibronectin in the presence of metal ions.

Incubation of human plasma fibronectin in the presence of low concentrations of FeCl3 or CuSO4 led to the formation of disulfide-bonded multimers as revealed by analysis in sodium dodecyl sulfate-polyacrylamide gel electrophoresis under nonreducing or reducing conditions. The polymers induced by FeCl3 did not enter the spacer gel, and those induced by CuSO4 migrated to the top of the running gel, indicating that the former polymers were larger than the latter, which in gel filtration experiments appeared to be larger than Mr 670,000. The polymerization occurred between pH 7 and 9 and more rapidly at 22 or 37 degrees C than at 4 degrees C and was inhibited by metal-chelating reagents. NaCl, heparin, spermine, urea, or guanidine hydrochloride did not appreciably affect the reaction, whereas dithioerythritol enhanced the CuSO4-induced polymerization of fibronectin. When incubated in the presence of FeCl3, the Mr 30,000 NH2-terminal, Mr 40,000 gelatin-binding, and the Mr 120,000-140,000 COOH-terminal fragments of fibronectin formed disulfide-bonded polymers, whereas only the Mr 140,000 fragment was polymerized in the presence of CuSO4. Disulfide-bonded polymers were also formed in the presence of FeCl3 but not CuSO4 when the free sulfhydryl groups of fibronectin were blocked by N-ethylmaleimide. The results suggest that in the presence of CuSO4, disulfide-bonded polymerization of fibronectin may involve predominantly the free sulfhydryl groups, whereas in the presence of FeCl3, also the intramolecular disulfides may exchange to form disulfides between separate fibronectin molecules. Thus, under different conditions, different parts of fibronectin may be susceptible to disulfide-bonded polymerization.