Homocysteine binds to human plasma fibronectin and inhibits its interaction with fibrin

Objective-More than 70% of circulating homocysteine is disulfide-bonded to protein, but little is known about the specific proteins that bind homocysteine and their function as a consequence of homocysteine binding. Methods and Results-When human plasma was incubated with [S-35]L-homocysteine, most of the homocysteine bound to albumin. However, additional homocysteine-binding proteins were detected, and 1 of them comigrated with fibronectin. Treatment with 2-mercaptoethanol removed the bound homocysteine, demonstrating the involvement of disulfide bonding. In contrast, [S-35]L-cysteine did not bind to fibronectin. Purified fibronectin bound approximate to5 homocysteine molecules per fibronectin dimer. SDS-PAGE of a limited trypsin digestion of homocysteinylated fibronectin showed that several tryptic fragments contained [S-35]homocysteine. Sequence analysis demonstrated that the fragments containing bound homocysteine had localized mainly to the C-terminal region, within and adjacent to the fibrin-binding domain. Homocysteinylation of fibronectin significantly inhibited its capacity to bind fibrin by 62% (P<0.005). In contrast, neither the binding of fibronectin to gelatin nor its capacity to serve as an attachment factor for aortic smooth muscle cells was affected. Conclusions-These results suggest that homocysteine may alter normal thrombosis and delay or interfere with wound healing by impairing the interaction of fibronectin with fibrin.