Subunit structure of human fibrinogen, soluble fibrin, and cross-linked insoluble fibrin.

The three unique polypeptide chains of human fibrinogen differ significantly in molecular weight. Cross-linkage of fibrin by fibrin-stabilizing factor results in the rapid formation of cross-links between gamma-chains and a slower formation of cross-links between alpha-chains. beta-Chains are not involved directly in the cross-linking of fibrin. Reduced, cross-linked fibrin contains uncross-linked beta-chains, dimers of gamma-chain, and higher polymers of alpha-chain. Although it is uncertain whether the gamma-gamma dimers are formed by chains in different molecules of fibrin, the polymers of alpha-chain in fibrin can only be accounted for by cross-linkage of alpha-chains in different molecules. The nature of cross-linkage among the subunits in fibrin can account well for the three-dimensional, covalent structure of cross-linked, insoluble fibrin.