Subunit structure of fragment D from fibrinogen and cross-linked fibrin.

The different species of Fragment D were purified from plasmin digests of either fibrinogen or highly cross-linked fibrin and examined electrophoretically in sodium dodecyl sulfate on polyacrylamide gels before and after reduction by ,&mercaptoethanol. It was found that Fragment D from cross-linked fibrin is approximately twice the size of that from fibrinogen, which indicates that the COOH-terminal cross-links in the y chain remnants are not cleaved during fibrin digestion by plasmin. The first Fragment D to appear in plasmin digests of fibrinogen is designated Fragment D1 which is composed of a /3” chain (44,500 molecular weight), a y’ chain (42,000 molecular weight), and an (Y” chain (-15,000 molecular weight) linked together by disulfide bonds. Fragment D from fibrin differs in that it has a y’ dimer (81,000 molecular weight) in place of y’ chains and that it contains two each of the o(’ and 0” chains. Further digestion of Fragment D, from fibrinogen results in cleavage of the y’ chain to give first the y” chain (37,000 molecular weight) and finally the 7”’ chain (27,000 molecular weight). Neither the y” chain nor the y”’ chain contained the COOHterminal cross-links found in the intact y chain or the y’ chain. It was possible to show by sodium dodecyl sulfate polyacrylamide gel electrophoresis that fibrinogen Fragment D1 could be cross-linked in the presence or absence of native fibrinogen through the formation of y-y’ or y’y’ dimers.