Isolation, Characterization, and Location of a Donor-Acceptor Unit from Cross-Linked Fibrin^*

Abstract

The cross-linking systems of bovine and human fibrins were studied by the introduction of a radioactive substitute donor as an inhibitor of fibrin cross-linking, separation of the constituent polypeptide chains after sulfitolysis, and tryptic digestion of the labeled γ-chains. The information gathered from this approach enabled us to isolate and characterize the complete donor-acceptor unit in tryptic digests of fibrin γ-γ cross-linked systems. In both bovine and human fibrin, this kind of cross-linking is accomplished by reciprocal bridging between overlapping carboxy-terminal segments of neighboring γ-chains. The amino acid sequence of the carboxy-terminal heptadecapeptide of the bovine γ-chain was determined and an alignment of the corresponding region of the human γ-chain established.