Amino acid sequences of lamprey fibrinopeptides A and B and characterizations of the junctions split by lamprey and mammalian thrombins.

The amino acid sequences of the fibrinopeptides A and B from lamprey fibrinogen have been determined. The fibrinopeptide A is the shortest fibrinopeptide ever isolated, being comprised of only six amino acids. The fibrinopeptide B, on the other hand, is the largest fibrinopeptide characterized to date, having 36 amino acid residues and a cluster of covalently bound carbohydrate. As reported previously, lamprey fibrinogen is readily clotted by mammalian thrombins, but only the fibrinopeptide B is released during the process. Lamprey fibrinopeptide A is not released by mammalian thrombins and could only be removed with the use of lamprey thrombin. Firm proof that the lamprey fibrinopeptides A and B are the amino segments of the alpha and beta-chains respectively was obtained by a series of stepwise degradations on lamprey fibrinogen and lamprey fibrins produced in turn by the action of mammalian thrombin (fibrin B) and lamprey thrombin (fibrin A). These studies were supplemented by stepwise degradations on the individual Aalpha and Bbeta-chains. It the case of the lamprey Aalpha-chain it was also possible to release the 6-residue fibrinopeptide A from the isolated chain with lamprey thrombin and demonstrate that the newly exposed amino-terminal sequence begins with the Gly-Pro-Arg sequence characteristic of mammalian fibrin alpha-chains. In fact, the sequences on the fibrin side of both of the junctions split by thrombin(s) are highly conserved and virtually identical with those found in mammalian alpha and beta-chains.