The AH-site of plasminogen and two C-terminal fragments. A weak lysine-binding site preferring ligands not carrying a free carboxylate function.

Glu-plasminogen [native plasminogen (Glu-1-Asn-790)], Lys-plasminogen [plasmin-cleaved fragment of plasminogen (Lys-77-Asn-790)] and miniplasminogen [fragment of plasminogen (Val-440-Asn-790)] were all found to interact specifically with immobilized 6-aminohexyl ligands. The interactions apparently are mediated by a single weak lysine-binding site, termed the AH-site, as seen from the patterns of inhibition obtained from frontal-quantitative-affinity-chromatography experiments with 6-aminohexanoic acid and alpha-N-acetyl-L-lysine methyl ester as competing ligands. The AH-site, in contrast with the strong lysine-binding site of Glu-plasminogen and Lys-plasminogen, may prefer ligands not carrying a free carboxylate function and therefore may interact with lysine side chains of proteins. In Glu-plasminogen the AH-site is present, but is apparently only partially free to react. It is suggested that it participates in an intramolecular complex and that an equilibrium state between two Glu-plasminogen forms exists. It is further suggested that binding of the plasminogens to fibrin is mainly determined by the AH-site.