Primary Structure of Peptides Released during Activation of Human Plasminogen by Urokinase

Activation of human plasminogen by urokinase is a two‐stage process. Initially peptide material is released from the NH 2 ‐terminal part of the proenzyme during formation of an inactive intermediate compound, from which plasmin is formed in a second step by proteolytic cleavage of an internal peptide bond. The released peptide material is shown to consist of two peptides, API and APII, containing 63 and 5 amino acid residues, respectively. The amino acid sequences of these peptides, and of two cyanogen bromide fragments situated in the NH 2 ‐terminal part of the plasminogen molecule (CNBr II and CNBr III), have been determined. From these results it is shown that API and APII, in intact plasminogen, occupy the positions 1–63 and 64–68 respectively. The residue in position 69 is shown to be methionine, which also is the NH 2 ‐terminal amino acid found in the intermediate compound formed during activation.