FUNCTION OF STREPTOKINASE FRAGMENTS IN PLASMINOGEN ACTIVATION

Several peptide fragments of streptokinase (SK) were prepared by incub ating SK with immobilized human plasmin (hPlm) and purified by h.p.l.c . with a reverse-phase phenyl column. The N-terminal sequences, amino acid compositions and molecular masses of these peptide fragments were determined. The SK peptide fragment of 36 kDa consisting of Ser(60)-L ys(387) (SK-p), was the only peptide fragment that could be tightly bo und to immobilized hPlm. Another three large SK peptide fragments, SK- m, SK-n and SK-o, with molecular masses of 7 kDa, 18 kDa and 30 kDa, a nd consisting of Ile(1)-Lys(59), Glu(148)-Lys(333), Ser(60)-Lys(333) r espectively, were also obtained from the supernatant of the reaction m ixture. The purified SK-p had high affinity with hPlm and could activa te human plasminogen (hPlg) with a k(Plg) one-sixth that of the native SK. SK-o had low affinity with hPlm and could also activate hPlg, alt hough the catalytic constant was less than 1% of the native SK. SK-n, as well as SK-m, which is the N-terminal 59 amino acid peptide of the native SK, had no activator activity. However, SK-m could enhance the activator activity of both SK-o and SK-p and increase their second-ord er rate constants by two- and six-fold respectively. It was concluded from these studies that (1) SK-o, the Ser(60)-Lys(333) peptide of SK, was essential for minimal SK activator activity, (2) the C-terminal pe ptide of SK-p, Ala(334)-Lys(387), was essential for high affinity with hPlm, and (3) the N-terminal 59-amino-acid peptide was important in m aintaining the proper conformation of SK to have its full activator ac tivity.