Initial interaction between fibrin and tissue plasminogen activator (t-PA). The Gly-Pro-Arg-Pro binding site on fibrin(ogen) is important for t-PA activity.

Gly-Pro-Arg-Pro (GPRP) is a potent inhibitor of fibrin polymerization. It also causes a concentration-dependent inhibition of the activation of plasminogen by t-PA when soluble desAABB-fibrinogen (fibrin II) or fibrinogen are used as promoters of t-PA. Fibrinogen has a much weaker promoter activity than does fibrin II. Experiments were undertaken to explain the mechanism of action of GPRP on plasminogen activation. Kinetic data indicated that when GPRP was present in the assay system, the fibrin(ogen)-GPRP complex was ineffective as a t-PA promoter. Only the free forms of fibrin II or fibrinogen were promoters of t-PA. GPRP specifically eluted t-PA, which was previously bound to a fibrin-Sepharose column, and also inhibited t-PA binding to fibrin-Sepharose in a concentration-dependent manner. These experiments were compared to those with other tetrapeptides: GHRP, GPGG, and GRGD. Only GHRP, which is known to bind more weakly to fibrin(ogen) than GPRP, had any effect. These results suggest that the GPRP binding site on the fibrin(ogen) molecule is important for t-PA activation and is a binding site for t-PA in the initial interaction between t-PA and fibrin. We propose that at or near the GPRP binding site on fibrin there is an initial binding site for t-PA. We hypothesize that kringle 1, which contains the sequence G128RRP, may be involved in the initial binding of t-PA to fibrin.