Fibrin and plasminogen structures essential to stimulation of plasmin formation by tissue-type plasminogen activator

Abstract

Plasminogen activation catalysed by tissue-type plasminogen activator (t-PA) has been examined in the course of concomitant fibrin formation and degradation. Plasmin generation has been measured by the spectrophotometric method of Petersen et al. (Biochem. J. 225 (1985) 149–158), modified so as to allow for light scattering caused by polymerized fibrin. Glu 1-, Lys 77- and Val 442-plasminogen are activated in the presence of fibrinogen, des A- and des AB-fibrin and the rate of plasmin formation is found to be greatly enhanced by both des A- and des AB-fibrin polmer. Plasmin formation from Glu 1- and Lys 77-plasminogen yields a sigmoidal curve, whereas a linear increase is obtained with Val 442-plasminogen. The rate of plasmin formation from Glu 1- and Lys 77-plasminogen declines in parallel with decreasing turbidity of the fibrin polymer effector. In order to study the effect of polymerization, this has been inhibited (1) by the synthetic polymerization site analogue Gly-Pro-Arg-Pro, (2) by fibrinogen fragment D 1 or (3) by prior methylene blue-dependent protooxidation of the fibrinogen used. Inhibition of polymerization by Gly-Pro-Arg-Pro reduces plasmin generation to the low rate observed in the presence of fibrinogen. Antipolymerization with fragment D 1 or photooxidation has the same effect on Glu 1-plasminogen activation, but only partially reduces and delays the stimulatory effect on Lys 77- and Val 442-plasminogen activation. The results suggest that protofibril formation (and probably also gelation) of fibrin following fibrinopeptide release is essential to its stimulatory effect. The gradual increase and subsequent decline in the rate of plasmin formation from Glu 1- or Lys 77-plasminogen during fibrinolysis may be explained by sequential exposure, modification and destruction of different t-PA and plasminogen binding sites in fibrin polymer.