The release of B beta 1-42 from fibrinogen and fibrin by plasmin

The balance between thrombin and plasmin action has been postulated to be an important determinant of thrombosis. Measurement of plasma concentrations of fibrinopeptide A (FPA), which reflect thrombin action on the NH2-terminal end of the A alpha chain, and of B beta 1-42 (thrombin-increasable fibrinopeptide B immunoreactivity-TIFPB) which reflect plasmin action on the NH2-terminal end of the B beta chain have shown systematic changes in the relative concentrations of the two peptides in thrombotic states. This paper reports kinetic data for TIFPB release by plasmin using fibrinogen, fibrin I monomer, and fibrin I polymer as substrates. For fibrinogen and fibrin I monomer the data fit the Michaelis-Menten equation. Experiments were performed with human proteins in 0.15M Tris-buffered saline at pH 7.4 and at 37 degrees C. With fibrinogen as substrate the Km was calculated to be 0.87 microM and the Vmax 3.75 X 10(-5) M/min/unit of plasmin. With fibrin I monomer as the substrate the Km was calculated to be 1.25 microM and the Vmax 5.5 X 10(-5) M/min/unit of plasmin. With fibrin I polymer as substrate the data did not fit the Michaelis-Menten equation but there appeared to be no dramatic differences in rates from those obtained with the other two substrates. The influence of factor XIIIa-induced cross-linking of fibrin was not examined. It is concluded from these findings that fibrinogen and non-cross-linked fibrin I are equally good substrates for plasmin cleavage of the NH2-terminal end of the B beta chain.