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 chain, and of Bβ 1–42 (thrombin-increasable fibrinopeptide B immunoreactivity - TIFPB) which reflect plasmin action on the NH₂-terminal end of the Bβ 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 Trisbuffered saline at pH 7.4 and at 37°C. With fibrinogen as substrate the K_m was calculated to be 0.87 μM and the V_max 3.75 × 10⁻⁵ M/min/unit of plasmin. With fibrin I monomer as the substrate the K_m was calculated to be 1.25 μM and the V_max 5.5 × 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 Bβ chain.