Stabilization of plasmin by lysine derivatives

Plasmin is a serine protease with trypsin-like specificity and is activated from plasminogen by several plasminogen activators. Since plasmin has lysine binding site in its heavy chain, lysine derivatives react with plasmin and then modify its activity. The effects of lysine derivatives such as epsilon-aminocaproic acid (EACA) and tranexamic acid on bovine plasmin activity were investigated. In the absence of lysine derivatives, the bovine plasmin activity which was evaluated as the amidolytic activity was reduced in a time- or temperature-dependent manner. However, the bovine plasmin activity became stable upon adding EACA or tranexamic acid. When plasmin was incubated at 4 degrees C for 1, 3 or 5 days without lysine derivatives, the plasmin activity decreased to 43.9%, 19.9% and 11.9% of the initial activity, respectively. On the other hand, when plasmin was incubated at 37 degrees C for 1, 3 or 5 days with tranexamic acid, its activity remained at 110%, 95.6% and 85.9%, respectively. After bovine plasmin had been incubated for 5 days at 4 degrees C in the absence of tranexamic acid, the plasmin activity declined to less than 20%. However, when bovine plasmin had been incubated for 5 days at 37 degrees C in the presence of tranexamic acid, the residual plasmin activity was more than 80%. A similar effect of EACA on bovine plasmin was observed, but it was weaker than that of tranexamic acid. Reversed-phase HPLC followed by SDS-PAGE demonstrated that bovine plasmin was degraded into several fragments. Amino acid sequencing of these fragments revealed that the Lys77-Arg78 or Arg78-Ile79, Arg342-Met343 and Arg557-Ile558 peptide bonds in the bovine plasmin molecule were cleft, respectively. Only the fragment consisting of the amino acid region from Met343 to the C-terminal amino acid, Asn786, exhibited amidolytic activity. In proportion to inactivation of the bovine plasmin, this fragment disappeared. The above findings suggest that lysine derivatives react with bovine plasmin and then stabilize the activity of plasmin by preventing the degradation of active fragment (Met343-Asn786).