Comparison of the effects of fibrin binding on the biochemical properties of single-chain tissue-type plasminogen activator (t-PA) and single-chain chimeric plasminogen activator (t-PA/scu-PA).

The effect of the binding of the single-chain chimeric plasminogen activator t-PA/scu-PA, which contains amino acids 1 to 274 of tissue-type plasminogen activator (t-PA) and amino acids 138 to 411 of single-chain urokinase-type plasminogen activator (scu-PA), to fibrin on its biochemical properties was investigated in a purified system. In contrast to the binding of single-chain tissue-type plasminogen activator (sct-PA) on fibrin, which causes an increase in its intrinsic activity, t-PA/scu-PA enzyme activity is not elevated. In contrast to sct-PA which retains its single-chain form during fibrin-binding, t-PA/scu-PA is converted to its more active two-chain form. The activating process of t-PA/scu-PA is accelerated by increasing fibrin concentrations. With constant concentrations of fibrin monomer, the activation velocity also increases with time. Since this effect is inhibited by epsilon-aminocaproic acid and by a monoclonal antibody directed against the fibrin-binding site of t-PA, the activation process depends on the fibrin-binding of the molecule. The results point to the fact that t-PA/scu-PA is autocatalytically converted to its two-chain form during fibrin-binding. The conspicuous differences of the effect of the fibrin-binding on the biochemical properties of sct-PA and t-PA/scu-PA are caused obviously by small differences in the structures of the protease-domains and/or by different communications between the identical A-chains and the protease domains of the enzymes.