THE STRUCTURE BASIS OF THE POOR FIBRIN SPECIFICITY OF UROKINASE(Ⅱ)——THE INHIBITION OF UROKINASE A CHAIN 149--157 ON THE FIBRIN STIMULATED ACTIVATION OF PLASMINOGEN BY TISSUE TYPE PLASMINOGEN ACTIVATOR

In view of the similarity of the charge distribution between fibrin A_α148--161 and Achain 149--157 of urokinase,the latter might compete with fibrin A_α148--161 when singlechain pro-urokinase is converted to double chain urokinase.To test this, the stretch of uro-kinase A chain 135--157 was separated from the low molecular weight urokinase, a competi-tive binding between this stretch and fibrin to tPA kringle-2 was shown by radio-bindingassay. The inhibition of the stretch on the fibrin stimulated activation of plasminogen wasdemonstrated in the caseinolytic system. The synthesized novapeptide urokinase A chain 149--157 (R-peptide) showed a significant inhibition on the activation of plasminogen in the pres-ence of fibrin. By contrasting finely with R-peptide, a synthesized novapeptide in which Arg154and Arg156 were replaced by Asp (D-peptide) did not show any inhibition effect on the fi-brin stimulated activation of plasminogen by tPA. These results suggest that the positivelycharged residues in

In view of the similarity of the charge distribution between fibrin A_α148--161 and A chain 149--157 of urokinase,the latter might compete with fibrin A_α148--161 when single chain pro-urokinase is converted to double chain urokinase.To test this, the stretch of uro- kinase A chain 135--157 was separated from the low molecular weight urokinase, a competi- tive binding between this stretch and fibrin to tPA kringle-2 was shown by radio-binding assay. The inhibition of the stretch on the fibrin stimulated activation of plasminogen was demonstrated in the caseinolytic system. The synthesized novapeptide urokinase A chain 149 --157 (R-peptide) showed a significant inhibition on the activation of plasminogen in the pres- ence of fibrin. By contrasting finely with R-peptide, a synthesized novapeptide in which Arg154 and Arg156 were replaced by Asp (D-peptide) did not show any inhibition effect on the fi- brin stimulated activation of plasminogen by tPA. These results suggest that the positively charged residues in the stretch 149--157 of urokinase are crucial for the inhibition of fibrin binding with the kringle domain of urokinase.