New Fluorogenic Peptide Substrates for Plasmin1

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Abstract
Fluorogenic peptides, peptidyl-4-methylcoumaryl-7-amides (MCA), containing COOH-ter-minal lysine residues, were newly synthesized and tested as substrates for plasmin. Among six peptidyl-MCA's, Boc-Val-Leu-Lys-MCA and Boc-Glu-Lys-Lys-MCA were found to be useful for the specific and sensitive assay of plasmin. The Km values estimated from Line-weaver-Burk plots for these substrates using human and bovine plasmins were in the region of 10−4 m. Boc-Glu-Lys-Lys-MCA was slightly hydrolyzed by bovine plasma kallikrein, and Boc-Val-Leu-Lys-MCA was slightly hydrolyzed by human and hog urinary kallikreins and hog pancreatic kallikrein. However, both of the fluorogenic peptides were essentially unaffected by urokinase, α-thrombin, Factor Xa, Factor DCa, Factor XIa, and Factor Xlla. It was confirmed that plasmin hydrolyzed Boc-Glu-Lys-Lys-MCA, cleaving the lysyl-MCA bond, but not the Iysyl-lysyl bond. These fluorogenic peptides were resistant to human plasmin activated by streptokinase. Boc-Glu-Lys-Lys-MCA was not hydrolyzed by human plasmin or plasminogen in the presence of more than a 5-fold molar excess of streptokinase. The sensitivity of Boc-Val-Leu-Lys-MCA to human plasmin was also reduced, but plasmin retained 35 % of the maximum activity even in the presence of a 20-fold molar excess of streptokinase. These results suggest that streptokinase-plasmin complex has essentially no activity towards Boc-Glu-Lys-Lys-MCA.