Characterization Of The Active Site Of Urokinase With A Series Of Potent Inhibitory Amidines
Twenty amidine-substituted indole-like heterocycles were synthesized and examined for their blocking effect against human urokinase and a number of related arginine- or lysine- directed proteases. Kinetic analyses were carried out with the help of peptide anilide substrates and revealed a reversible competitive inhibitory pattern with each compound. The Ki values were therefore interpreted to reflect binding conditions at the active site of the enzymes.A highly potent inhibitor of urokinase was discovered in 5-amidino-l-(4-amidinobenzyl)indole which proved to be 18 times more effective on a molar basis than p-aminobenzamidine and 150 times more effective than benzamidine. The Ki value at 37°C and pH 8.3 was determined as 3.2 × 10-6 M. In striking contrast to the findings with the other proteases studied, urokinase was very sensitive to inhibition by 6-amidinoindoline (Ki 1.8 × 10-5 M), yet was much less susceptible to inhibition by the fully unsaturated analog 6-amidinoindole. Steric factors resulting from the difference in planarity between the two compounds are held responsible for this observation. In plasminogen activation assays the antiurokinase effect of the heterocycles mirrored their potency in the assays employing the synthetic urokinase substrate.The significant differences in the inhibitory activities of amidines against urokinase, on the one hand, and plasmin, thrombin and factor Xa, on the other hand, will be useful for experiments where selective inhibition of plasminogen activation is to be achieved. The compounds will also be of help in characterizing other tissue activators with respect to urokinase.