Complex-formation and inhibition of urokinase by blood plasma proteins

We have studied the formation of covalent complexes between 125I-urokinase (125I-UK) and proteins in human plasma. Although 125I-UK reacts with many proteinase inhibitors in purified systems, the predominant complexes formed in plasma are with antithrombin III (ATIII) and alpha 2-macroglobulin (alpha 2M). 125I-UK interacts with purified alpha 2M or alpha 2M in plasma to form a characteristic pattern of multiple complexes whose Mr values by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis are in the range of 380 000-720 000, under non-reducing conditions, and 180 000-430 000 after reduction. We also examined the inhibition of UK amidolytic activity by plasma and by purified ATIII. In the presence of saturating concentrations of ATIII and heparin, an apparent first-order rate constant of 6.8 X 10(-1) s-1 was calculated for the inhibition of urokinase. In contrast, the rate constant for the formation of covalent ATIII-UK complexes was lower, suggesting the inhibition of UK proceeds first via the formation of transient non-covalent intermediates that are then transformed more slowly into covalent end products. The observed rate constants for enzyme inhibition or complex-formation with plasma or purified inhibitors are insufficient to account for the reported clearance rate of injected UK in vivo.