CHARACTERIZATION OF A MACROPHAGE-DERIVED PLASMINOGEN-ACTIVATOR INHIBITOR - SIMILARITIES WITH PLACENTAL UROKINASE INHIBITOR

Human and mouse macrophages release a fibrinolytic inhibitor after stimulation by endotoxin in vitro. The released mouse inhibitor was indistinguishable in size by molecular-sieve chromatography from an intracellular form (approx. 50 kDa), and both inhibitors blocked urokinase directly as judged by a 125I-plasminogen conversion assay. The intracellular inhibitor was found mostly to dissociate from 125I-urokinase during sodium dodecyl sulphate/polyacrylamide-gel electrophoresis under reduced conditions, but a dodecyl sulphate-stable complex at 65-67 kDa was observed. Because of similarities in the reported size, stability and urokinase-binding properties of a placental urokinase inhibitor, the kinetic properties of the two inhibitors were compared. Under the reaction conditions employed (37.degree. C at pH 7.4 in the presence of 0.2% Triton X-100), the association rate constants and equilibrium dissociation constants of the two inhibitors were indistinguishable, 3 .times. 105 M-1 .cntdot. s-1 and 4 .times. 10-10 M, respectively. These data show that peritoneal macrophages contain a plasminogen-activator very similar to a previously recognized placental inhibitor. Although the inhibitor appears to be a trace protein in macrophages, placental macrophages may account for the accumulation of the inhibitor in placental tissue.