[No authors listed]
We have analyzed the occurrence of components of the plasminogen activation system in bovine milk. Zymographic analyses showed that tissue-type plasminogen activator (t-PA) occurred in association with casein micelles, partially as a complex with type-1 plasminogen activator inhibitor (PAI-1), whereas urokinase-type plasminogen activator (u-PA) was confined to milk leukocytes. Whey contained a component with a plasminogen dependent proteolytic activity which was shown to be plasma prekallikrein (PPK). The u-PA in the milk leukocytes was shown to be bound to urokinase receptor (u-PAR). A purification to near-homogeneity of the bovine u-PAR was undertaken. Investigating the novel t-PA binding to casein micelles by ligand blotting and Sepharose immobilized casein, multimeric forms of kappa-casein and dimeric alpha s2-casein were identified as t-PA binding components. The kappa-casein gene and the fibrinogen gene are believed to have evolved from a common ancestor. Thus, the recent finding that casein enhances t-PA catalyzed plasminogen activation (Marcus, G., Hitt, S., Harvey, S.R. and Tritsch, G.L. (1993) Fibrinolysis 7, 229-236), and the observed t-PA/casein binding suggests that the casein micelle, which also contains plasminogen, may serve as a matrix for t-PA-catalyzed plasminogen activation in milk.
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