Plasma urokinase, a plasminogen activator immunochemically related to urinary urokinase (UK), was removed from human plasma (3.5 ng/ml) by immuno-depletion with antibodies raised against UK. The remaining plasminogen activator activity of the depleted plasma could not be inhibited by anti-UK antibodies and a sensitive ELISA for UK did not detect any UK levels that were higher than the background of the assay (0.1 ng/ml). However, when the depleted plasma was subjected to SDS-PAGE, substantial amounts of protein were found hereafter around 110 and 46 kD which now gave a positive reaction in the ELISA (35-350 ng/ml plasma). From these observations it is concluded that in human plasma two types of UK-related protein occur: Type I, among which the plasma urokinase, has antigenic determinants which are directly accessible to the anti-UK antibodies, Type II has determinants in a latent form. The function of the 110 kD type-II protein is that of a plasminogen activator; that of the 46 kD protein is not yet clear.
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