Immunoblatting analysis of purified human urokinase plasminogen activator (u‐PA), gives a positive signal when reacted with anti‐phosphotyrosine monoclonal antibodies (MoAb anti P‐Tyr); competition with α‐phospha‐DL‐tyrosine (P‐Tyr) but not α‐phosphia‐DL‐threonine or serine (P‐Treo, P‐Ser) completely suppresses this signal. Either the 55 kDa u‐PA form and the lower M r form (33 kDa) derived from the 55 kDa u‐PA are Tyrphosphorylated also the u‐PA secreted in the culture media of human fibrosarcoma cells (HT‐1080) is phosphorylated in tyrosine as well as u‐PA present in tissue extracts of tumors induced in nude mice by HT‐1080 cells. These data show that urine purified human u‐PA and u‐PA produced by humans fibrosarcoma cells, in vitro and in vivo, are phosphorylated in tyrosine; furthermore our data show that u‐PA is the major Tyr‐phosphorylated protein present in these human tumor cells.

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