Characterization of two different cytoplasmic protein tyrosine kinases from human breast cancer

Two different protein tyrosine kinases were detected in the cytosolicfraction of different human tumor tissues. After partial purification, thetwo enzymes, which were highly active in breast tumor tissues, werecharacterized. One of them, soluble tyrosine kinase-1 (STK-1), represents asoluble form of the c-Src protein, which is apparently underphosphorylatedon its C-terminal tyrosine residue whereas the other (STK-2) is a 48-kDaprotein tyrosine kinase (PTK), which is molecularly and functionallyrelated to the C-terminal Src kinase (Csk). These two protein tyrosinekinases clearly exhibit a different substrate specificity, and areresponsible for the high tyrosine kinase activity present in the cytosolicfraction of human breast cancer. In addition, it was observed that STK-1and STK-2 are also expressed in the breast cancer cell line, CAL-51.