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

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