Characterization of tyrosine protein kinase associated with subcellular components of human colonic epithelium

Tyrosine protein kinase (TPK) co-isolated with subcellular components derived from human colonic epithelium. The highest TPK activity, measured in the Triton X-100-insoluble cytoskeletal pellet, was directly related to the degree of malignancy of colonic tissue. TPK activity was assayed by measuring the incorporation of [γ-³²P] from [γ³²]ATP into the synthetic polymer [Glu⁸⁰Tyr²⁰](n) substrate. Lineweaver-Burk plots yielded an apparent Km of 167 μg/ml for [Glu⁸⁰Tyr²⁰](n) and of 19μM for ATP: V(max) for the phosphate donor was 0.9 nmol/min/mg protein. TPK activity was markedly stimulated by the metal ions Mg²⁺ and Mn²⁺ and significantly suppressed by tyrphostins, potent specific TPK inhibitors, shown to interfere with TPK-dependent growth processes. This is the first report to present evidence for TPK activity associated with cytoskeleton-enriched subcellular preparations harvested from human colonic epithelium.