Characterization of protein-tyrosine kinase activity in the canine prostate

Following the measurement of the phosphorylation of the substrate poly(Glu80Na,Tyr20) and the analysis of the alkali-resistant phosphorylation of endogenous proteins, the protein-tyrosine kinase of the canine prostate was partially characterized with regard to its subcellular localization, as well as certain kinetic and molecular properties. This kinase was mainly found in the cytosolic fraction (75%); however, its specific activity was similar to that of the residual enzyme present in the particulate fraction. Conditions for optimal activity of both fractions were determined. Under these conditions, several endogenous phosphoproteins (44-63 kilodaltons upon electrophoresis) were alkali resistant and phosphotyrosine was present in all of the major ones (pp63, pp57, pp52, and pp44). The particulate protein-tyrosine kinase activity was partially solubilized (58%) with 0.5% Triton X-100; this percentage was increased to 85% in the presence of 0.25 M KCl. Upon gel filtration, both cytosolic and particulate kinases showed an apparent molecular mass of 44 kilodaltons; these enzymes also phosphorylated similar major alkali-resistant phosphoproteins. The soluble protein-tyrosine kinase, with a sedimentation coefficient of 4.0S and an isoelectric point of 5.5, could be separated from arginine esterase and prostatic acid phosphatase.