Characterization of extracellular binding sites for [3H]-staurosporine on capillary endothelial cells.

[3H]-staurosporine, a non-specific protein kinase inhibitor, bound with high affinity and in a reversible manner to specific and saturable binding sites in cultured bovine cerebral cortex capillary endothelial cells. Scatchard analysis revealed the presence of one class of non-interacting binding sites with an equilibrium dissociation constant (KD) of 9.2 nM and Bmax of 19.3 fmol/10(5) cells. The binding of [3H]-staurosporine was fully displaced by unlabelled staurosporine, H-7 and ATP with IC50 values of 6.9 nM, 3 microM and 0.4 microM respectively. Mild trypsinization of cells after [3H]-staurosporine binding revealed the presence of membrane-associated, extracellular binding sites which could be an ecto-protein kinase.