Zn²⁺ enhances protein tyrosine kinase activity of human platelet membranes

In human platelet membranes enhanced tyrosine phosphorylation of certain proteins was observed when Zn²⁺ instead of Mg²⁺ or Mn²⁺ was used as a divalent cation for the kinase reaction. An enhanced level of phosphate incorporation into tyrosine residues occurred into a 68 kDa polypeptide besides the 45 kDa and 105 kDa proteins. Preincubation of platelet membranes with TBR-IgG showed a concentration-dependent inhibition of the phosphorylation of the 45, 68 and 105 kDa proteins. Moreover, pp60^c-src, representing the major protein tyrosine kinase activity in platelets, was found to be stimulated by Zn²⁺. The data, thus, support the assumption that pp60^c-src kinase is responsible for Zn²⁺ stimulated tyrosine phosphorylation.