Blood platelets express high levels of the pp60c-src-specific tyrosine kinase activity.

TLDR

The study examined whether human and rabbit platelets express the pp60c‑src tyrosine kinase, the cellular homolog of the Rous sarcoma virus transforming protein. pp60c‑src activity was measured by an immune‑complex kinase assay using enolase as substrate, while protein levels were quantified by immunoblotting. Platelets displayed markedly elevated pp60c‑src kinase activity and protein, with pp60c‑src being a major tyrosine‑phosphorylated protein in platelet membranes, demonstrating that high tyrosine kinase activity is not exclusive to proliferating cells.

Abstract

We have examined human and rabbit blood platelets for expression of pp60c-src, the normal cellular homolog of the transforming protein of Rous sarcoma virus. pp60c-src kinase activity was determined by an immune-complex kinase assay that uses enolase as the substrate, and pp60c-src protein levels were determined by an immunoblot assay. Lysates from platelets expressed high levels of pp60c-src-specific kinase activity and pp60c-src protein compared to the levels found in other tissues. pp60c-src was also found to be one of the major proteins phosphorylated in vitro in membranes isolated from platelets. Multiple protein species other than pp60c-src were also phosphorylated on tyrosine in the membrane phosphorylation reactions, and phosphotyrosine represented approximately equal to 80% of the total phosphoamino acid residues phosphorylated in the membranes. These results indicate that tyrosine kinases represent the major protein phosphorylating enzymes detected in isolated platelet membranes. Although the association of tyrosine kinase activity with many viral oncogene products and cellular growth hormone receptors has suggested a role for these enzymes in the regulation of cell proliferation, these results indicate that the expression of high levels of tyrosine kinase activity is not exclusively associated with proliferating cells.