Detection of a tyrosine kinase in human sera and blood cells by pp60src antiserum

Using sera of Rous sarcoma virus-tumor bearing rabbits (TBR-sera) as a tool to detect pp60src kinase in immunoprecipitates, we report here that about 10% of our TBR-sera revealed tyrosine kinase activity in human serum, plasma and in soluble extracts of human blood cells. The activity found in serum represents 5-12% of the total kinase activity in blood. Most of the enzyme activity was detected in lymphocytes and polymorphonuclear cells rather than in platelets and erythrocytes. We also demonstrate that the tyrosine kinase in serum is inhibited by quercetin, a potent inhibitor of the viral pp60src protein kinase.