Quercetin inhibits tyrosine phosphorylation by the cyclic nucleotide-independent, transforming protein kinase,...

The bioflavonoid quercetin is a potent inhibitor of a cyclic nucleotide-independent, tumor virus-coded protein kinase which phosphorylates tyrosine residues and acts as a cellular transforming protein. Half-maximal inhibition of the protein kinase occurred at 3-4 microM quercetin whereas rutin was much less effective. The finding, that quercetin inhibits a cyclic nucleotide-independent protein kinase activity, may provide clues to the diverse pharmacological effects of the bioflavonoids.