The effect of quercetin on the phosphorylation activity of the Rous sarcoma virus transforming gene product in vitro and in vivo.

The phosphotransferase activity of the Rous sarcoma virus src gene product, pp60src, was inhibited both in vitro and in vivo by the bioflavonoid quercetin. The Ki for the inhibitory effect was in the range of 6-11 microM under conditions in vitro. The inhibitory effect of quercetin was competitive towards the nucleotides ATP and GTP as substrates for pp60src and was non-competitive towards alpha-casein as the protein substrate of this kinase activity. In contrast, studies in vitro of the phosphotransferase activity of the catalytic subunit of the cAMP-dependent protein kinase showed that this flavonoid did not inhibit the phosphorylation of physiological substrates of this enzyme. In cultured cells the half-maximal inhibition of tyrosine phosphorylation of pp60src as well as the phosphorylation of the Mr = 34000 protein, a physiological substrate of pp60src, was in the range 0.06-0.08 mM.