Tyrosine protein kinase activity in the DMBA-induced rat mammary tumor: inhibition by quercetin.

Tyrosine protein kinase activity was measured in membranes from DMBA-induced mammary tumors, with Angiotensin II as substrate. The apparent Km for the peptide was 3.3 mM. This enzymatic activity is inhibited by Ca+2; Mn+2 can replace Mg+2 with an increase in the Km for ATP from 47 /microM to 172 microM. The enzymatic activity was not affected by cyclic AMP but was inhibited in dose dependent manner by quercetin, a bioflavonoid which is known to inhibit proliferation of malignant cells in vitro.