Inhibitory influence of natural flavonoids on human protein kinase CK2 isoforms: effect of the regulatory subunit.

CK2 is a pleiotropic, constitutively active protein kinase responsible for the phosphorylation of more than 300 physiological substrates. Typically, this enzyme is found in tetrameric form consisting of two regulatory subunits CK2β and two catalytic subunits CK2α or CK2α'. Several natural occurring flavonoids were tested for their ability to inhibit both CK2 holoenzymes, CK2α2β2 and CK2α'2β2. We identified few substances selectively inhibiting only the α' subunit. Other compounds showed similar effect towards all four isoforms. In some cases, like chrysoeriol, pedalitin, apigenin, and luteolin, the α2β2 holoenzyme was at least six times better inhibited than the free α subunit. Otherwise, we have found a luteolin derivative decreased the kinase activity of CK2α' with an IC50 value of 0.8 μM, but the holoenzyme only with 9.5 µM.