Emodin, an anthraquinone derivative isolated from the rhizomes of Rheum palmatum, selectively inhibits the activity of casein kinase II as a competitive inhibitor.

Ser/Thr protein kinases play important roles in signal transduction pathways that control the proliferation and differentiation of eukaryotic cells. In this paper, we present evidence that emodin, an anthraquinone derivative, selectively inhibits casein kinase II (CKII), a Ser/Thr kinase, as a competitive inhibitor. The results with ethyl acetate extracts of the rhizomes of Rheum palmatum showed that emodin significantly inhibited the activity of cyclin B/cdc2 protein kinase (cdc2). We measured IC50 values for emodin on the activities of several Ser/Thr protein kinases, including cAMP-dependent protein kinase (PKA), protein kinase C (PKC), cdc2, casein kinases I (CKI) and CKII. Interestingly, emodin inhibited CKII activity with an IC50 value of 2 microM, which was two to three orders of magnitude lower than those against the other kinases. Enzyme kinetic assays showed that emodin inhibited CKII activity as a competitive inhibitor against ATP with a Ki value of 7.2 microM. Collectively, we suggest that emodin is a selective CKII inhibitor, whose action mechanism is mediated through competitively binding to the ATP binding site.