Differential inhibition of eukaryote protein kinases by condensed tannins

Condensed tannins, isolated from a variety of plant sources, were characterized according to the constituent flavans, being based on procyanidin and/or prodelphinidin and having a cis or trans stereochemistry at positions 2 and 3. All the tannin preparations are potent inhibitors of rat liver cyclic AMP-dependent protein kinase catalytic subunit (cAK) with IC 50 values (concentrations for 50% inhibition) ranging from 0.009 to 0.2 μM. The tannin preparations are very good inhibitors of rat brain Ca 2+- and phospholipid-dependent protein kinase C (PKC) (IC 50 values in the range 0.3–7 μM), wheat embryo Ca 2+-dependent protein kinase (CDPK) (IC 50 values in the range 0.8–7 μM) and of calmodulin (CaM)-dependent myosin light chain kinase (MLCK) (IC 50 values in the range 7–24 μM). One of the most effective preparations, that from the leaves of Ribes nigrum, has IC 50 values with respect to cAK, PKC, CDPK and MLCK of 0.009, 0.6, 2.0 and 16 μM, respectively. In general, the order with respect to sensitivity to inhibition by these condensed tannins is cAK > PKC > CDPK > MLCK. The Ribes nigrum preparation is a competitive inhibitor of cAK with respect to both ATP and synthetic peptide substrate. These condensed tannin preparations are the most potent plant-derived inhibitors of cAK yet found.