Inhibition of signal-regulated protein kinases by plant-derived hydrolysable tannins

A variety of hydrolysable tannins purified from Phyllanthus amarus are potent inhibitors of rat liver cyclic AMP-dependent protein kinase catalytic subunit (cAK) with IC₅₀ values (concentrations for 50% inhibition) in the range 0.2-1.7 μM. The three most effective compounds of this series of hydrolysable tannins have five phenolic substituents. These three compounds are also the most effective inhibitors of wheat embryo Ca²⁺-dependent protein kinase (CDPK), rat brain Ca²⁺- and phospholipid-dependent protein kinase C (PKC) and Ca²⁺-calmodulin-dependent myosin light chain kinase (MLCK). The order of sensitivity for protein kinase inhibition by the hydrolysable tannins studied is cAK>CDPK>PKC>MLCK. Thus the IC₅₀ values for protein kinase inhibition by the most potent compound are 0.2 μM (for cAK), 1.8 μM (for CDPK), 26 μM (for PKC) and 56 μM (for MLCK) when protein kinase affinity is measured using synthetic peptide substrates. These hydrolysable tannin inhibitors found are the most specific and potent plant-derived inhibitors of cAK yet found.