Specific inhibition of cyclic AMP-dependent protein kinase by warangalone and robustic acid

The prenylated isoflavone warangalone from the insecticidal plant Derris scandens is a selective and potent inhibitor of rat liver cyclic AMP-dependent protein kinase catalytic subunit (cAK) (IC50 3.5 microM). The inhibition of rat liver cAK by warangalone is non-competitive with respect to both ATP and the synthetic peptide substrate (LRRASLG) employed in this study. Warangalone is a poor inhibitor of avian calmodulin-dependent myosin light chain kinase (MLCK), rat brain Ca(2+)- and phospholipid-dependent protein kinase C (PKC) and wheat embryo Ca(2+)-dependent protein kinase (CDPK). The related plant derived prenylisoflavones are also potent cAK inhibitors. Thus, 8-gamma-gamma-dimethylallylwighteone, 3' -gamma-gamma-dimethlallylwighteone and nallanin are inhibitors of cAK with IC50 values in the range 20-33 microM. The prenyl-substituted isoflavones tested in this study are ineffective or poor as inhibitors of PKC. Thus nallanin is a poor PKC inhibitor (IC50 value of 120 microM). The related isoflavones biochanin A and genistein are poor inhibitors of cAK (IC50 values 100 microM and 126 microM, respectively). Genistein inhibits MLCK (IC50 value 14 microM) but biochanin A is a poor MLCK inhibitor (IC50 value 300 microM). The D. scandens prenyl-isoflavones and related isoflavones are ineffective inhibitors of wheat embryo Ca(2+)-dependent protein kinase (CDPK). The 4-methoxy-3-phenyl-coumarin robustic acid is a potent inhibitor of rat liver cAK (IC50 value 10 microM) but is a poor inhibitor of rat brain PKC, avian MLCK and wheat embryo CDPK. The coumarins 5-methoxypsoralen and 4,4'-di-O-methyl scandenin are poor cAK inhibitors (IC50 values 240 and 248 microM, respectively). All of the non-prenylated coumarins examined are ineffective as inhibitors of the eukaryote signal-regulated protein kinases cAK, MLCK, PKC and CDPK. The selective, high affinity interaction of warangalone and robustic acid with cAK may contribute to their biological effects in vivo and to the insecticidal activity of the plant D. scandens.