Selective inhibition of eukaryote protein kinases by anti-inflammatory triterpenoids.

The ursane triterpenoid alpha-amyrin and the lupane triterpenoid lupeol are potent inhibitors of the catalytic subunit (cAK) of rat liver cyclic AMP-dependent protein kinase (PKA) with IC50 values of 8 and 5 microM, respectively. The palmitate and linoleate esters of alpha-amyrin and lupeol are also potent inhibitors of cAK (IC50 values in the range of 4-9 microM). alpha-Amyrin, lupeol and lupeol linoleate are much less potent as inhibitors of rat brain Ca(2+)- and phospholipid-dependent protein kinase (PKC) (IC50 values 32, 82 and 35 microM; respectively) and alpha-amyrin linoleate and the palmitate esters of lupeol and alpha-amyrin are ineffective or very poor inhibitors of this protein kinase. These compounds are very poor or ineffective as inhibitors of chicken gizzard calmodulin-dependent myosin light chain kinase (MLCK). alpha-Amyrin inhibits plant Ca(2+)-dependent protein kinase (CDPK) (IC50 52 microM) but lupeol and the triterpenoid esters tested are ineffective. alpha-Amyrin and the linoleate and palmitate esters of alpha-amyrin and lupeol inhibit cAK in a fashion that is competitive with respect to both peptide substrate and ATP (Ki values in the range 2-6 microM). However, while lupeol is competitive with respect to ATP it is apparently non-competitive with respect to peptide substrate. alpha-Amyrin also inhibits CDPK competitively and alpha-amyrin, lupeol and lupeol linoleate are competitive inhibitors of PKC. alpha-Amyrin and the palmitate esters of lupeol and alpha-amyrin are competitive inhibitors of the potato high affinity cyclic AMP-binding phosphatase (Pase) but lupeol inhibits the Pase non-competitively. These hydrophobic triterpenoids are further examples of anti-inflammatory triterpenoids that are cAK inhibitors.