Inhibition of serine proteases by anti-inflammatory triterpenoids

The lupane triterpenoid lupeol, the ursane triterpenoid alpha-amyrin and esters of these compounds are present in the bark of roots of Alstonia boonei (Apocynaceae) and have anti-inflammatory properties. alpha-Amyrin is a competitive inhibitor of bovine trypsin and chymotrypsin (K-i values 29 mu M and 18 mu M, respectively). Lupeol linoleate, lupeol palmitate and alpha-amyrin linoleate are non-competitive inhibitors of trypsin (Ki values 7 mu M, 10 mu M and 16 mu M, respectively). alpha-Amyrin linoleate is also a non-competitive inhibitor of chymotrypsin (Ki value 28 mu M). Lupeol is a competitive inhibitor of both trypsin and chymotrypsin (K-i values 22 and 8 mu M, respectively). alpha-Amyrin palmitate is a potent non-competitive inhibitor of chymotrypsin (K-i 6 mu M). Lupeol, alpha-amyrin and the palmitic and linoleic acid esters of these compounds are ineffective or very weak as inhibitors of porcine pancreatic elastase and of Lucilia cuprina and Helicoverpa punctigera leucine aminopeptidases. These hydrophobic triterpenoids represent further examples of anti-inflammatory triterpenoids that are PKA inhibitors as well as being selective protease inhibitors.