Gentiana scabra Bunge (Gentianaceae) is an important traditional Chinese medicine commonly used as a stomachic or appetite stimulant. In this study, 21 triterpenoids (1-21) were isolated from a methanol extract of the rhizomes and roots of G. scabra. Their structures were elucidated by comparing spectroscopic data with reported values. Among the isolated triterpenoids, scabanol (2) was firstly isolated from natural sources. All isolated compounds were evaluated for their inhibitory activity against indoleamine 2,3-dioxygenase (IDO), which catalyzes the rate limiting reaction for the conversion of tryptophan to kynurenine. Compounds 10 and 11 showed significant inhibitory activities, with IC(50) values of 12.5 and 9.5 μM, respectively. Compound 12 showed a moderate inhibitory effect, with an IC(50) value of 18.7 μM. Compounds 2 and 13 showed weaker inhibitory effects, with IC(50) values of 56.8 and 60.6 μM, respectively. Kynurenine is a potent immune modulator to suppress the functions of a variety of immune cells including T cells and natural killer cells. Given that, our results that a few selected triterpenoids inhibit IDO warrant further studies on their effects on the host immune system as natural immune stimulators.
Keywords: Gentiana scabra; Gentianaceae; IDO inhibition; Triterpenoid.