Mechanism of the superoxide scavenging activity of neoandrographolide - a natural product from Andrographis paniculata Nees.

It was hypothesized that neoandrographolide might scavenge free radicals by donating the allylic hydrogen of the unsaturated lactone ring. It was found that the stoichiometry of the reaction between neoandrographolide and superoxide radical generated from KO(2) in DMSO was 2 to 1. One major reaction product was isolated and determined to be a diacid formed by the opening of the lactone ring. It was concluded that the antiradical activity of neoandrographolide proceeded by hydrogen abstraction from carbon C-15. A reaction mechanism was proposed.