|Ricky Emery Kamdem
Advisor: Dr. Chi-Tang Ho
April 26, 2002
Final Ph.D. Seminar
Antioxidant Mechanism of Diterpene Lactone Andrographolides from Andrographis paniculata Nees
The medicinal plant Andrographis paniculata Nees plant is found in many Asian countries where it is used in traditional medicine against a variety of diseases including cold, fever, snakebite, diarrhea, liver diseases, diabetes, cancer, inflammation and malaria (1,2).
In an earlier pharmacological study Kapil et al. (3) demonstrated that andrographolides protected rats’ liver against the hepatotoxins carbon tetrachloride (CCl4) and tert-butyl hydroperoxide (tBHP) by reducing the levels of the lipid oxidation product malondialdehyde (MDA) and by maintaining high levels of the reduced form of glutathione (GSH). They suggested that inhibition of malondialdehyde formation revealed the free radical scavenging properties of diterpene lactone andrographolides and that they would act so through their glucosidic moiety. On the basis of the vast amount of literature now available on the antioxidant mechanism of polyphenols it is hard to believe that the glucosidic moiety would account for free radical scavenging of a compound.
We hypothesized that neoandrographolide, andrographolide, and 14-deoxy-11,12-didehydroandrographolide might scavenge free radicals by donating the allylic hydrogen of the unsaturated lactone ring. Under this hypothesis our objective was to study the reaction kinetics and mechanism of andrographolides with free radicals.
We found that all three isomers were effective scavengers of ABTS +. and superoxide radicals. The reaction of these diterpene lactones with superoxide yielded two new diacids, neoandrographic and didehydroandrographic diacids. We proposed a model reaction mechanism in which the initial attack of superoxide radical consisted of deprotonation of carbon C-11 or C-15 depending on the isomer. A key feature of the model proposed is that andrographolide hydroperoxides were formed as reaction transients, but spontaneously dehydrated into cyclic anhydrides that in turn hydrolyzed into water soluble diacids. We elucidated the structures of two new dimers purified from the superoxide system and we found that the dimers were termination products of transient radicals that had their lone electrons on carbon C-11 or C-15, which provided an unambiguous validation of the proposed reaction mechanism. We concluded that diterpene lactone andrographolides offered an interesting model that could be used in the design of a new class of nutritional antioxidants.
Zhang, X.F. and Tan B. K.H. 2000. Antihyperglycemic and antioxidant properties of Andrographis paniculata in normal and diabetic rats. Clinical and experimental pharmacology and physiology 27, 358-363.
Puri, A., Saxena, R., Saxena, R. P., Saxena, K. C. 1993. Immunostimulant agents from Andrographis paniculata. Journal of natural products 56 (7), 995 – 999.
Kapil,A, Koul, I.B., Banerjee, S.K., and Gupta, B.D. 1993. Antihepatotoxic effects of major diterpenoide constituents of Andrographis paniculata. Biochemical pharmacology 46 (1), 182-185.