Antioxidant Activity of two Abietane Diterpenoids from Plectranthus spp.

1. Antioxidant Activity of two AbietaneDiterpenoids from Plectranthus spp. The genus Plectranthus (Lamiaceae) consists of about 350 species, distributed from Africa through Asia to Australia. Several species are used as folk medicine for skin irritations and nausea, also as antiseptics and vermicide.1 The main metabolites of this genus Plectranthus L’Herit belong to phenolics and terpenoids classes. Within this last class, diterpenes are well-represented by a great variety of oxidized abietanes, namely with phenolic functions. Several similar diterpenes, have shown antioxidant activities. The knowledge of free radical mediation in several human pathological processes encouraged the use of diterpenes as natural drugs to prevent and treat some human diseases, namely atherosclerosis and cancer cells proliferation, besides its known utility in food industry.2 Here we report the isolation, structure elucidation of two known diterpenoids, parvifloron D (1) and 7α-acetoxy-6β,12-dihydroxy-8,12-abietadiene-11,14-dione (2) from P. eckloniiBenth. and P. grandidentatus, respectively, and their antioxidant abilities. The activities of the compounds were measured by the α,α-diphenyl-β-picrylhydrazyl (DPPH) method. The percentage of DPPH radical reduction by abietanes1 and 2 was evaluated at different concentrations in a spectrophotometric assay1. Both diterpenoids1 and 2 showed a dose dependent scavenging activity. Measurement of DPPH Radical Scavenging Activity: A solution of test compound in methanol (500 μL) was added to an methanol solution of DPPH radical (0.100 mM, 500 μL). The reaction mixture was vortexed thoroughly and left in the dark at room temperature for 30 min. The absorbance of the mixture was measured spectrophotometrically at 516 nm. Quercetin and BHT were used as the standards. The ability to scavenge DPPH radical was calculated by the following equation: DPPH radical scavenging activity (%) = [(Abscontrol - Abssample)/(Abscontrol)] x 100 where Abscontrol was the absorbance of DPPH radical + methanol, Abssample was the absorbance of DPPH radical + sample/standard. The scavenging activity was expressed in terms of IC50, the concentration of the samples required to give a 50% reduction in the intensity of the signal of the DPPH radical. The results were done at least in triple. P. Rijoa,b, J. Chumbob, A. Oliveirab, M.C. Costab,d, B. Rodríguezc and M.F. Simõesa a Fac. Farmácia, iMed-UL, Univ. de Lisboa, Lisboa, Portugal, b Escola Superior de Saúde Ribeiro Sanches (ERISA), R. Fernando Palha, 69, 1900-693, Lisboa, Portugal, cInst. Química Orgânica,CSIC, c. Juan de la Cierva 3, E-28006, Madrid, Spain, d Laboratório Nacional de Energia e Geologia, I.P., Estrada da Portela, Bairro do Zambujal – Alfragide, Apartado 7586 - 2720-866, Amadora. Parvifloron D (1) and 7α-acetoxy-6β,12-dihydroxy-8,12-abietadiene-11,14-dione (2) Parvifloron D1 showed antioxidant properties equivalent (IC50 0.1125 ± 0.0177 mM) to BHT but lesser than quercetin reference. The IC50 of royleanone 2 (1.8500 ± 0.0707 mM) revealed antioxidant ability inferior to butylated hydroxytoluene (BHT) reference. We intend to improve antioxidant properties of Abietane 1 and some of its new derivatives through synthetic strategies targeting relevant biological activities which are under study. References: 1 Y. Narukawa, et al., 2001, Chem. Pharm. Bull.49, 9, 1182—1184. 2 C. Gaspar-Marques et al., 2008, Nat. Prod. Res., 22, 167–177. Acknowledgments: To the students Helena Santos, Telma Gama e Mª Inês Carvalho (ERISA), FCT-MCES (I&D nº 8/94, grant nº SFRH/BD/19250/2004); CYTED (Project X.11: PIBATUB); CICYT (grant nº CTQ2006-15279-C03-02).