Antioxidant Activity of two Abietane Diterpenoids from Plectranthus spp.

1
Antioxidant Activity of two Abietane Diterpenoids
from Plectranthus spp.

• 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.

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
LHerit 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 7a-acetoxy-6ß,12-dihydroxy-8
,12-abietadiene-11,14-dione (2) from P. ecklonii
Benth. and P. grandidentatus, respectively, and
their antioxidant abilities. The activities of
the compounds were measured by the
a,a-diphenyl-ß-picrylhydrazyl (DPPH) method. The
percentage of DPPH radical reduction by abietanes
1 and 2 was evaluated at different concentrations
in a spectrophotometric assay1. Both diterpenoids
1 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)/(Abscont
rol) 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.
Parvifloron D (1) and 7a-acetoxy-6ß,12-dihydroxy-8
,12-abietadiene-11,14-dione (2)
Parvifloron D 1 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.
Compounds IC50 (mM)
BHT 0,1050 0,0070
Quercetin 0,0075 0,0010
Parvifloron D (1) 0,1125 0,0177
7a-acetoxy-6ß,12-dihydroxy-8,12-abietadiene-11,14-dione (2) 1,8500 0,0707
References 1 Y. Narukawa, et al., 2001, Chem.
Pharm. Bull. 49, 9, 11821184. 2 C.
Gaspar-Marques et al., 2008, Nat. Prod. Res., 22,
167177.
Acknowledgments To the students Helena Santos,
Telma Gama e Mª Inês Carvalho (ERISA), FCT-MCES
(ID nº 8/94, grant nº SFRH/BD/19250/2004) CYTED
(Project X.11 PIBATUB) CICYT (grant nº
CTQ2006-15279-C03-02).