The evolution of aerobic metabolic processes unavoidably led to the production of reactive
oxygen species (ROS). ROS have the ability to cause harmful oxidative damage to
biomolecules. Increased ROS generation and subsequent oxidative stress have been
associated with aging and neurodegenerative disorders such as Parkinson’s and Alzheimer’s
diseases as a result of the extreme sensitivity of the central nervous system to damage from
ROS. Antioxidant defence systems have co–evolved with aerobic metabolic processes to
counteract oxidative damage inflicted by ROS. The impact of neurodegenerative disorders
on society is increasing rapidly as the life expectancy of the global population increases. In
this day and age, a much younger group of the population is also experiencing
neurodegenerative symptoms as a result of the harmful effect of the human
immunodeficiency virus (HIV) on the central nervous system.
Plants are an invaluable source of medicinal compounds. The use of plants for their healing
properties is rooted in ancient times. The aim of this study was to select from twenty one
plants, the plant with the most promising antioxidant activity and to determine whether
extracts of this plant could act as free radical scavengers, comparing the results to Trolox, a
known free radical scavenger. The next step was to isolate and characterize a compound
from an extract exhibiting promising antioxidant activity. Bioassay–guided fractionation was
followed to achieve this.
During screening trials, twenty one plants, namely Berula erecta, Heteromorpha
arborescens, Tarchonanthus camphoratus, Vernonia oligocephala, Gymnosporia buxifolia,
Acacia karroo, Elephantorrhiza elephantina, Erythrina zeyheri, Leonotis leonurus,
Plectranthus ecklonii, P. rehmanii, P. venteri, Salvia auretia, S. runcinata, Solenostemon
latifolius, S. rotundifolius, Plumbago auriculata, Clematis brachiata, Vangueria infausta,
Physalis peruviana and Lippia javanica were selected from literature, based on reported
antioxidant activity within the plant families, for screening of their antioxidant activity. One
hundred and ten extracts were prepared from the leaves, using Soxhlet extraction and the
solvents petroleum ether (PE), dichloromethane (DCM), ethyl acetate (EtOAc) and ethanol
(EtOH), consecutively.
The focus during initial screening trials was on chemistry–based assays. The oxygen radical
absorbance capacity (ORAC) and ferric reducing antioxidant power (FRAP) assays were
employed for the primary screening of the one hundred and ten leaf extracts. The ORAC
assay was used to determine whether the plant extracts were able to scavenge peroxyl
radicals and the FRAP assay was used to determine the reducing abilities of the extracts.
Quantification of the peroxyl radical scavenging activity by the ORAC assay revealed that
activity was observed for most of the extracts, with the ethyl acetate and ethanol extracts of
L. javanica exhibiting the most promising activity. This pattern of activity was also found with the reducing capacity evaluated by the FRAP assay in which the EtOAc and EtOH extracts of
L. javanica also exhibited the most promising activity.
L. javanica was selected for further study by screening for biological activity, employing the
nitro–blue tetrazolium (NBT) assay and thiobarbituric acid reactive substances (TBARS)
assay. Using a cyanide model to induce neurotoxic effects in rat brain homogenate, the
neuroprotective properties of the extracts of L. javanica leaves were examined using the NBT
assay and compared to that of Trolox. The NBT assay determines the level of superoxide
anions. All the extracts of L. javanica significantly reduced superoxide anion generation at all
concentrations used. The petroleum ether and ethyl acetate extracts, at all concentrations,
reduced superoxide anion generation to values lower than that of the control, suggesting that
these extracts may be able to attenuate normal free radical processes in the brain. The
petroleum ether extract exhibited the most promising activity at a concentration of 1.25 and
2.5 mg/ml and also exhibited similar results as the ethyl acetate extract at a lower
concentration than the ethyl acetate extract (2.5 mg/ml compared to 5 mg/ml).
A toxin–solution consisting of hydrogen peroxide (H2O2), iron(III)chloride (FeCl3) and ascorbic
acid was used to induce lipid peroxidation and the ability of the extracts of the leaves of
L. javanica to attenuate lipid peroxidation was investigated in rat brain homogenate and
compared to that of Trolox. All of the extracts of L. javanica significantly attenuated toxininduced
lipid peroxidation at all concentrations used. All of the extracts were also able to
significantly attenuate toxin–induced lipid peroxidation to values lower than that of the control.
These results suggest that all of the extracts of L. javanica possess the ability to attenuate
not only toxin–induced lipid peroxidation, but also lipid peroxidation that occurs during normal
processes in the brain.
The petroleum ether extract was subjected to bioassay–guided fractionation using column
and thin–layer chromatography and the NBT and TBARS assays. Fraction DD1 was
investigated by means of nuclear magnetic resonance, infrared and mass spectrometry. The
exact structure of fraction DD1 was not elucidated.
Considering all the results, it is clear that L. javanica shows great potential as a medicinal
plant with antioxidant activity and may therefore be beneficial in diminishing the destructive
oxidative effects inflicted by free radicals. There are however still many compounds to be
isolated from L. javanica.
Key words: Verbenaceae, Lippia javanica, antioxidant, neurodegeneration, oxygen radical
absorbance capacity (ORAC), ferric reducing antioxidant power (FRAP), nitro–blue
tetrazolium assay (NBT), thiobarbituric acid reactive substances assay (TBARS). / Thesis (M.Sc. (Pharmaceutical Chemistry))--North-West University, Potchefstroom Campus, 2011.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:nwu/oai:dspace.nwu.ac.za:10394/4726 |
| Date | January 2010 |
| Creators | Pretorius, Corlea |
| Publisher | North-West University |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
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