Phytochemical and Antioxidant studies on Stem Bark of Aubrevillea kerstingii

Antioxidants are believed to play a role in preventing the development of some chronic diseases. Medicinal plants with antioxidant activity have continuously been utilized both in traditional and contemporary medicine for management of free radical related diseases. There is need to search for medicinal plants with antioxidant potentials and to isolate compounds responsible for the activity. The aim of this research was to carry out the phytochemical and antioxidant studies on stem bark extract of Aubrevillea kerstingii. The powdered stem bark of Aubrevillea kerstingii (1.5kg) was extracted with n-hexane, ethyl acetate and methanol using sequential maceration. Qualitative screening for antioxidant activity was carried out using thin layer chromatographic TLC autographic screening on pre-coated silica plate. The determination of 2,2-diphenyl-1- picrylhydrazyl (DPPH) radical scavenging activity of the 3 stem bark extracts was carried out according to a standard method. The ethyl acetate extract (5g) was pre-adsorbed on silica gel and loaded into a column pre-packed with 100g silica gel (60-120 Merck). The extract was eluted with n-hexane: ethyl acetate in a gradient elution starting from n-hexane (100%) to ethyl acetate methanol (90:10). The DPPH scavenging activity of the extracts revealed a concentration dependent increase in free radical scavenging activity. The n-hexane, ethyl acetate and methanol extracts showed IC50 values of 98.58μg/ml, 36.79μg/ml and 49.27μg/ml respectively compared with ascorbic acid which had IC50 of 17.89μg/ml. The isolated compound was identified as oleanolic acid using NMR. The 1H-NMR and13C-NMR spectra obtained in CDCl3 with NMR spectrometer showed chemical shifts characteristic of oleanolic acid. The methanol extract and ethyl acetate extracts showed significant antioxidant activity. Oleanolic acid isolated from the ethyl acetate extract may be responsible for the antioxidant activity.