A comparative evaluation of the biological activities and phytochemical properties in Ehretia obtusifolia and Ehretia rigida.

Abstract

Ehretia from the Boraginaceae family is predominantly found in parts of Asia and North America, with fewer species found in Africa, Europe, and Australia. The genus consists of more than 150 species, and species such as E. microphylla, E. accuminata, E. laevis have been reported on their medicinal prowess. Distribution of Ehretia in Southern Africa is found among a variety of habitats such as the lush forests of the Eastern Cape and Arid parts of Namibia. In South Africa, two species have been identified, namely E. rigida and E. obtusifolia and are used in traditional medicine. African and Asian countries traditional medicine is highly recommended because of its affordability. The aim of the study was to establish a baseline and compare different biological activities and phytochemical properties exhibited by the two South African species. In the study, phenolics, saponins, flavonoids, and tannins were detected in bark, roots, and leaves, of both species, but no detection of alkaloidsf. E. obtusifolia had a higher quantity of flavonoids than E. rigida. Both species exhibited high phenolic quantities in leaves with E. rigida having the highest quantity. Condensed tannins were found with a higher content in leaves than roots and bark for both species, with E. rigida containing higher quantities. E. rigida had the lower MIC’s compared to E. obtusifolia (0.195 mg/ml against M. luteus from ethyl-acetate root extracts). E. rigida had more samples with a MIC lower than 1 mg/ml than E. obtusifolia. The lowest MIC for leaves was 0.39 from ethyl-acetate extracts against S. aureus while methanol bark extracts also achieved 0.39 mg/ml against M. luteus. E. obtusifolia’s lowest MIC was 0.195 mg/ml from methanol leaf extracts against K. pneumoniae. Activity against C. albican was not as good as against the bacterial strains, as the lowest MIC was 0.78 mg/ml for both species. E. rigida and E. obtusifolia had dose-dependent antioxidant activity, with methanol and ethyl-acetate bark, leaf, and root extracts having the highest activities for both species. This study revealed that in comparison to literature, the activity achieved was similar or better when compared to the likes of E. laevis extracts. The α-glucosidase inhibitory activity reported in this study was dose-dependent. The relationship between antioxidant activity and antidiabetic activity is well documented and this study found that extracts with high antioxidant activity also had similarly high α-glucosidase inhibitory activity. It was E. rigida methanol extracts from bark and roots which exhibited the highest activities compared to E. obtusifolia. However, based on the dosedependent activity, E. obtusifolia is more potent because of the higher activity observed at the lowest concentrations. The study demonstrated that both species have good ethnopharmacological properties and were rich in phytochemicals, particularly phenolics and flavonoids. With E. rigida being the least studied of the two species with only one reported study, it was important to carry out this investigation as it has yielded further evidence that the genus Ehretia has multiple species with medicinal potential.