Thirty plant species traditionally used to treat skin pathologies were chosen from the readily available ethnobotanical literature. Four plants (aqueous or methanol extracts) displayed promising 5-lipoxygenase inhibitory activity with IC50 values below 61 ppm. These included Aloe greatheadii, Melianthus comosus, Pentanisia prunelloides and Warburgia salutaris. Essential oils generally displayed superior 5-lipoxygenase inhibitory activity with IC50 values between 22 and 75 ppm. These included the essential oils of Ballota africana, Helichrysum odoratissimum, Heteropyxis natalensis and Lippia javanica. A large proportion of the plants exhibited dose-dependent DPPH anti-oxidant activity with IC50 values between 5 and 94 ppm for the most active. These included Halleria lucida, Croton sylvaticus, Melianthus comosus, Lippia javanica and Pentanisia prunelloides. Aqueous extracts of Melianthus comosus exhibited the most potent anti-inflammatory and anti-oxidant activity.
The methanol extract of the leaves of Halleria lucida was subjected to activity guided fractionation and two anti-oxidant molecules were isolated, namely luteolin-5-O-glucoside and verbascoside (acteoside). Isobologram construction resulted in a concentration-dependent additive and antagonistic interaction being recognised between the two isolated compounds.
Warburgia salutaris displayed promising 5-lipoxygenase inhibitory activity. Two isolated compounds, mukadiaal and warburganal were found to partially contribute to the anti-inflammatory activity of the plant. The essential oils of Helichrysum odoratissimum, Heteropyxis natalensis and Lippia javanica were subjected to gas chromatography and major compounds contributing to possible anti-inflammatory effects identified. These included β-caryophyllene, 1,8-cineole, limonene and α-humulene. Enantiomers and racemic mixtures of limonene displayed significantly different 5-lipoxygenase inhibitory activity suggesting stereoselectivity of the enzyme-catalysed reaction. The monoterpene 1,8-cineole appeared to cause partial potentiation of the anti-inflammatory activity displayed by limonene. These results provide some in vitro scientific rationale for their traditional use as dermatological agents.