Phytochemical analysis and in vitro anti-diabetic activity of selected South African medicinal plants traditionally used to treat diabetes mellitus

Abstract

Diabetes Mellitus (DM) is a chronic disease that requires constant treatment and its prevalence in South Africa continues to increase. With many people not having access to proper health care or are unable to afford appropriate treatments, the need to find alternative, more affordable treatments are required. South Africa is well known for its diverse flora and for using herbal remedies to treat a wide variety of ailments, however, even though many of these medicinal plants used to treat DM are well documented, many still lack any form of scientific evidence to support its use. In this study, South African medicinal plants were selected that are currently being used to treat DM, namely; Artemisia afra, Bulbine natalensis, Cnicus benedictus, Elephantorrhiza elephantina, Heteromorpha arborescens and Tulbaghia violacea. Extracts of these plants were screened for their in vitro antidiabetic activity using a bioassay that tested the extracts’ ability to inhibit the α-glucosidase enzyme. For B. natalensis, this was the first report of its in vitro antidiabetic activity. A methanol extract made from the leaves of E. elephantina showed the most promise, obtaining a % inhibition higher than that of the positive control, with the second highest % inhibition shown by the A. afra tea infusion extracts. Although herbal remedies are extensively used, they are also still widely critiqued. One aspect of contention is that plants are known to react to their environment, and as a result, chemical variation within and between plants can be expected, which will also likely lead to variation in biological activity. Therefore, the chemical variation of the major compounds within A. afra samples collected monthly over a 1- year period was investigated to better understand the expected chemical variation. Thirteen compounds were identified and comparatively quantified and were also individually tested for their antidiabetic bioactivity. The identified compounds showed substantial chemical variation over the course of the year, with the most notable observation that an interchangeable trend between the dicaffeoylquinic acids and monocaffeoylquinic acids concentrations were observed. The former being higher during summer whilst the latter was found to be higher during winter. It is believed that the dicaffeoylquinic acids, which have higher molar attenuation coefficients, than the monocaffeoylquinic acids (34.0 ϵ vs 18.3 ϵ at 330 nm), are therefore mainly biosynthesised during summer when UV radiation is most intense. The α-glucosidase bioassay also revealed that two dicaffeoylquinic acids were far more active (85% and 89% inhibition) than the positive control, acarbose (48% inhibition), whilst the monocaffeoylquinic acids were far less active (ranging from inactive to 31% inhibition). These differences in bioactivity, in combination with the seasonal chemical variation, indicates that chemical variation can have a profound influence on bioactivity. This again highlights the

importance of quality control of herbal medicine, especially pertaining to the treatment of chronic diseases such as DM.