Chemical profiling, antioxidant, enzyme inhibitory and molecular modelling studies on the leaves and stem bark extracts of three African medicinal plants

Africa is famous for its floral biodiversity, exploited by local people for therapeutic purposes. However, such plants need to be scrutinised scientifically for the presence of bioactive compounds and possible biological properties. This study attempts for the first time to highlight the pharmacological and phytochemical profile of extracts prepared from leaves and stem barks of three African plants (Macaranga hurifolia Beille, Sterculia tragacantha Lindl. and Zanthoxylum gilletii (De Wild.) P. G. Waterman. The extracts were tested for antioxidant and enzyme inhibitory effects. Free radical scavenging, metal chelator, reducing power and phosphomolybdenum assays were performed to evaluate antioxidant effects. To identify enzyme inhibitory effects, cholinesterases (acetylcholinesterase (AChE) and butrylcholinesterase (BChE)), tyrosinase, α-amylase and α-glucosidase were selected as target enzymes. High performance liquid chromatography-Electrospray tandem mass spectrometry (HPLC-ESI-MS) technique was also used for chemical profiling. ABTS (2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) and DPPH (1,1-diphenyl-2-picrylhydrazyl) radical scavenging assays showed that the stem barks of all three African plants were better scavenger than leaf extracts. Sterculia tragacantha was found to be a better metal chelator (64.10 ± 4.66 mg EDTAE/g) among the studied plants. All extracts exhibited good clinical enzyme inhibitory activities. The stem bark of S. tragacantha exhibited the best acetylcholinesterase activity compared to the other plants. HPLC-ESI-MS characterization showed that the most abundant compounds in stem bark were flavonoids in M. hurifolia (4.2 ± 0.2 mg/g DE), proanthocyanidins in S. tragacantha (42 ± 1 mg/g DE) and similar concentrations of phenolic acids and flavonoids in Z. gilletii (2.8-3.1 mg/g DE). Based on the biological activity, the most abundant and relevant bioactive compounds in the extracts were studied using molecular modelling approach against tyrosinase. The studied African plants showed good antioxidant and enzymatic propensities and thus can be considered as potential bioresources for future development of nutraceuticals and/or for pharmaceutical applications.