This finding further validates our methodology (in addition to the experimental validation of the original VS workflow [1]) and suggests that the mode of action of these molecules could involve DPP-IV inhibition. The remaining 6 natural compounds not previously reported to have antidiabetic properties represent new molecules that may also exhibit this bioactivity. In the next paragraphs, the most significant compounds found in these 12 antidiabetic extracts are discussed:N-Nororientaline (CAS number 29079-44-5, see Table S1) has been isolated from five plant species from genus Erythrina (i.e., Erythrina variegata [22], Erythrina crystagalli, Erythrina indica, Erythrina poeppigiana [22], and Erythrina arborescens) and was identified as a DPP-IV inhibitor by our VS procedure. Extracts from Erythrina variegata, Erythrina senegalensis, Erythrina addisoniae, Erythrina abyssinica, and Erythrina mildbraedii are reported to enhance antidiabetic activity [23?7]. Isoprenylated flavonoids isolated from Erythrina mildbraedii and prenylflavonoids isolated from the Erythrina senegalensis roots have been described as inhibitors of two other proteins frequently targeted in T2DM treatment (i.e., protein tyrosine phosphatase-1B [25,26] for the former class of molecules and acyl CoA:diacylglycerol acyltransferase [24] for the latter class). All of these flavonoid compounds are chemically related to our hit. Therefore, theantidiabetic action of extracts from these plants may be the result of more than one bioactive component and mode of action. Tecostamine, which is found in Tecoma stans, has hypoglycemic properties similar to those of tecomine [28]. Our results therefore suggest that the hypoglycemic properties of tecostamine could be mediated by the inhibition of DPP-IV. In addition, the Tecoma stans aqueous extract posses at least four antidiabetic-related activities (i.e., intestinal a-glucosidase in?hibition, post-prandial antihyperglycemic, hypocholesterolemic and hypotriglyceridemic effects). Therefore, although it is possible that most of these activities are exerted by the phenolic compounds present in the Tecoma stans aqueous extract, bioguided studies are necessary to confirm this hypothesis [29]. Epinephrine (also known as adrenaline), which is found in Scoparia dulcis, has been reported to improve hypoglycemia [30]. Scoparia dulcis has been described as a folk-medicinal plant and has been traditionally used as a remedy for diabetes mellitus in India and for hypertension in Taiwan [31]. From Indian Scoparia dulcis, an antidiabetic compound named amellin was also isolated and characterized by Nath [32]. From the same family as epinephrine, two additional compounds were predicted by our VS to be potential DPPIV inhibitors. These compounds are (+)-pseudoephedrine (CAS number 90-82-4) and (2)-ephedrine (CAS number 299-42-3), and both have also been reported to have hypoglycemic activity [33]. Both molecules are found in several Ephedra species (Ephedra alata [34], Ephedra distachya [35], Ephedra equisetina [36], Ephedra gerardiana [35], Ephedra shennungiana [37], Ephedra sinica, Ephedra vulgaris and Ephedra pflanze). However, Ephedra distachya and Ephedra alata are the only species reported to have antidiabetic properties [33,38]; therefore, we proposed that the remaining Ephedra species may be new sources of antidiabetic extracts. The molecules ajmaline and isosandwichine, which are enantiomers with the same CAS number, 509-37-5, are found in several Rauwolfia species. Rauwolfia vomitoria has been investigated for the content of alkaloids, especially those alkaloids with hypotensive and anti-inflammatory properties [39] in addition to the antidiabetic properties. Rauwolfia serpentina is also used as antidiabetic extract [40]. The remaining species that contain these molecules, such as Rauwolfia canescens [41], Rauwolfia degeneri [42], Rauwolfia densiflora [43], Rauwolfia heterophylla [44], Rauwolfia indecora, Rauwolfia obscura [45] and Rauwolfia tetraphylla [42], are putative antidiabetic extracts. Serpinine (CAS number 509-38-6) is isolated from Vinca major and several Rauwolfia species (Rauwolfia obscura [45], Rauwolfia tetraphylla, Rauwolfia serpentina [46] and Rauwolfia sellowii [1]), belongs to the same cluster as ajmaline and isosandwichine (i.e., cluster 78; see Table S1).
Therefore, they share similar chemical structures and natural sources. Moreover, Vinca major organic leaf extract strongly stimulates glucose utilization [47]. One interesting hit predicted to be a DPP-IV inhibitor is an epicatechin derivate that is found in Vitis vinifera. An antihyperglycemic effect in streptozotocin-induced diabetic rats and insulinomimetic activity in insulin-sensitive cell lines have been described for grape seed procyanidin extracts (GSPE) [48]. In addition, it has been demonstrated that oligomeric procyanidins from GSPE activate the insulin receptor by interacting with and inducing the phosphorylation of the insulin receptor and that this interaction leads to increased glucose uptake [49]. Moreover, several epicatechinderivates have been reported to have antidiabetic properties (the most studied of which is epigallocatechin gallate [50]). Some findings demonstrate that epigallocatechin gallate may be a novel, plant-derived compound capable of reducing the risk of type 1 diabetes [51]. Therefore, the DPP-IV inhibition induced by this epicatechin derivate may contribute to the antihyperglycemic effect of GSPE [48]. The remaining 3 molecules predicted to be DPP-IV inhibitors through our VS workflow and that are found in extracts with described antidiabetic properties are hydroxysmirnovine from Galega orientalis [52], (2)-halosaline (CAS number 26648-71-5) from Haloxylon salicornicum [53] and isochanoclavin-(I) (CAS number 1150-44-3) from Pennisetum typhoideum [54] (see Table S1). Our results suggest that these molecules could be DPP-IV inhibitors and that extracts containing these molecules could potentiate the glucose-induced insulin response by prolonging the half-lives of GLP-1 and GIP incretins, due to the inhibition of DPP-IV. This information is novel and relevant, as no mechanism that explains the antidiabetic properties of these extracts has been previously suggested.
Virtual Screening Hits in Natural Extracts with No Described Antidiabetic Activity
Taking into account that extracts from closely related species of the same genus may share a high number of components, we also determined if any of our VS hits that were isolated from plants with no described antidiabetic activity belong to the same genus as species with known antidiabetic properties. We identified 6 molecules isolated from 6 different plants, Aconitum japonicum, Ervatamia officinalis, Solanum nudum, Solanum sodomaeum, Stephania cepharantha and Tabernaemontana eglandulosa (see Table S2), that meet these criteria. The related species with described antidiabetic properties are Aconitum carmichaelii [33], Aconitum moschatum [2], Aconitum violaceum [2], Ervatamia microphylla [55], Solanum lycocarpum [56], Solanum nigrum [57], Solanum xanthocarpum [58], Stephania hernandifolia [59], Stephania glabra [60], Stephania tetrandra [61], and Tabernaemontana divaricata [55]. Therefore, it is plausible to hypothesize that these 6 plants could also have antidiabetic properties mediated, at least partially, by the inhibition of DPP-IV.