Abstract
Background: Natural extracts play an important role in traditional medicines for the treatment of diabetes mellitus and are also an essential resource for new drug discovery. Dipeptidyl peptidase IV (DPP-IV) inhibitors are potential candidates for the treatment of type 2 diabetes mellitus, and the effectiveness of certain antidiabetic extracts of natural origin could be, at least partially, explained by the inhibition of DPP-IV. Methodology/Principal Findings: Using an initial set of 29,779 natural products that are annotated with their natural source and an experimentally validated virtual screening procedure previously developed in our lab (Guasch et al.; 2012) [1], we have predicted 12 potential DPP-IV inhibitors from 12 different plant extracts that are known to have antidiabetic activity. Seven of these molecules are identical or similar to molecules with described antidiabetic activity (although their role as DPP-IV inhibitors has not been suggested as an explanation for their bioactivity). Therefore, it is plausible that these 12 molecules could be responsible, at least in part, for the antidiabetic activity of these extracts through their inhibitory effect on DPP-IV. In addition, we also identified as potential DPP-IV inhibitors 6 molecules from 6 different plants with no described antidiabetic activity but that share the same genus as plants with known antidiabetic properties. Moreover, none of the 18 molecules that we predicted as DPP-IV inhibitors exhibits chemical similarity with a group of 2,342 known DPP-IV inhibitors. Conclusions/Significance: Our study identified 18 potential DPP-IV inhibitors in 18 different plant extracts (12 of these plants have known antidiabetic properties, whereas, for the remaining 6, antidiabetic activity has been reported for other plant species from the same genus). Moreover, none of the 18 molecules exhibits chemical similarity with a large group of known DPP-IV inhibitors.
?Citation: Guasch L, Sala E, Ojeda MJ, Valls C, Blade C, et al. (2012) Identification of Novel Human Dipeptidyl Peptidase-IV Inhibitors of Natural Origin (Part II): In Silico Prediction in Antidiabetic Extracts. PLoS ONE 7(9): e44972. doi:10.1371/journal.pone.0044972 Editor: Vladimir N. Uversky, University of South Florida College of Medicine, United States of America Received September 22, 2011; Accepted August 16, 2012; Published September 21, 2012 Copyright: ?2012 Guasch et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. ?Funding: This research was supported by grants from the Ministerio de Educacion y Ciencia of the Spanish Government (AGL2011-25831/ALI and AGL2011?23879/ALI) and from the Generalitat de Catalunya (ACC1O/TECCT11-1-0012 program and XRQTC grant). N.G.-A. was supported by a pre-doctoral fellowship BES2009-026450 from the Spanish Government. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist.

Introduction
Medical plants play an important role in the management of type 2 diabetes mellitus (T2DM) by delaying the development of diabetic complications and correcting metabolic abnormalities [2]. Traditional plant-based remedies have been and are being used by T2DM patients around the world (e.g., patients belonging to the Chinese [3], Indian [4] and Mexican [5] populations), and many scientific studies have confirmed the benefits of medicinal plants with hypoglycemic effects on these patients [6?]. Furthermore, during the past few years, some of the new bioactive drugs isolated from hypoglycemic plants have been demonstrated to have antidiabetic activity with greater efficacy than synthetic oral hypoglycemic agents used in clinical therapy regimens [10].

The most commonly studied hypoglycemic plants are Opuntia streptacantha, Trigonella foenum-graecum, Momordica charantia, Ficus bengalensis, Polygala senega and Gymnema sylvestre [10]. Despite their long tradition of use worldwide, few of these plants have been tested in modern, large-scale, clinical-type trials to determine their efficacies. However, it is clear that more research needs to be undertaken on these and other medicinal plants with hypoglycemic effects because, in most cases, the bioactive compounds and their modes of action still remain unclear. Numerous mechanisms of antidiabetic action have been proposed for extracts of the previous mentioned plants, some of them relate to their ability to stimulate insulin secretion [11]. Regarding the stimulation of insulin secretion, one target of interest for the antidiabetic action of these extracts is the serine protease dipeptidyl peptidase-IV (DPP-IV; EC 3.4.14.5) because theinhibition of DPP-IV has been shown to be an appropriate treatment for T2DM [12]. DPP-IV specifically removes Nterminal dipeptides from substrates containing proline or alanine as the second residue, transforming them into inactive or even antagonistic species. The most important substrates of DPP-IV are incretins, such as glucagon-like peptide-1 (GLP-1) and glucosedependent insulinotropic polypeptide (GIP), which stimulates insulin secretion [13]. Incretin hormones are intestinal hormones that are released in response to nutrient ingestion and that potentiate the glucose-induced insulin response. Therefore, GLP-1 stimulates insulin biosynthesis and secretion, reduces glucagon release, slows gastric emptying, reduces appetite, and stimulates the regeneration and differentiation of islet B-cells [14]. On the other hand, GIP is involved in glucose metabolism by enhancing insulin secretion [15]. Both peptides have short half-lives because of their rapid degradation by DPP-IV. Therefore, inhibiting DPPIV prolongs the action of GLP-1 and GIP, which, in turn, improves glucose homeostasis with a low risk of hypoglycemia [12]. The first DPP-IV inhibitor on the market was sitagliptin (by Merck & Co.) [16], which was followed by the structurally similar vildagliptin (by Novartis) [17] and saxagliptin (by Bristol-Myers Squibb and AstraZeneca) [18]. The efficacy and safety profile of DPP-IV inhibitors have been promising and advantageous to date.

In contrast to sulfonylureas and other antidiabetic drugs, DPP-IV inhibitors do not have an intrinsic risk of inducing hypoglycemia, and they are body-weight neutral. Their tolerability profile is good, and no specific adverse reactions have been reported [12]. The DPP-IV binding site is highly druggable in the sense that tight, specific binding to the enzyme can be achieved with small molecules with drug-like physicochemical properties [19,20]. The two key binding-site areas for the intermolecular interaction of DPP-IV and reversible inhibitors of non-peptide nature are the lipophilic S1 pocket (formed by Tyr631, Val656, Trp659, Tyr662, Tyr666 and Val711) and the negatively charged Glu205/206 pair [20].