Chemical Composition and Biological Evaluation of Natural Products with Antioxidant Properties

An abnormal oxidative status is linked to the development of several diseases [1]. Oxidative stress, as described by Helmut Sies, refers to an imbalance between the production of pro-oxidants and antioxidant defenses that may result in damage to biological systems [2]. In mammals, these defenses rely on enzymes such as the superoxide dismutase (SOD) that catalyze the quenching of radicals upon transformation to neutral molecules. Essential nutrients, such as tocopherols and ascorbic acid, contribute to the scavenging of radicals and protect our body from oxidative stress, hence forming a part of the endogenous scavenging machinery. Many exogenous compounds can exert the same activity in different compartments of our organism, hence representing valuable aids against oxidative stress. Other compounds may lack red-ox properties, but are able to induce antioxidant endogenous defenses [3]. Natural products afford a wide plethora of such compounds [4,5]. For instance, several polyphenols have been reported as effective antioxidants, at least in vitro [6,7]. Usually, the antioxidants effects depend on the scavenging properties of such compounds, or to their ability to chelate metals such as Fe and Cu that can catalyze the formation of ROS [8]. In other cases, their ability to induce the cellular antioxidant defense systems or inhibit radical and oxidant production by regulating different types of oxidases has been documented [9]. The well-known stilbene resveratrol is one of such compounds. It has an inhibitory effect on excessive ROS production, aberrant mitochondrial distribution, and lipid peroxidation [10]. Furthermore, it can enhance the activity of glutathione peroxidase, glutathione S-transferase, and glutathione reductase [11]. Currently, resveratrol is undergoing clinical trials to assess its usefulness as a therapeutic agent for cardiovascular disorders, neuronal disorders, and tumors. The antioxidant properties of resveratrol are involved in its effect on diseases [11,12].

Assessing the antioxidant properties of plant extracts or purified natural compounds may not be straightforward. From one side, the chemical characterization of extracts is often challenging due to their high complexity, which also affects the isolation of pure bioactive components in high yields and purity [13]. From the other side, natural products often have a poor bioavailability that make it difficult to determine their antioxidant effects especially in vivo [14]. In light of these considerations, the chance of finding novel natural antioxidants is accompanied by the need to develop innovative and efficient methods for their chemical analysis, extraction and purification, as well as semi-synthetic and technological solutions to increase their bioavailability. The aim of this Special Issue was to gather the latest research regarding the identification of novel natural sources of antioxidants, their extraction, chemical characterization and pre-clinical evaluations. Many Authors contributed with research articles on natural products from different Regions of the World. For instance, three plant species from Galicia, Spain, namely Tradescantia fluminensis, Arundo donax, and Eucalyptus globulus were considered for their content in antioxidants (anthocyanins), whose extraction was achieved in high yields using optimized methods; ethanolic extracts of Calamintha incana leaves harvested in Jordan were shown to contain relevant amounts of p-cymene, a known antioxidant [15], as well as to exert significant antioxidant and antidiabetic properties; the antioxidant activity of Sasa borealis, a traditional natural medicine from South-Korea, was determined through different assays that highlighted its radical-scavenging (ABTS and DPPH) properties, and its ability to inhibit intracellular ROS production and induce the translation of the antioxidant enzymes superoxide dismutase 2 (SOD2), catalase (CAT), and glutathione peroxidase (GPx). Other Authors revised the literature regarding other natural sources of interest for their relevant content in antioxidants. Shirodkar and collaborators focused on Clitoria ternatea, a plant used in Ayurvedic medicine in South Asia. The flowers of this plant contain peculiar anthocyanins such as ternatins and delphinidin derivatives, which characterize their bright color. Other secondary metabolites with antioxidant properties such as kaempferol, quercetin and myricetin derivatives are present [16]. Like many other plant secondary metabolites, these compounds suffer of poor stability. Microencapsulation has been proposed as a strategy to preserve their chemical properties and their antioxidant potential for application in nutraceuticals as bioactive agents [17], or in food as additives [18]. An exhaustive review on the physicochemical properties and therapeutic potential of plant-derived extracellular vesicles isolated from seeds, leaves, and fruits has also been included in this Special Issue. These assemblies are gaining interest as drug delivery carriers thanks to their versatility, their ability to transport bioactive molecules and antioxidants and prevent their degradation, and their negligible toxicity [19].

Overall, the original articles and reviews published in this Special Issue may afford a potential contribution to future investigations on natural antioxidants. Novel potential sources of antioxidants have been uncovered, and methods for their effective extraction have been described. For some species, preliminary data on their antioxidant effects have been also presented. As Guest Editor of this Special Issue, I hope that its content will be of interest for many readers involved in the multifaceted study of antioxidants. As a conclusive note, I would like to sincerely thank all the authors for choosing this Special Issue to share the results of their research work, as well as the reviewers and Assistant Editors for their valuable support.