**1. Introduction**

Reactive oxygen species (ROS) are chemically unstable oxygen-containing molecules such as superoxide anions and hydroxyl radicals that are able to readily react with and inflict damage to cellular constituents such as nucleic acids, proteins, and lipids [1–3]. ROS are continuously formed in the body during metabolic reactions involving oxygen such as the mitochondrial electron transport chain, in activated white blood cells in order to eliminate bacteria and other invaders, and as products of various intracellular enzymatic reactions such as those catalyzed by nitric oxide synthase and xanthine oxidase, which yield nitric oxide radicals and superoxide radicals, respectively [1–3]. ROS are also produced following exposure of the body to various noxious agents ranging from car exhaust and cigarette smoke to γ-radiation and certain medical drugs [1–3].

ROS play important roles in the cells of the body, for instance, as elements of intracellular signaling pathways for several normal physiological functions including those associated with the regulation of immunity, cell differentiation, and longevity [4–6]. However, a buildup of these species may cause oxidative stress, cell and tissue injury, and cell death [4–6] and is probably at the basis of several ailments such as heart conditions, Alzheimer's disease, and cancer, as well as premature aging and cerebrovascular accidents [7–11]. For this reason, the body has a variety of innate antioxidant defense mechanisms to its disposal to mitigate potential damage by ROS, including enzymatic antioxidant systems (for instance, superoxide dismutase, catalase, and glutathione peroxidase) and non-enzymatic systems (for instance, bilirubin, glutathione, and certain vitamins) [12]. In addition to these innate defense systems, exogenous antioxidants provided through the diet and/or nutritional supplements may help protect the body from oxidative stress [13]. Thus, the consumption of compounds rich in antioxidants may decrease the risk of developing the abovementioned diseases [14–16].

An important class of plant-derived antioxidants is represented by phenolic compounds, secondary plant metabolites made up of one or more aromatic ring(s) coupled to one or more hydroxyl group(s) [17]. Phenolic compounds help protect plants from pathogens, animal and insect attack, as well as ultraviolet radiation; provide plants their characteristic colors; and contribute to the organoleptic properties of plants [18]. There are tens of thousands of plant phenolic compounds including the main dietary constituents flavonoids, phenolic acids, and tannins, in addition to coumarins, naphthoquinones, stilbenes, anthraquinones, and lignans [13, 17]. Their mitigating effect on oxidative stress has been attributed to their ability to eliminate potentially harmful oxidizing free radical species by acting as reducing agents, hydrogen donors, quenchers of singlet oxygen, or chelators of metal ions that catalyze oxidation reactions [13, 17].

The pea family Fabaceae is a large family of flowering plants that include various economically important plants such as the soybean *Glycine max* (L.) Merr., the cowpea *Vigna unguiculata* (L.) Walp.), and the peanut *Arachis hypogaea* L. [19, 20]. The Fabaceae family also includes many species that represent important sources of a wide variety of ethnobotanical medicines against a myriad of diseases (see, for instance, references [20, 21]). This may be attributable to their relatively high contents of various pharmacologically active constituents including phenolic compounds with antioxidant properties [22, 23]. In addition, the Fabaceae is considered a plant family that hyperaccumulates selenium, a key constituent of selenoproteins such as the antioxidant enzyme glutathione peroxidase [24].

*Phenolic Compounds and Antioxidant Activities of Eight Species of Fabaceae… DOI: http://dx.doi.org/10.5772/intechopen.106076*

The Republic of Suriname (South America) has a land area of roughly 165,000 km2 , about 80% of which consists of sparsely inhabited, dense, pristine, and highly biodiverse tropical rain forest [25]. Conversely, about 80% of the country's population of just over 600,000 lives in the relatively narrow northern coastal zone of the country [26]. Mostly because of the variety of habitats and the humid tropical temperature, the biodiversity in Suriname is high, encompassing roughly 5100 different plant species [27]. As in other parts of the world, the Fabaceae plant family represents a substantial part of Suriname's plant diversity, with estimations of over 400 different species in more than 100 genera from the northern coast all the way up to the expansive forested mountain ranges [28]. The Fabaceae are also ingredients of a large variety of traditional medicines in Suriname. So far, it is not clear whether this is because of their remarkably high phenolic content and antioxidant activity. In this chapter, we have addressed this topic by assessing whether the traditional uses and pharmacological activities of eight medicinally commonly employed Fabaceae in Surinamese traditional medicine may be associated with their phenolic content and antioxidant activity.

### **2. ROS and oxidative stress**

ROS can be defined as oxygen-containing reactive species and include oxygen-free radicals with unpaired electrons such as superoxide, hydroxyl, peroxyl, and alkoxyl radicals, as well as non-radical species such as hydrogen peroxide, peroxynitrite, hypochlorous acid, and ozone [1–3]. Reactive nitrogen species (RNS) such as nitric oxide, peroxynitrite, and nitrogen dioxide radical, as well as reactive chlorine species (RCS) such as hypochlorous acid, are also classified as ROS [1–3]. As mentioned in the preceding section, ROS are able to readily react with and cause damage to biomolecules including proteins, lipids, and nucleic acids, leading to cell and tissue injury [4–6]. The high reactivity of ROS derives from the presence of a single unpaired electron in their outer orbit formed as a result of incomplete reduction of the oxygen metabolites [4–6].

ROS can be generated from either endogenous or exogenous sources. Endogenous sources of ROS are cellular organelles where oxygen metabolism is high, such as mitochondria, phagocytic cells, endoplasmic reticulum, and peroxisomes [12]. For instance, during oxidative phosphorylation in the mitochondria, the electron transport chain produces electrons for the reduction of molecular oxygen into superoxides. The superoxides are transformed into the much less reactive hydrogen peroxide by superoxide dismutase. However, when hydrogen peroxide interacts with ions of transition metals such as Fe2+ and Cu2+, the most reactive ROS, hydroxyl radicals are formed through Fenton's reaction [29]. And phagocytized bacteria, bits of necrotic tissue, other harmful cells, and foreign particles are destroyed by macrophages and neutrophils by the so-called respiratory burst (or oxidative burst), involving the rapid release of superoxides and hydrogen peroxide following the supply of electrons by NADPH [30].

Other important endogenous (enzymatic) sources of ROS are the cytochrome P450 superfamily of enzymes that produce ROS during the detoxification and excretion of xenobiotics [31], cyclooxygenase and lipoxygenase that generate ROS from arachidonic acid [32], and xanthine oxidoreductase that produces superoxide anions during the breakdown of purines to uric acid [33]. And as mentioned before, in the Fenton and Haber-Weiss reactions, molecular oxygen is reduced to form superoxide

anions, which dismutates to form hydrogen peroxide that can react with traces of iron or copper to form more highly reactive hydroxyl ions and subsequently hydroxyl radicals [34].

Exogenous sources of ROS are γ-radiation and UV radiation; air pollutants such as car exhaust, cigarette smoke, and industrial contaminants; peroxides, aldehydes, oxidized fatty acids, and transition metals in foods; a large variety of xenobiotics including toxins, pesticides, and herbicides; as well as various medical drugs such as narcotics, anesthetizing gases, and antineoplastic agents [1–3]. Gamma radiation, for instance, interacts with water molecules to form water radical cations and free electrons, which react with other water molecules to form highly active hydroxyl radicals, superoxides, and organic radicals. These ROS are then converted into organic hydroperoxides and hydrogen peroxide, which subsequently react with Fe2+ and Cu2+ ions, generating even more ROS, eventually resulting in massive damage to cellular biomolecules such as DNA, proteins, and lipids [35].

Iron and copper, along with cadmium, nickel, arsenic, and lead, not only generate ROS by Fenton or Haber-Weiss type reactions, but also by direct reactions with cellular constituents, producing, for example, thiol-type radicals [36]. For instance, arsenic induces the production of peroxides, superoxides, and nitric oxide and inhibits antioxidant enzymes such as glutathione-transferase, glutathione-peroxidase, and glutathione-reductase by binding to the sulfhydryl group [37]. And lead triggers lipid peroxidation and increases glutathione peroxidase concentration in brain tissue [38]. The free radicals generated from these reactions can affect DNA, with substitutions of some DNA bases such as guanine with cytosine, guanine with thymine, and cytosine with thymine [39].

An example of a medical drug that generates ROS is the antitumor antibiotic doxorubicin, both the antineoplastic activity and the cardiomyopathy of which are probably based on its reduction to a semiquinone-derivative that can autoxidize in the presence of oxygen and then produces superoxide anions following electron donation by oxidases such as mitochondrial NADPH and nitric oxide synthases [40].

### **3. Defenses against oxidative stress**

At non-cytotoxic levels, ROS and their secondary electrophilic species perform important functions in the human body, among others, by acting as redox signaling messengers required for the normal physiological functioning of cells [41]. In general, ROS are messengers in the transduction of certain metabolic and environmental cues, which affect diverse signaling pathways, culminating in the activation of transcription factors and other proteins, determining cell fate [5]. A well-described example is redox signaling involving the oxidation of cysteine residues of proteins by hydrogen peroxide, and converting a thiolate anion in cysteine (Cys-S-) into the sulfenic form (Cys-SOH), causing the protein to undergo allosteric changes that alter its function [42]. The sulfenic form can be reduced to thiolate anions by the disulfide reductases thioredoxin and glutaredoxin, to return the protein function to its original state [43]. Comparable reversible ROS-operated mechanisms are involved in the regulation of several key signal transduction pathways such as the PI3K-AKT and RAS-MEK-ERK pathways involved in the promotion of cell proliferation, nutrient uptake, and cell survival [44, 45].

Whether ROS cause oxidative stress and cellular damage is determined by the net result of their production and elimination by antioxidant defenses. Thus, oxidative

stress is a consequence of "a disturbance in the prooxidant to antioxidant balance in favor of the former, leading to potential damage" [3]. The antioxidant defenses prevent the formation of ROS or interrupt their propagation, eliminate ROS by scavenging them, slow down redox reactions by removing free-radical intermediates, inhibit oxidation reactions by being oxidized themselves, and repair the oxidized molecules [46]. These mechanisms can be distinguished into innate defense systems and exogenous antioxidants provided through the diet and/or nutritional supplements.

#### **3.1 Innate antioxidant defenses**

The innate antioxidant defenses of the body comprise enzymatic and non-enzymatic systems. The main enzymatic antioxidant systems are superoxide dismutase, catalase, and glutathione peroxidase. The metalloprotein superoxide dismutase catalyzes the dismutation of superoxides, that is, the formation of one molecule of oxygen and one molecule of hydrogen peroxide from two superoxides [47, 48]. Hydrogen peroxide can subsequently be converted into highly reactive hydroxyl radicals in the presence of transition metal ions such as Fe2+ or Cu2+ in the Fenton reaction, propagating the damage inflicted to cellular DNA, proteins, and lipids [47, 48]. Superoxide dismutase prevents this process through its three isoforms, cytosolic copper/zinc-superoxide dismutase (Cu/Zn-SOD, SOD1), mitochondrial manganese superoxide dismutase (Mn-SOD, SOD2), and extracellular copper/zinc-superoxide dismutase (Cu/Zn-EC-SOD, SOD3) [47, 48]. The isoforms are located in distinct cellular compartments and/or have different metal components, but all three convert and neutralize superoxides as mentioned above [47, 48].

Catalase acts as a catalyst for the conversion of hydrogen peroxide into oxygen and water. It mitigates the effect of intracellular hydrogen peroxide [49]. Glutathione peroxidases are a family of at least eight oxidoreductases that contain seleno-cysteine in the active site [50, 51]. These enzymes catalyze the reduction of organic hydroperoxides into alcohol and water groups using reduced glutathione as a co-substrate [50, 51]. They can also catalyze the reduction of hydrogen peroxide to water and oxygen by oxidation of reduced glutathione to its disulfide [50, 51]. Oxidized glutathione can be reduced to glutathione by the enzyme glutathione reductase by using NADPH as a reducing substrate [50, 51]. In this way, glutathione peroxidase protects cells from oxidative damage and helps detoxify hydrogen peroxide [50, 51].

Non-enzymatic endogenous antioxidant mechanisms are, among others, bilirubin and albumin. Bilirubin is produced from the enzymatic degradation of hemoglobin and other heme proteins to first yield biliverdin, and then bilirubin following reduction of biliverdin by the enzyme biliverdin reductase [52]. Bilirubin prevents lipid oxidation by removing peroxyl radicals whereby it is oxidized itself to biliverdin, after which it is rapidly reduced by biliverdin reductase to bilirubin [52]. And serum albumin represents an abundant circulating antioxidant defense system [53]. It is able to bind transition metals such as copper and iron, preventing the formation of hydroxyl radicals *via* the Fenton reaction after their interaction with hydrogen peroxide, directly scavenge hydroxyl radicals, and bind and transport bilirubin, which then acts as an inhibitor of lipid peroxidation [53].

#### **3.2 Exogenous defenses: dietary nutrients**

Exogenous antioxidants are mainly derived from dietary sources and include, among others, a variety of phenolic compounds, essential minerals, vitamins, small peptides, and certain fatty acids [13]. Their health-promoting and preventive effects against diseases associated with oxidative stress are now well established [7–11]. The most common phenolic compounds in the diet are phenolic acids and various subclasses of flavonoids, which together account for on average 60 and 30%, respectively, of the total dietary intake of phenolic compounds [54].

Phenolic compounds are able to act as antioxidants in multiple ways, among others, because of their redox properties, which enable them to adsorb and neutralize free radicals, quench singlet and triplet oxygen, or decompose peroxides [55, 56]. These processes are accomplished by hydrogen atom transfer, transfer of a single electron, sequential proton loss electron transfer, or chelation of transition metals [55, 56]. In addition, phenolic compounds are able to act synergistically with other antioxidants such as ascorbic acid, β-carotene, and α-tocopherol [57] and are presumably also involved in the regulation of intracellular glutathione levels [58].

Other dietary constituents with antioxidant properties are certain essential minerals, vitamins, small peptides, and fatty acids. Copper, iron, manganese, zinc, and selenium are indirectly involved in the body's antioxidant defenses by enhancing the activities of antioxidant enzyme. For instance, selenium is a cofactor of glutathione transferase and other selenoproteins [59]. It has notable antioxidant activity [60] and may be beneficial in chronic conditions such as cancer [61], heart disease [62], and cognitive disorders [63]. And copper, zinc, and manganese are cofactors of superoxide dismutase [64].

Antioxidant vitamins such as ascorbic acid are able to quench ROS by donating electrons to them; α-tocopherol inhibits ROS generation, preventing lipid peroxidation of cellular membranes; thiamin is a cofactor of NADPH that is required for the production of glutathione reductase and the activity of catalase; and the retinol precursor β-carotene reacts with peroxyl, hydroxyl, and superoxide radicals [65, 66]. The common dietary small peptide glutathione is able to directly scavenge ROS [67]. And polyunsaturated fatty acids in, for instance, fish oil, are able to eliminate ROS and inhibit cellular processes that generate ROS, decreasing the risk of cardiovascular diseases by reducing triacylglycerol production in the plasma [68].
