Antioxidant_Properties_of_Leafy_and_Non-Leafy_Vegetables

CHAPTER ONE 1.0 Introduction Oxidation, which is essential for the production of energy to fuel biological process usually produces free radicals and other reactive oxygen species that can damage tissues and causes cell death. Although almost all organisms possess antioxidant defence and repair systems that have evolved to protect them against oxidative damage, these systems are insufficient to prevent the damage activity entirely (Simic et al., 1992). However, antioxidant supplements or foods containing antioxidants may be used to help human body reduce oxidative damage. In recent years, there has been particular interest in the antioxidant and health benefits of phytochemicals in vegetables. Vegetables and herbs were the basis of nearly all medicinal therapy until synthetic drugs were developed in the nineteenth century but the use of these vegetables along with fruits and other herbs is still on the increase because of the numerous phytochemicals in addition to antioxidants present in them (Wei and Shiow, 2001). The presence of phytochemicals, in addition to vitamins and pro-vitamins, in fruits and vegetables has been considered of crucial nutritional importance in the prevention of chronic diseases, such as cancer, cardiovascular disease and diabetes (Doll and Petro, 1981). Organisms are endowed with endogenous (catalase, superoxide dismutase, glutathione peroxidase/reductase) and exogenous (vitamin C, E, ß-Carotene) antioxidant defence system against reactions of free radicals. However, the generation of free radicals in the body beyond its antioxidant capacity leads to oxidative stress which has been implicated in the aetiology of several pathological conditions such as lipid peroxidation, protein oxidation, DNA damage and cellular degeneration related to cardiovascular disease, diabetes, inflammatory disease, cancer and Parkinson disease. When left unpaired, it can cause base mutation, single and double-strand breaks, DNA cross-linking, and chromosomal breakage and rearrangement (Ames et al., 1993). As a result of this, much attention is been focused on the use of antioxidants especially natural antioxidant to inhibit and protect damage due to free radicals and reactive oxygen species. Synthetic antioxidant such as butylated hydroxyanisole (BHA), tert-butylated hydroxyquinone and butylated hydroxytoluene have been of utmost concern to many researchers because of their possible activity as promoters of carcinogenesis (Atiqur et al., 2008). Plant based antioxidant are now preferred to the synthetic ones because of their safety. Epidemiological studies have shown that the consumption of vegetables and fruits can protect humans against oxidative damage by inhibiting or quenching free radicals and reactive oxygen species (Ames et al., 1993). Many plants including fruits and vegetables are recognized as sources of natural antioxidants that can protect against oxidative stress and thus play an important role in the chemoprevention of diseases that have their aetiology and pathophysiology in reactive oxygen species (Odukoya et al., 2001). These positive effects are believed to be attributable to the antioxidants; particularly the carotenoids, flavonoids, lycopene, phenolics and β-carotene (Amin et al., 2004). Thus, the objective of this paper is to discuss the antioxidant properties of some leafy and non-leafy vegetables. CHAPTER TWO 2.0 Vegetables Vegetables are a large class of plants. They give variety of flavours and colour to feed and food and they include leaves, stems, seeds and flowers (Tindall, 1983). There has been a particular interest in the antioxidants and health benefits of phytochemicals in vegetables. This was as a result of their potential effect on human health (Wei and Shiow, 2001). Vegetables have been used for a large range of purpose including nutrition, medicine, flavourings, beverages and industries. Since pre-historic times, vegetables and herbs were the basis of nearly all therapies until synthetic drugs were developed in the nineteenth century (Wei and Shiow, 2001). However, vegetables along with fruits and other herbs are increasingly gaining popularity again over synthetic drugs because of the dreaded side effects of chemical accumulation in the body as a result of taking too much of synthetic drugs. On the other hand, vegetables and fruits contain phytochemicals that acts as antioxidants in the body. They also help to retain stronger bones. It has been shown to decrease the amount of calcium excreted in the urine (Wei and Shiow, 2001). Vegetables acts as an ‘alkaline buffer’ neutralizing acid produced when fish and meat are digested. These acids would otherwise tend to increase the amount of calcium host in the urine but, sufficient vegetable consumption neutralizes this effect (Appel et al., 1997). 2.1 Classification of Vegetables Vegetables are classified based on morphological features. They are: i. Non-leafy vegetable types • Earthy vegetable roots e.g. sweet potatoes (Ipeoma batata), Carrots (Daucus carota), etc. • Buds bulbs e.g. Onions (Allium pa), Garlic (Allium salvum), etc. • Vine fruits e.g. Cucumber (Cucumis sativus), pumpkin (Cucurbita maxima), etc. • Berry fruits e.g. African eggplant (Solanum macrocarpon), Tomato (Lycopersium macrocarpon), etc. • Legumes e.g. Garden pea (Psium sativum), green beans (Vigna unguiculata), etc. • Sprout e.g. Asparagus (Brassica oleraceae), etc. ii. Leafy vegetable type • Leafy vegetables e.g. Jute Mallow (Corchorus olitorius), bitterleaf (Vernonia amygdalina), etc. (Tindall, 1983). 2.2 Antioxidants Antioxidants are substances that are capable of counteracting the damaging, but normal effects of the physiological process of oxidation in animal tissues (Gey, 1998). Oxidative stress occurs when the production of harmful free radicals is beyond the protective capability of the antioxidant defences. Antioxidant works to protect lipids from peroxidation by radicals. Antioxidants are effective because they are willing to give up their own electrons to free radicals. When a free radical gains the electron from an antioxidant; it is no longer capable of attacking the cell and the chain reaction of oxidation is broken (Dekkers et al., 1996). Antioxidants are nutrients (vitamins and minerals) as well as enzymes (proteins), they are manufactured within the body and can also be obtained from the food humans eat such as fruits, vegetables, seeds, nuts, meat and oil (Dekkers et al., 1996). Antioxidants acts as radical scavenger, hydrogen donors, electron donors and peroxide. A characteristic feature of antioxidants is their effectiveness at very low concentrations (0.001-0.1%). Some of them e.g. tocopherols, have definite concentrations, which if exceeded can cause a decrease in its antioxidant activity and higher concentrations can cause a pro-oxidant effect. Antioxidants can be divided into two groups: natural and synthetic. In recent years, many investigations suggest limitation of synthetic antioxidant use, with regard to their toxicity. Also, the consumer show larger interest in food products with natural sources of antioxidative compounds (Pszola, 2001). Dietary antioxidants are considered beneficial because of their potential protective role against oxidative stress, which is involved in the pathogenesis of multiple diseases such as cancer and coronary heart disease. Antioxidants in vegetables appear to be of great importance in controlling damage by free radicals. Each antioxidant is unique in terms of its structure and functions. These antioxidants are: a) Vitamin E Vitamin E is a fat-soluble vitamin present in nuts, seeds, vegetables, fish oils, whole grains and cereals. Alpha-tocopherol is the most widely available isomer, which has the highest biopotency or strongest effect in the body. It safeguards cell membrane from damage by free radicals. Alpha-tocopherol also protects low-density lipoproteins (LDLs) from oxidation (Morrisey, 1999). Vitamin E is the major hydrophobic chain-breaking antioxidant that prevents the propagation of free radical reactions in the lipid components of membranes, vacuoles and plasma lipoproteins (Ricciarelli et al., 2001). b) Vitamin C Vitamin C also known as ascorbic acid, is a water-soluble vitamin present in citrus fruits and juices, green peppers, cabbage and strawberries (Kendall, 2000). It scavenges free radicals that are in the aqueous phase of cell. Vitamin C works synergistically with vitamin E to quench free radicals of the smokers. Vitamin C protects the body against cancer of the oesophagus, oral cavity and stomach. It also regenerates the reduced form of vitamin E (Mimica-Dukie, 2000). c) β-Carotene It is a precursor to Vitamin A (retinol) and is present in dark vegetables. In theory, β-carotene has remarkable antioxidant chemistry. β-carotene can interact with a free radical in the presence of oxygen to form peroxyl radicals. It is very effective in the protection against oxidative changes. β-carotene is a quencher of singlet oxygen and is also especially excellent at scavenging free radicals in low oxygen concentration (Bray, 1999). d) Selenium It is a trace element. It is a mineral that humans need to consume only in small quantities. It forms the active site of several antioxidant enzymes including glutathione peroxidase. Similar to selenium, the minerals manganese and zinc are trace elements that form an essential part of various antioxidant enzymes (Kendall, 2000). e) Other antioxidants In addition to vitamins and minerals, there appear to be many other compounds that have antioxidant properties. Among them is co-enzyme Q10 (or ubiquinone) which is essential for energy production and can also protect the body from destructive radicals. Also, uric acid, a metabolic product of purine nucleotides, has become increasingly recognized as important antioxidant (Robin, 2004). 2.3 Mode of Action of Antioxidants Antioxidant defense system against oxidative stress is composed of several lines and the antioxidants are classified into four categories based on function: i. Preventive antioxidants, which suppress formation of free radicals (enzymes such as glutathione peroxidase, catalase, selenoprotein, carotenoids etc.). ii. Radical scavenging antioxidants suppressing chain initiation and/or breaking chain propagation reactions. iii. Repair and de novo antioxidants (some proteolytic enzymes, repair enzyme of DNA etc.) and iv. Adaptation where the signal for the production and reactions of free radicals induces formation and transport of the appropriate antioxidant to the right side (Bray, 1999). 2.4 Antioxidant Properties of Leafy Vegetables 2.4.1 Antioxidant Properties of Vernonia amygdalina (Bitter leaf) Vernonia amygdalina (plate 1) is a perennial shrub that belongs to the Asteraceae family and is popularly called bitter leaf in English. It is known as Ewuro in yoruba, Etidot in Ibibio, Onugbu in Igbo, Ityuna in Tiv, Ilo in Igala, Oriwo in Edo and Chusar-doki in Hausa. It has petiolate leaves of about 6mm diameter and ellicptic in shape. The leaves are green with a characteristic odour and bitter taste (Akpaso et al., 2011). They are well distributed in tropical African and Asia and are commonly found along drainage lines and in natural forest or commercial plantation. In most part of Africa, the leaves of V. amygdalina are used as soup condiments after washing or boiled to get rid of the bitter taste. Specifically, it is used to prepare the popular Nigerian bitter leaf soup, Onugbo and as spice in the Cameroon dish called Ndole (Yeap et al., 2010). Huffman and Seifu (1989) reported the use of V. amygdalina in the treatment of parasite related disease in wild chimpanzee in Tanzania. This necessitated quite a great number of researches to test the efficacy of different part of the plant in managing a wide array of ailments. Many traditional medicine practitioners use different parts of the plants in treating various ailments for instance the whole plant is being used as antihelminth, antimalaria and as a laxative. Their traditional use is not limited to human alone, in northern Nigeria it has been added to horse feed to provide a strengthening or fattening tonic Chusan-Dokin in Hausa (Igile et al., 1994). Different extracts of V. amygdalina has been shown to possess antioxidant properties both in vitro and in vivo. Ayoola et al., (2008) showed the in vitro antioxidant properties of the ethanolic extract of leaves of V. amygdalina using the 2,2-diphenyl-1-picrylhydrazyl radical (DPHH) scavenging test. V. amygdalina was shown to have moderate inhibition of 77.7% thus indicating the scavenging ability of the vegetable. Also, the aqueous and ethanolic extract of V. amygdalina has further been shown to have potent antioxidant properties as they were able to inhibit bleaching of β-carotene, oxidation of linoleic acid and lipid peroxidation induced by Fe2+/ascorbate in a rat liver microsomal preparation. This study showed that the antioxidant activity of the ethanolic extracts was higher than that of the aqueous extracts, and compared favourably with synthetic antioxidant BHT and BHA (Owolabi et al., 2008). However, another study reported that methanol extract displayed highest antioxidant activity followed by acetone and water extract (Erasto et al., 2007). Further confirmation of the antioxidant activities of V. amygdalina was reported by Oloyede and Ayila (2012). They investigated the antioxidant activity of different extracts, aqueous, methanol, hexane, ethylacetate and butanol extracts of Vernonia amygdalina using three methods: scavenging effect on 2,2-diphenyl-1-picryhydrazyl radical (DPPH), hydroxyl radical and peroxide oxidation by ferric thiocynate method. All fractions showed significant antioxidant activity (p