Plants as a Source of Natural Antioxidants

By Sang-Uk Chon, Feng Chen, S E Atawodi and 11 more

A comprehensive overview of both traditional and current knowledge on the health effects of plant based antioxidants, this book reviews medicinal and aromatic plants from around the world. It covers the different sources of antioxidants including essential oils, algae and marine microorganisms, as well as the role of abiotic and biotic stresses, endophytes, transgenic approaches in scavenging ROS and antioxidant plants used in different therapeutic systems.

• Language: English
• Publisher: CAB International
• Release date: Dec 18, 2014
• ISBN: 9781789244366

Book preview

Preface

Reactive oxygen species (ROS), which are also known as active oxygen species (AOS) and ­reactive oxygen intermediates (ROI) are formed as by-products of oxidative metabolism. In addition to metabolism, harmful radiation and attacks by pathogens also induce the formation of ROS. These free radicals, as is evident from their various names, are highly reactive and many can start chain reactions that form yet more free radicals. All types of cell components are at risk of oxidative damage from free radicals. In humans, this type of damage can cause various degenerative conditions that may lead to cancer and cell ageing. Hence, antioxidants have a positive effect on general health in humans who, in addition to their endogenous antioxidants, take in a considerable amount of antioxidants with the diet. As these molecules are not food per se, but have health effects, they are called nutraceuticals.

There is presently an increased interest worldwide in identifying antioxidant compounds that are pharmacologically effective and have low or no side effects for use in preventive medicine and the food industry. Plants are susceptible to damage caused by active oxygen, and produce a significant amount of various antioxidant (or potentially antioxidant) compounds (in addition to tocopherols). These compounds include flavonoids, other phenolic compounds and polyphenolics (condensed and hydrolysable tannins, lignin precursors). Such compounds can prevent the oxidative stress caused by the production of ROS, act as ROS-scavenging compounds and provide broad-spectrum protection against oxidative radicals. Ayurveda, Unani, Chinese and other traditional medicine systems provide a substantial lead into finding active and therapeutically useful antioxidant compounds from plants, as does research on the phytochemistry of plants with antioxidant activity. Indeed, many aromatic, medicinal and spice plants have been confirmed to contain compounds with strongly antioxidative components.

The aim of the book is to provide up-to-date basic information on antioxidant plants from different sources and on the role of different abiotic and biotic stresses, endophytes and mycorrhizal fungi in the development of antioxidant compounds in plants. There is also discussion of transgenic approaches to the scavenging of ROS, and of the antioxidant plants used in different therapeutic systems. Overall, the book throws light on the different medicinal and aromatic plants that have the potential to be used as antioxidants. It will be an excellent reference for medical practitioners, botanists, phytochemists, pharmacologists, microbiologists, biotechnologists and herbal drug researchers and practitioners. The book will also serve as a comprehensive overview of traditional and current knowledge on the health effects of plant-based antioxidants and, bearing in mind the side effects of synthetic antioxidants, will be relevant to the advancing back to nature movement of today’s world.

The book has been devised as a ‘one-stop platform’ comprising a perfect blend of comprehensive information on plants as a source of natural antioxidants. It has 14 chapters contributed by eminent scientists working in the field of antioxidants and natural products. These cover most aspects of plant-based antioxidants, focusing on up-to-date information contributed by world experts in the field and taking a global look at the subject. The chapters include information on traditionally used antioxidants from different biodiversity rich countries, and on the antioxidant potential of algae, endophytic fungi, marine microorganisms, mushrooms and mycorrhizal fungi, as well as plants themselves. In addition, pharmacological, biochemical, biotechnological and industrial aspects have also been covered, Further, as a result of the interdisciplinary specialization that there is within various fields, an attempt has been made to provide a pertinent collection of references on the subject of natural antioxidants within a single volume.

I am very grateful to the contributors for their timely responses in the production of the book, in spite of their busy academic schedules, and wish to express my gratitude to them all for providing their excellent chapters. Without their full cooperation, this work would not have been possible.

My wife, Dr Nirmala Kishore, has always been my intellectual companion and provided me with constant inspiration in bringing out the book. My beloved daughter, Dr Vatsala Kishore MD, and my son, Navneet Kishore, have always provided me with unmatched help and sacrifices. I also bow my head to my father, Sri G.N. Dubey, mother, Smt Shanti Devi, and father-in-law, Prof. Ram Deo Shukla, for their blessings and encouragement. My sincere thanks are also due to my research students, Archana, Priyanka, Bhanu, Prashant, Akash, Abhishek and Manoj, for their help and cooperation.

Thanks are also due to CABI Publishers for publishing the book, taking the utmost interest and providing helpful assistance and understanding. Special thanks go to Dr Sreepat Jain, the Commissioning Editor, who initially motivated me to bring out this book and has provided his full support, and also to Alexandra Lainsbury, Editorial Assistant at CABI.

N.K. Dubey

1 Plants of Indian Traditional Medicine with Antioxidant Activity

Nawal Kishore Dubey, * Akash Kedia, Bhanu Prakash

and Nirmala Kishore

Department of Botany, Banaras Hindu University,Varanasi, India

1.1 Introduction

Free radicals are chemical species that have one or more unpaired electrons, as a result of which they are highly unstable and can cause damage to other molecules by extracting electrons from them in order to attain stability. Among them are reactive oxygen species (ROS) that include superoxide radicals, ­hydroxyl radicals, singlet oxygen and hydrogen peroxide, which are often generated as by-products of biological reactions but can also be derived from exogenous factors (Cerutti, 1991). Some ROS have positive biological roles, in processes such as energy production, phagocytosis, regulation of cell growth, intercellular signalling and synthesis of biologically important compounds (Halliwell, 1997). Often though, they can induce the oxidation of lipids, causing membrane damage and decreasing membrane fluidity. ROS can also lead to cancer via DNA mutations (Cerutti, 1991, 1994; Pietta, 2000), and to abnormal ageing and neurodegenerative diseases (Beal, 1995).

The amounts of ROS present in an organism can be regulated by synthesizing enzymes such as endogenous superoxide dismutase, glutathione peroxidase and catalase, or by non-enzymatic antioxidants such as ascorbic acid (vitamin C), α-tocopherol (vitamin E), glutathione (GSH), carotenoids, flavonoids, etc. Sies (1993) has examined these strategies. As already noted, the overproduction of reactive species, induced by exposure to external oxidant substances, or by a failure in the usual defence mechanisms, can lead to the development of degenerative diseases (Shahidi et al., 1992); these include cardiovascular diseases, cancers (Gerber et al., 2002), neurodegenerative diseases (for instance Alzheimer’s disease; Di Matteo and Esposito, 2003) and inflammatory diseases (Sreejayan and Rao, 1996). In particular, the hydroxyl radical is known to react with all of the components of DNA (Halliwell and Gutteridge, 1999), with the polyunsaturated fatty acid residues of phospholipids (Siems et al., 1995) and with the side chains of all amino acid residues of proteins, especially cysteine and methionine residues (Stadtman, 2004).

One solution to this major problem is to supplement the diet with antioxidant compounds that are found in natural plant sources (Knekt et al., 1996). Plants produce antioxidants to counter the oxidative stress caused by the production of ROS during photosynthesis and thus represent a source of new anti­oxidant compounds. The traditional Indian medicine system of Ayurveda has a special branch called rasayana in which disease is prevented and the ageing process counteracted through the optimization of home­ostasis. Some of the plants used in rasayana preparations have been found to be 1000 times more potent than ascorbic acid, α-tocopherol, and probucol in their antioxidant activity (Scartezzini and Speroni, 2000).

In recent years, the use of natural antioxidants present in traditional medicinal plants has become of special interest in the scientific world due to their presumed safety and nutritional and therapeutic value (Ajila, In recent years, the use of natural antioxidants present in traditional medicinal plants has become of special interest in the scientific world due to their presumed safety and nutritional and therapeutic value (Ajila, et al., 2007). This contrasts with the synthetic antioxidants that are commonly used in processed foods, such as butylated hydroxytoluene (BHT) and butylated hydroxyanisole (BHA), which have side effects and have been reported to be carcinogenic (Ito et al., 1983). The majority of the antioxidant activity of plants is due to the presence of phenolic compounds (flavonoids, phenolic acids and alcohols, stilbenes, tocopherols, tocotrienols), ascorbic acid and carotenoids. Recent reports have indicated that there is an inverse relationship between the dietary intake of antioxidant-rich foods and the incidence of human disease, so it seems that natural plant antioxidants can serve as a type of preventive medicine. A large number of plants worldwide have been found to have both strong antioxidant activity (Baratto et al., 2003) and powerful scavenger activity against free radicals (Kumaran and Karunakaran, 2007).

India is a land of multiple geographical regions, and its flora, with more than 45,000 plant species, represents 7% of the world’s flora. Out of this vast number of plant species, medicinal plants comprise approximately 8000 species, and account for about 50% of all the Indian higher flowering plant species and 11% of total known world flora (Ali et al., 2008). A number of these Indian medicinal plants have been used in the traditional Ayurveda system of medicine for thousands of years. Ayurveda (literally ayus, life, and veda, knowledge; hence science of life) is the oldest medical system in the world and has been practised in India for more than 3500 years. The first recorded book on Ayurvedic medicine was Acharya Charak’s Charaka Samhita (600

bc

), and traditional healers have used this resource since time immemorial for the benefit of humankind. Other ancient Indian literature is also a source of information on the medicinal properties of herbal plants and preparations that have been found to be effective in the treatment of various diseases, as detailed in the Glossary of Indian Medicinal Plants (Chopra et al., 1956). The more modern manifestation of Ayurveda is Maharishi Ayurveda (Glaser, 1988).

The World Health Organization (WHO) has estimated that almost 80% of the earth’s inhabitants believe in traditional medicine for their primary health care needs, and that most of this therapy involves the use of plant extracts and their active components (­Winston, 1999). A number of plants and plant products have medicinal properties that have been ­validated by recent scientific developments throughout the world, owing to their potent pharmacological activity, low toxicity and economic viability. A plethora of literature is available on traditional Indian medicinal plants with antioxidant activity (Scartezzini and Speroni, 2000; Ali et al., 2008). This chapter reviews the antioxidant activity of such traditional Indian medicinal plants based on a literature survey.

1.2 Some Traditionally used Antioxidant Plants and Methods Used for ­Screening Them

Ayurveda, whose efficacy has been approved by the WHO (Zaman, 1974) provides an approach to prevention and treatment of different diseases by a large number of medical procedures and pharmaceuticals. There is a long list of traditional Indian medicinal plants that show antioxidant activity when screened by different methods. Table 1.1 presents a selection of such plants as reported by different researchers, with brief details of the assay methods and plant preparations used for each; further information on the methods mentioned in the table is given below.

A number of methods have been described by different workers for testing the antioxidant activity of medicinal plants (see Ali et al., 2008 and Krishnaiah et al., 2011). They include the following in vitro enzymatic and non-enzymatic antioxidant assays:

•    1,1-diphenyl-2-picrylhydrazyl (DPPH, also designated 2,2-diphenyl-1-picrylhydrazyl) radical scavenging (Brand-­Williams 1,1-diphenyl-2-picrylhydrazyl (DPPH, also designated 2,2-diphenyl-1-picrylhydrazyl) radical scavenging (Brand-­Williams et al., 1995);

•    β-carotene linoleic acid bleaching (Koleva et al., 2002);

•    inhibition of linoleic acid peroxidation (Osawa and Namiki, 1981);

•    ferric reducing antioxidant power (FRAP) (Benzie and Strain, 1996);

•    total radical trapping antioxidant potential (TRAP) (Krasowska et al., 2001);

•    oxygen radical absorbance capacity (ORAC) (Huang et al., 2002);

•    15-lipoxygenase inhibition (Lyckander and Malterud, 1992);

•    lipid peroxidation (LPO) (Ramos et al., 2001);

•    nitroblue tetrazolium (NBT) reduction or superoxide anion scavenging activity (Kirby and Schmidt, 1997);

•    hydroxyl radical scavenging activity (­Jodynis-Liebert et al., 1999);

•    non-site- and site-specific deoxyribose degradation assay (Maulik et al., 1997);

•    hydrogen peroxide scavenging activity (Ruch et al., 1989);

•    2,2′-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) radical scavenging (Re et al., 1999);

•    reducing power assay (Oyaizu, 1986);

•    Briggs Rauscher (BR) method (Cervellati et al., 2002);

•    Trolox equivalent antioxidant capacity (TEAC) method (Rice-Evans et al., 1996) – Trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid) is a water-­soluble vitamin E analogue used as a standard antioxidant;

•    phenazine methosulfate–nicotinamide adenine dinucleotide reduced (PMS–NADH) system superoxide radical scavenging (Lau et al., 2002);

•    linoleic acid peroxidation–ammonium thiocyanate (ATC) method (Masuda et al., 1992); and

•    ferric thiocyanate (FTC) and thiobarbituric acid (TBA) reaction methods (Mackeen et al., 2000).

Of these methods, the most widely used and reliable methods are the ABTS and DPPH methods.

Auddy et al. (2003) screened the antioxidant activity of the ethanolic extracts of three Indian medicinal plants traditionally used for the management of neurodegenerative diseases, viz. Sida cordifolia, Evolvulus alsinoides and Cynodon dactylon, and found IC50 (half maximal inhibitory concentration) values 16.07, 33.39 and 78.62 mg/ml, respectively, when tested with the ABTS assay. Using the same assay, the relative antioxidant capacity (IC50) for water infusions of the same three plants was as follows: E. alsinoides, 172.25 mg/ml; C. dactylon, 273.64 mg/ml; and S. cordifolia 342.82 mg/ml. When tests were performed of the effects of the water infusions on lipid peroxidation, the IC50 values were as follows: E. alsinoides 89.23 mg/ml; S. cordifolia, 126.78 mg/ml; and C. dactylon. 608.31 mg/ml.

Naik et al. (2003) examined the antioxidant potential of four aqueous extracts from different parts of medicinal plants used in Ayurvedic medicine, viz. Momordica charantia, Glycyrrhiza glabra, Acacia catechu and Terminalia chebula, using the ABTS and DPPH methods. The T. chebula extract showed the maximum potency and was equivalent to that of ascorbic acid. The IC50 value of the methanolic leaf ­extract of Amaranthus viridis (14.25 μg/ml) was greater than that of BHT (15.7 μg/ml) when tested with the DPPH assay (Iqbal et al., 2012). In a study by Reddy et al. (2005), three plant foods, viz. dried amla (Indian gooseberry, ­Emblica officinalis) fruits, dried drumstick (­Moringa oleifera) leaves and raisins (from Vitis vinifera) exhibited a high percentage of antioxidant activity when evaluated using the β-carotene–linoleic acid assay in an in vitro system and compared with BHA.

Ali et al. (2008) reviewed 24 Indian ­medicinal herbs reported to have antioxidant properties. Gupta and Sharma (2006) provided a brief account of research reports on common plants found in India, including traditional medicinal plants with antioxidant potential. Scartezzini and Speroni (2000) reviewed the antioxidant activity of Curcuma longa, Mangifera indica, M. charantia, P. emblica, Santalum album, Swertia chirata and Withania somnifera, all of which are used in Indian traditional medicine. Rathee et al. (2007) found that the acetone-soluble fraction of the ethyl acetate extract of Nyctanthes arbor-tristis (harsingar) leaf had impressive antioxidant activity as shown by the DPPH, hydroxyl and superoxide radical and H2O2 scavenging assays. Tanti et al. (2010) showed that the methanolic leaf extract of Dendrocnide sinuata, a medicinal plant used by the different tribal communities of north-east India, exhibited high free radical scavenging activity in the DPPH assay at concentrations of 75 and 100 μg/ml.

1.3 Phytochemistry of Antioxidant Plants

Several studies have been carried out to identify antioxidant compounds that are pharmacologically potent and have a low profile of side effects. The Ayurveda system provides many leads for finding active and therapeutically useful compounds from plants. Poly-­herbal preparations in Indian traditional medicine may have antioxidant activity arising from their constituent plants, and these may act synergistically to prevent ageing and related degenerative diseases. Several Indian medicinal plants have been extensively used in the Ayurveda system as rejuvenators, slowing the process of ageing and related disorders. Plants and plant products are also part of the vegetarian diet and may exhibit their medicinal properties in this way. Moreover, the active principles have been isolated from a large number of medicinal plants; examples include mangiferin from M. indica (Ghosal, 1996), the tannins emblicanin A and B from P. emblica (Ghosal et al., 1996) and curcumin from C. longa (Ammon and Wahl, 1991).

The antioxidant activity of medicinal plants may be attributed to the presence of various phytochemicals (often secondary metabolites) that have been identified. Natural plant antioxidants are mainly in the form of phenolic compounds (flavonoids, phenolic acids and alcohols, stilbenes, tocopherols, ­tocotrienols), ascorbic acid and carotenoids. Of these, the flavonoids, tannins and plant phenolics are the major group of compounds that act as primary antioxidants or free radical scavengers. Furthermore, some of these natural phenolic compounds are more efficacious as antioxidants than synthetic antioxidants (Rice-Evans, 1996). Terpenoids (which include the carotenoids) can both act as regulators of metabolism and physiology and play a protective role as antioxidants (Graßmann, 2005). The antioxidant properties of plants then may well be a strong contributing factor to the use of plants in the management and treatment of various diseases and to their use in traditional medicine (Scartezzini and Speroni, 2000). Within the plants themselves, these same antioxidants are important in protecting cells from damage caused by free radicals and in offering protection against cellular oxidation reactions.

Mathur et al. (2011) screened the phytochemical constituents and the antioxidant properties of methanolic and aqueous extracts of the fruits of W. coagulans, which is one of the most commonly used plants among traditional practitioners. The phytochemical screening showed the presence of alkaloids, steroids, phenolic compounds, tannins, saponins, carbohydrates, proteins, amino acids and organic acids. Both the methanolic and aqueous extracts showed high in vitro antioxidant activity compared with standard ascorbic acid, although the aqueous extracts showed higher antioxidant potential.

Leaf extracts of N. arbor-tristis are also extensively used in Indian traditional medicine. The acetone-soluble fraction of the ethyl acetate extract showed impressive antioxidant activity in several in vitro experiments, e.g. the DPPH, hydroxyl and superoxide radical and H2O2 scavenging assays. It also exhibited preventive activity against the Fe(II)-induced lipid peroxidation of liposomes and γ-ray-­induced DNA damage. The strong reducing power and high phenolic and flavonoid contents could be responsible for the antioxidant activity that was found(Rathee et al., 2007).

Tanti et al. (2010) suggested that the presence of terpenoids, tannins and flavonoids could be responsible for antioxidant activity of methanolic leaf extracts of D. sinuata. ­Kumaran and Karunakaran (2007) found a correlation between the antioxidant activity and total phenolic content of five Phyllanthus species from India; the species with a greater phenolic content showed more antioxidant activity and vice versa. Iqbal et al. (2012) showed that the methanolic extract of leaves of A. viridis had a higher phenolic content (5.4–6%) and greater antioxidant activity than the methanolic extract of the seeds, which contained 2.4–3.7% phenolics, i.e. phenolic content seems to be correlated with antioxidant activity. Katalinic et al. (2006) screened 70 medicinal plant extracts for antioxidant capacity (measured by the FRAP assay) and total phenolic content and found a significant linear correlation between the two.

As already noted, the antioxidant activity of these traditional medicinal plants may come in part from antioxidant vitamins, ­phenolics or tannins. Phenolics, in particular flavonoids, are often directly linked to antioxidant activity (Abu-Amsha et al., 1996; Rice-Evans, 1996; Dreosti, 2000) and tannins, which are astringent antioxidants, are known to occur in Abies, Picea, Tsuga, Thuja, Juniperus, Nuphar, Quercus, Populus, Gaultheria, Dirca, Rhus, Prunus, Sorbus and Smilacina (Arnason et al., 1981). Flavonoids are recognized to have beneficial effects on plants protecting them against ultraviolet light and even herbivores (Harborne and Williams, 2000). Using a variety of experimental model systems, it has been found that the protective effects of flavonoids are due to their capacity to transfer electrons to free radicals and to chelate metal catalysts (Ferrali et al., 1997), activate antioxidant enzymes (Elliot et al., 1992), reduce α-­tocopherol radicals (Hancock et al., 2001) and inhibit known free radical producing enzymes, such as myeloperoxidase and NADPH oxidase (Middleton and Kandaswami, 1992) and xanthine oxidase (Nagao et al., 1999). Flavonoids have also been demonstrated to have exceptional cardioprotective effects, essentially because of their capacity to inhibit LDL peroxidation (Mazur et al., 1999).

Tannins, which are astringent antioxidants, are a prominent component of some of plants (Arnason et al., 1981; Haslam, 1996), and are known to occur in Abies, Picea, Tsuga, Thuja, Juniperus, Nuphar, Quercus, Populus, Gaultheria, Dirca, Rhus, Prunus, Sorbus and Smilacina, which are traditionally used as food, beverage and medicinal plants in eastern Canada (Arnason et al., 1981). Along with anthocyanins, tannins could be contributory factors in the antioxidant activities of medicinal plants. In addition, tannins could have a combined or synergistic effect with other antioxidants (particularly ascorbic acid) within the plant extract (Saucier and Waterhouse, 1999).

Tea contains tannin, with most of its antioxidant activity attributed to catechins (flavanol derivatives, also known as condensed tannins) (Nanjo et al., 1996). Rather than containing a single chemical, however, tea contains many different flavonoids, viz. catechins, theaflavins and flavonols (Wiseman et al., 1997), which together can lead to enhanced antioxidant activity. Both green tea (Matsumoto et al., 1993) and black tea (Gomes et al., 1995) have shown antidiabetic activity in the reduction of blood glucose. Black tea has lower antioxidant activity than green tea, probably as a result of a factor of the fermentation process that reduces its catechin content to 9% in contrast to green tea’s 30% (Wiseman et al., 1997).

Coriandrum sativum (coriander) is an annual herb that originates from the Mediterranean region and is now extensively cultivated in India. The seeds are aromatic, bitter and have anti-inflammatory and diuretic properties. The herb helps in digestion and is useful in treating burning sensations, coughs, bronchitis, vomiting, dyspepsia, diarrhoea, dysentery, gout, rheumatism, intermittent fever and giddiness (Varier, 1994). Coriander seeds have been shown to have anti-peroxidative properties (Chitra and Leelamma, 1999). The activity of polyphenolic compounds from coriander seeds in protecting against oxidative damage induced by H2O2 in human lymphocytes has been reported by Hashim et al. (2005). The compounds quercetin 3-­glucuronide, isoquercitrin and rutin identified in coriander fruits (Kunzemann and Herrmann, 1977) have also been reported to have antioxidant properties as measured by the DPPH assay (Wangensteen et al., 2004; Wong and Kitts, 2006).

Furthermore, various complications of diabetes, including retinopathy and atherosclerotic vascular diseases (the leading cause of mortality in diabetics) have been linked to oxidative stress (Baynes, 1991). Antioxidants (vitamin E or C) have been used for treatments of these diseases (Cunningham, 1998). Different plants often contain substantial amounts of tocopherols (vitamin E), carotenoids, ascorbic acid (vitamin C), flavonoids and tannins, which are beneficial as antioxidants for treatment of these diseases. Vitamin C is an important dietary antioxidant (Rock et al., 1996), and vitamin E is another dietary antioxidant that has been investigated for its effect on diabetes (Paolisso et al., 1993; Cotter et al., 1995). The combined antioxidant activity of these two dietary antioxidants vitamin E and C is greater than their individual activities (Cotter et al., 1995, so it has been suggested that this type of interaction may be an important property of plant medicines associated with diabetes (­Cunningham, 1998).

Many studies have been performed to identify antioxidant compounds with phar­macological activity and a limited toxicity. In this context, ethnopharmacology represents the most important way possible of finding interesting and therapeutically helpful molecules.

1.4 Reverse Pharmacology with Traditionally used Antioxidant Plants

Reverse pharmacology is defined as the science of integrating documented clinical ­experiences and experiential observations into leads by trans-disciplinary exploratory studies and further developing these into drug candidates or formulations through robust preclinical and clinical research. The traditional knowledge-inspired reverse pharmacology described here relates to ­reversing the routine ‘laboratory to clinic’ progress of discovery pipeline to change it to ‘clinics to laboratories’. This means that traditional medicine all over the world is now being re-­evaluated by extensive research on different plant species and their therapeutic principles. The hidden potential of many medicinal plants is yet to be discovered as they were formerly intended only for traditional use. A salient feature of reverse pharmacology is the combination of knowledge from traditional or folk medicine with modern technologies to provide better and safer leads.

An example is given by studies that have been conducted on Trichopus zeylanicus (arogyappacha), a wild plant from a rare genus that grows in the hilly Agasthyar forests of Kerala. The tribal inhabitants (Kani tribe) of this area use the plant as a health tonic and rejuvenator (Sharma et al., 1989; Evans et al., 2002). Singh et al. (2005) have explored and identified the constituent(s) of the plant that is(are) active in increasing the non-specific resistance of the body to combat the harmful influence of stress. The antioxidant properties of T. zeylanicus were established using the free radical assays DPPH and ABTS, and by measuring its ability to reduce iron, lipoxygenase activity and hydrogen peroxide-induced lipid peroxidation. In another study, Tharakan et al. (2005) demonstrated that T. zeylanicus contains polyphenols and sulfhydryl compounds that have the ability to scavenge ROS.

Although in vitro antioxidant assays have been carried out on many plants with ­reported medicinal properties, in vivo tests ­remain to be done on the majority of them, and the clinical efficacies of many plant preparations that are in use have not yet been validated. In addition, while the mechanism of action of some of the antioxidants that have been identified in plants is known, the active ingredients in many plant extracts with antioxidant properties remain to be identified. A further elucidation of both known and yet to be identified natural antioxidants in concert with the newly emerging technology of metabolomics could help disease prevention and provide information on cures associated with the use of simple herbs.

Herbs such as Amaranthus paniculatus, Aerva lanata, Coccinia indica and C. sativum are used as vegetables and could be a source of dietary antioxidant supplies. Data on the phytochemistry of these medicinal plants could provide promising molecules for pharmacotherapy. As well as using such reverse pharmacological studies on traditional medicinal plants to provide an economic and time- saving approach to drug development, reverse pharmacology can also be applied for determination of the hidden therapeutic ­potential of traditional medicinal plants for new indications. Here, it would be cheaper and perhaps more productive to re-examine plant remedies described in ancient texts. In addition, it should be borne in mind that the active antioxidant principles of medicinal plants may be distributed in specific plant parts, and may be affected by seasonal variation, geographical factors, other environmental factors and plant age. Hence, such factors should be considered during reverse pharmacological studies on antioxidant plants. Another consideration is the proper standardization of postharvesting processing of raw materials from antioxidant plants.

1.5 Bioprospecting for Traditionally ­Antioxidant Plants

Although modern medicine may be available in countries like India, the traditional systems of medicine are often used for various historical, cultural and ecological reasons (Kunwar et al., 2010). Quantitative intracultural and intercultural comparisons of medicinal plant knowledge analyses are believed to be a valid ethnobotanical research approach ­towards uncovering generalized knowledge (Vandebroek, 2010).

Furthermore, each nation has rights over its biodiversity, in spite of which a situation called biopiracy (or gene robbing) has developed in which the genetic resources of ­biodiversity-rich developing countries are being exploited by biotechnologically rich ­developed countries. C. longa and W. somnifera are examples of antioxidant plants from India that have been patented by outsiders on the basis of secondary research. In such cases, indigenous knowledge is being exploited for commercial gain, with no compensation to the indigenous peoples themselves. Many ­believe that biopiracy contributes to the inequality between the developing countries that are rich in biodiversity and the developed countries that host the companies engaging in biopiracy.

This situation has given rise to the process of bioprospecting, which deals with the issues related to the protection of the legal status of indigenous knowledge and compensation to indigenous herbal practitioners for that knowledge. Bioprospecting is an urgent issue for a biodiversity-rich nation like India, which needs to identify its useful plants, their phytochemicals and the genes controlling them, and to document these bio-resources. Such an approach to India’s traditionally used medicinal plants would no doubt be helpful in a manyfold enhancement of Indian herbal medicines in the global herbal market.

1.6 Conclusion

India is a rich home to rare medicinal plants of high medicinal importance with antioxidant activity. Several studies are ongoing throughout the world to identify antioxidant compounds that are pharmacologically potent with a low profile of side effects, and Ayurveda, the oldest medical system in the world, provides many leads to finding active and therapeutically useful compounds from plants. Recent research has centred on various strategies to protect crucial tissues and organs against oxidative damage induced by free radicals, and many novel approaches and significant findings have been made in the last few years.

The traditional Indian diet includes medicinal plants that are rich sources of natural antioxidants, and a higher intake of foods with a high level of antioxidants could be a strategy that is gaining in importance for preventing diseases that are caused by the generation of free radicals. This antioxidant capacity can be explored in the food industry by using plants as a source of antioxidants to prevent the development of rancidity and oxidation in lipids. In fact, in recent years, research has focused on the use of medicinal plants to extract natural and low-cost antioxidants that are also safe and have nutritional and therapeutic value to replace synthetic additives such as BHA and BHT that might be carcinogenic and/or otherwise toxic. Such nutraceuticals are likely to hold the key to a healthy society in the future.

Further, many herbs that are used as spices also have antimicrobial activity that is of use in preventing the growth of food-borne pathogens, while the herbal mixture preparations of Indian traditional medicine may have an antioxidant activity arising from their plant constituents that may well act in a synergistic way. This hypothesis, along with their lack of toxicity, is important for understanding both their past and present use.

Natural antioxidants mainly come from plants in the form of phenolic compounds (flavonoids, phenolic acids and alcohols, stilbenes, tocopherols, tocotrienols), ascorbic acid and carotenoids. The quest for such natural antioxidants, not only for their pharmaceutical uses, but also for dietary and cosmetic uses, has become a major industrial and scientific research challenge over the last two decades. Efforts to gain extensive knowledge on the power of plant antioxidants and to tap their potential are therefore on the increase.

* Corresponding author. E-mail address: [e-mail]

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2 Natural Antioxidants from Traditional Chinese Medicinal Plants

Li Sha, ¹ Li Shu-Ke,¹ Li Hua-Bin, ¹ * Xu Xiang-Rong,² Li Fang, ¹

Wu Shan1¹ and Li An-Na1¹

¹ Guangdong Provincial Key Laboratory of Food,