Importance of Flavonoid as Secondary Metabolites

Abstract

Flavonoids are broad-spectrum secondary metabolites with cosmetics, pharmaceutical, nutraceutical, and medicinal applications. They play a crucial role in life span shortening complications, including diabetes, CVS disorder, and cancer. They are the secondary metabolites essential natural products due to their anti-inflammatory, antioxidant, anticancer, anti-cholinesterase, disease combating, antimicrobial, hepatoprotective, neuroprotective, cardioprotective, antiallergic, and many more pharmacological activities causing substantial economic and social burdens. They have the ability to scavenge superoxide, hydroxyl, and lipid radicals. They are a group of polyphenolic compounds having 15 carbon skeleton consisting of two benzene rings with heterocyclic pyran ring, which are classified as anthocyanins, flavonols, isoflavonols, and flavanones, and present in vegetables, fruits, flowers, seeds, stems, and leaves.

1. Introduction

Flavonoids are the largest group of naturally occurring phenols as phytochemicals. They have the capacity to occur both in the free state and as glycosides (largest naturally occurring phenols). They are the polyphenolic compounds that are biosynthesized by the polypropanoid pathway having a precursor as a phenylalanine molecule. The Latin word “flavus” means yellow, which is responsible for colors in flowers, fruits, and leaves. They are widely distributed in plants having color component properties. They are commonly distributed in the plant kingdom, bryophytes, and in pteridophytes, but not distributed in algae.

The precursor of biosynthesis of flavonoid involves condensation of 2 units of malonyl CoA, 1 unit of Acetyl CoA, and cinnamic acid (biosynthesized by shikimic acid) that results in C₁₅ intermediate, which results in various kinds of flavonoids.

Flavonoids are considered as a major group of plant polyphenols having potential for cosmeceuticals and biomedical applications. They are present in food materials and plants. They are responsible for protection against pathogens, herbivores, and also ultraviolet radiation. They are absorbed from the small intestine and the colon for complete absorption. They are widely used as chemotaxonomic markers and belonging to Polygonaceae, Umbelliferae, Rutaceae, Rosaceae, Leguminosae, Lamiaceae, and also Compositae. Solubility properties include soluble in water and alcohol, whereas insoluble in organic solvents [1].

Pharmacological properties include anti-inflammatory, antiallergic effects, antithrombotic, vasoprotective, antioxidant activity, diuretic, antispasmodic, antibacterial, antifungal properties, and many more.

Quercetin, kaempferol, quercitrin, flavones, dihydroflavons, flavans, flavonols, anthocyanidins, proanthocyanidins, calchones, catechins, and leucoanthocyanidins are some of the classes that show biological and pharmacological activities, such as anticancer, antimicrobial, antistress, antiallergic, oestrogenic activity, vascular activity, and hepatoprotective activity. The antioxidant property is due to the suppression of reactive oxygen species (ROS) formation. They are also involved in the inhibition process of enzymatic activity in reactive oxygen species synthesis.

Hepatoxicity is generally treated by flavonoids, such as quercetin, catechin, rutin, and venoruton. They act by increasing hepatic Gclc expression by increasing cAMP levels, which increases Gclc and helps in the transcription. The increased Gclc expression helps in the depletion of hepatic ROS levels and proapoptotic signaling, which protects the hepatic cells. Clinical trials have shown a significant effect of flavonoids in the treatment of liver diseases. Table 1 shows various chemical tests of flavonoids.

S.No.	Identification test	Procedure	Observation
1	Ammonia test	Filter paper strip was dipped in the alcoholic solution of extract. Ammoniated with ammonia solution.	Color changed from white to orange.
2	Shinoda/Pew Test	Test solution (5 ml) + 5 ml. 95% alcohol + few drops of conc. HCl + 0.5 g magnesium turning.	Appearance of pink color.

Table 1.

Identification of flavonoids.

Naringenin, apigenin, galangin, isoflavones, chalcones, and flavanones are the flavonoids that have been observed to have antibacterial activity might be due to complex formation with protein by bonding viz. hydrophobic effects or covalent bond that inhibit DNA synthesis and RNA synthesis. Vegetables and fruits are the major source of flavonoids and have been observed to cure and prevent cancer. Flavonoids produce anticancer activity may be due to inhibition of tyrosine kinase activity, which is one of the growth factor signaling to the nucleus present in the cell membrane. Flavonoids inhibit fatty acid synthase activity and lipogenesis in the prostate cancer cell. It may also be due to inhibition of cell cycle arrest or inhibition of heat shock protein or inhibition of nuclear type II estrogen binding sites. Quercetin, genistein, daidzein, epigallocate-chin-3-gallate, biochanin, and hesperidin are flavonoids acting as anticancer [2].

1.1 Classification

The classification is based on the degree of oxidation of the central pyran ring where Figure 1 shows a basic structure of flavonoids with the structure of each example of various types of flavonoids. The classification is as follows:

1. Flavandiol (3, 4-Hyroxyflavane), for example, leukoanthocyanidine.

2. Flavanones (4-Oxo-flavane), for example, naringenin.

3. Flavanols (3-Hydroxy-4-Oxo-flavane, Catechine), for example, catechin.

4. Flavones (4-oxo-flav-2-ene), for example, apigenin.

5. Flavonols (3-Hydroxy-4-Oxo-flav-2-ene), for example, quercetin.

6. Flavylium (Anthocyanidin), for example, cyanidin.

Figure 1.

Structure of basic flavonoids and their various types.

2. Flavonoids as “cosmeceutical”

Cosmetics products are products that are intended to apply on hair and skin to enhance appearance, promote attractiveness, and beautify and cleanse the properties of skin and hair. In 1990, the term “Cosmeceuticals” was described as the over-the-counter skin care products that involve therapeutic properties by addition of plant active ingredients, such as alpha-hydroxy acid, retinoic acid, ascorbic acid, and coenzyme Q10, which helps in skin elasticity, reduction of wrinkles in skin as antiaging effect, to check degradation of collagen, and also protection against UV radiation. The presence of multi-active properties in flavonoids provides protection to skin blood vessels, which telangiectasias and petechias cause by rupturing blood vessels. The main activity in cosmetic is capillary permeability reduction, blood vessel protection, and platelet aggregation prevention [3].

Phytochemicals, such as phenolic compounds, substances structurally characterized by having one or more hydroxyls attached to an aromatic ring, are classified into simple phenolics and polyphenols, which may be subdivided into tannins and flavonoids. Both flavonoids and non-flavonoids are associated with various interesting cosmetic properties, such as photoprotection, antiaging, moisturizing, antioxidant, astringent, anti-irritant, and antimicrobial activity [4].

The polyphenol nature is responsible for the colors in many fruits, vegetables, and flowers, which protect from environmental stress that act as an antioxidant. Flavonoid in cosmetics provides antioxidant protection against UV radiation protection for our skin [2, 5].

Southeastern Asian medicinal plant Alpinia galanga belonging to Zingiberaceae in traditional medicine used to relieve stomach pain, indigestion, and to treat skin diseases. Antioxidant, anti-inflammatory, and antibacterial are medicinal properties. Flavonoids, phenolic acids, and volatile compounds are present as phytochemicals in several parts, such as leaves, seeds, and rhizomes. Flavonoid in A. galanga plays a crucial role in the cosmetic area. Isolated and identified flavonoids from seeds and the rhizomes of A. galanga are found as 11 flavonols, 4 dihydroflavonols, one flavan 3-ol, and flavanone. Galangin (3,5,7-trihydroxyflavone) is the largest compound of the 11 flavonols, whereas kaempferol, quercetin, and myricetin are popular non-methylated flavonols. Alpinone, pinobanksin 3-acetate, and 3-cinnamate are three dihyroflavonols isolated and identified in rhizomes (Figure 2).

Figure 2.

Leaves, rhizome, and whole plant of Alpinia galanga.

In Thailand, Vietnam, and many Southeast Asian countries, A. galanga was used traditionally as a major ingredient in cosmetics, which include body soap and skin care products, by developing its extract using an easy hot extraction method where water was used as a solvent. Figure 2 shows the leaves, rhizome, and whole plant of A. galangal.

Recently, people use to order online the dried plant and its extract easily. In cosmeceuticals, the authentication process and identification of plants should be carried out previously for the development processes. The research on the potential of extracts and/or phytochemicals from this medicinal plant is insufficient; a greater number of studies focusing on flavonoid identification of the potential extracts from this medicinal plant should be conducted. Easy availability, easy cultivation, and low price are major reasons to promote the flavonoid bioactive ingredients of A. galanga in the field of cosmetics. The anti-wrinkle and antioxidant properties in cosmetics are always remembered [6].

Water lily (Nymphaea lotus L.) is a traditional ornamental medicinal and cosmetic plant having perennial aquatic flowering nature which is in many countries of Asia and Africa, especially in Thailand, Nepal, Vietnam, Indonesia, China, Bangladesh, and Sri Lanka. Roots, rhizome, stolon, petiole, young leaves, and flower parts were used traditionally as homemade natural cosmetics products, such as skincare and perfume, whereas medicinally in the treatment of circulatory system syndrome. It is also considered as the symbol of the Hindu Goddess “Sarasvatiji” and “Laxmiji.” A high content of flavonoids in flowers help local people to use the ethanolic extract for homemade cosmetic products, especially for skincare and perfumery. Several publications have specifically revealed the potential in cosmetic and cosmeceuticals potential of N. lotus L. Flower and stamen are considered as the richest source of flavonoids, which conclude as important bioactive constituents for cosmetic applications. Chalcone glycoside chalcononaringenin-2”-O-galactoside, flavonol glycosides, isorhamnetin-7-O-galactoside, isorhamnetin-7-O-xyloside, isorhamnetin-3-O-xyloside, myricetin-3-O-xyloside quercetin-3-O-rhamnoside, quercetin-3-O-xyloside, and kaempferol-3-O-galactoside are major flavonoids present in the N. lotus L. For antioxidant activity, in vitro assays include DPPH (2,2-diphenyl-1-picrylhydrazyl), ABTS {2,20-azino-bis(3 ethylbenzothiazoline-6-sulfonic acid)}, FRAP (Ferric Reducing Antioxidant Power Assay), BHT (Butylated hydroxytoluene), cellular assays and animal studies were performed and showed a positive result [7].

Anti-wrinkle, skin whitiliser, coolant, anti-acne, and relaxing activities are reported. The anti-acne property is due to antibacterial properties. The literature revealed that N. lotus L. has properties to improve skin complexion, reduce skin pigmentation, and improve skin with soothing and emollient activity.

The genera lotus of varieties N. lotus, Nymphaea caerulea, Nelumbo nucifera, and Nelumbo lutea have potential that can be utilized in herbal cosmetics due to the presence of a large amount of flavonoids, which reported as anti-acne, skin whitiliser, and anti-pimple activity with improve skin texture. Figure 3 shows various varieties of N. lotus.

Figure 3.

(i) Nymphaea lotus, (ii) Nymphaea caerulea, (iii) Nelumbo nucifera, and (iv) Nelumbo lutea.

The market formulation includes Veet hair removal gel, AHAVA mineral botanic body lotion, Clarisonic daily acne cleanser, Lotus sunscreen, Neutrogena deep cleanser, face wash, skin tonner, and many more, which showed a positive response from people and increase the demand in the cosmetic industry. Mechanism of action includes inhibition of tyosinase which results in skin whitelising, inhibition of elastase and DOPA oxidase inhibition which results in antiwrinkle activity, antiradical property prevents inhibition of Ultraviolet radition which prevent skin tanning, and inhibition of melanin which also results skin whitilising and anti-ageing effects [8].

3. Flavonoids as nutraceuticals

Nutraceuticals may range from isolated nutrients, dietary supplements, diets to genetically engineered “designer” food, herbal products, and processed products, such as cereals, soups, and beverages. A nutraceutical is any nontoxic food extract supplement that has scientifically proven health benefits for both the treatment and prevention of diseases. According to Stephen DeFolice, nutraceuticals are food or parts of food that provide medical or health benefits, including the prevention and treatment of disease. In simple language “nutraceuticals” are food materials utilized for treatment and prevention of disease and may range from isolated nutrients, dietary supplements, diets to genetically engineered “designer” food, herbal products, and processed products, such as cereals, soups, and beverages. Flavonoid rich foods are considered as superfoods nutraceuticals include plant origin food mainly tea, fruits, grains, legumes, nuts, vegetables, and wine.

Flavonoids rich foods as nutraceuticals have explored the working mechanisms which include pharmacological activities, such as anti-wrinkle, antiaging, anticancer, antibacterial, hypoglycemic, anti-hypertension, anti-obesity, antiproliferative, anti-thrombotic, and anti-platelet aggregation. The presence of phenolic compounds is confirmed by potent antioxidant activity and metal chelators agent. Daily diets consumed are always rich in these flavonoids. Major active ingredients that are considered in the plant are flavonoids that have a long half-life with less side effects and are absorbed in the intestine after ingestion. They have a high absorption capacity in the intestine.

The mechanisms behind the activities are trapping of free radicals, decreasing leukocyte immobilization, and regulation of nitric oxide and xanthine oxidase activity. The pharmacokinetic studies of flavonoids are shown in Figure 4, which includes absorption, distribution, and biotransformation where hydrolysis is an important part of absorption in the cecum and colon by enterobacteria, aglycone is absorbed by gut epithelial cells and enter the circulation to metabolize in the liver [8, 9, 10, 11].

Figure 4.

Pharmacokinetic studies of flavonoids.

4. Flavonoids as pharmaceuticals

Flavonoids are the bioactive phytochemical constituents that are present in fruits, herbs, stems, cereals, nuts, vegetable, flower, and seeds and give biological activities. About 10,000 flavonoid compounds are isolated and identified, showing effective antioxidant, anticancer, antibacterial, cardioprotective agents, anti-inflammation, immune system promoting, and skin protectant for medical application.

4.1 Anticancer activity

Cancer is a major health problem that can be defined as impaired cell cycle and uncontrolled proliferation, which results in the growth of abnormal cells. Increased exposure to stress, pollution, radiation, ultraviolet rays, smoking, oxidative stress, genetic mutation, and lack of apoptotic function are the major causes of cancer. Anthocyanins, flavones, flavones, flavonols, and chalcones are major flavonoids having anticancer activities. The mixed mechanism of action of flavonoids as anticancer action includes down-regulation of mutant p53 protein, inhibition of expression of Ras proteins, estrogen receptor binding capacity, tyrosine kinase inhibition and cell cycle arrest. Quercetin was reported to exert a growth inhibitory effect on various tumor cells also cell cycle arrest in proliferating lymphoid cells. Dryopteris erythrosora Erythrina suberosa, Phaseolus vulgaris L., Medicago truncatula Gaertn, Ceratonia siliqua L., Butea monosperma, Glycyrrhiza glabra L. and many more plants reported as anticancer due to presence of flavonoids [12, 13, 14].

4.2 Antioxidant activity

Plants, animals, and human protection against the effect of reacting oxygen species by suppressing reacting oxygen species with chelation of the trace elements involved in the free radical generation and enzyme inhibition. They are found in the chloroplast having scavenging activity of singlet oxygen and stabilizers of the chloroplast outer envelope membrane [15].

The enzymatic and non-enzymatic systems involve detoxification and removal of oxidant species of glutathione [GSH], GSH peroxidase, GSH reductase, and GSH S-transferase. The prooxidant activity of flavonoids becomes cytotoxic that undergoes transition metal reactions resulting in the formation of highly reactive oxygen species that damages protein and DNA [1].

4.3 Antimicrobial activity

According to World Health Organization, multidrug-resistant pathogenic microorganisms are major global health complications, which involve various natural products. As one of the class of secondary metabolites of the natural class, flavonoids play a vast and crucial role to handle multidrug-resistant pathogenic microorganisms strains with their versatile pharmacological activities. Prenylation or geranylation at C6; and hydroxylation of C5, C7, C3’, and C4’ have reported to enhance bacterial inhibition of flavonoids, whereas methoxylation at C3’ and C5 has been studied to decrease antibacterial action of flavonoid. It is reported that the cell membrane is found at the major site of flavonoid action, which help in the inhibition of the respiratory chain and the ATP synthesis that also involves damage to phospholipid bilayers. Flavanone is acting as potent antibacterial activity by synthesizing a compound with halogenations of the B ring, as well as lavandulyl or geranyl substitution of the A ring [16, 17, 18].

4.4 Cardioprotective activity

More than 4000 flavonoids, which include chalcones, flavonols, dihyroflavonols, catechins, isoflavones, and catechins have the capacity for cardioprotective activity against myocardial ischemia or reperfusion as antihypertensive, anti-atherosclerotic, and anti-platelet. The significant role of flavonoids by preventing cardiovascular diseases, which may be due to antioxidant, antithrombotic, and antiatherogenic activity. For example, red wine consumption will protect against thrombosis and atherosclerosis by inhibition of platelet aggregation and LDL oxidation. The literature revealed that a daily diet of 100 mg of flavonoid helps in the reduction and possibility of cardiovascular diseases by inhibition of low-density lipoprotein oxidation and reduced platelet aggregability [19, 20, 21, 22].

5. Conclusion

For thousands of years, flavonoids in plants are utilized as traditional medicine. They are polyphenolic compounds that are biosynthesized by the polypropanoid pathway and have potential as cosmeceutical, nutraceutical, and pharmaceutical applications. They work in the multi-mechanism of action, such as protecting endothelial cell, inhibiting foam cell formation, regulating lipid metabolism, anti-inflammatory, and underlying molecular mechanism. In future prospective, flavonoids as nutraceutical, cosmeceutical, and pharmaceutical aspects would help to reduce the burden in urban and rural populations of developed and developing countries by elevation of its pharmacokinetic, metabolic, and pharmacodynamic characteristics and using in novel drug delivery system technology.