Secondary Metabolites in Fruit Crops: Health Benefits
Authors: Sunil Kumar, Arvind Nagar and Om Prakash Patidar

Introduction:

Fruits are an important part of the human diet and a major source of biologically active substances such as vitamins and secondary metabolites. Fruits provides not only essential nutrients needed for life but also other bioactive compounds for health promotion and disease prevention. Previous epidemiologic studies have consistently shown that diet plays a crucial role in the prevention of chronic diseases. Consumption of fruits has been strongly associated with reduced risk of cardiovascular disease, cancer, diabetes, Alzheimer disease, cataracts, and age-related functional decline. Prevention is a more effective strategy than treatment of chronic diseases. Plant-based foods, such as fruits contain significant amounts of bioactive phytochemicals, may provide desirable health benefits beyond basic nutrition to reduce the risk of chronic diseases. Metabolites are involved in the growth of organisms through the process of metabolism. The metabolism is referred to as the sum of the all the biochemical reactions carried out by an organism. Depending on the origin and function, metabolites can be divided into two major categories; namely, Primary and Secondary metabolites.

Primary Metabolites Secondary Metabolites
Primary metabolites are essential to the growth of the cell. They are produced continuously during the growth phase and are involved in primary metabolic processes such as respiration and photosynthesis. Primary metabolites, which are identical in most organisms, include sugars, amino acids, tricarboxylic acids, the universal building blocks, and energy sources. Secondary metabolites are the compounds which are derived by pathways from primary metabolic routs, and are not essential to sustain the life of cells. These compounds do not have a continuous production. Very often secondary metabolites are produced during non- growth phase of cells. Secondary metabolites are the end products of primary metabolites such as alkaloids, phenolics, steroids, essential oils, lignins, resins and tannins etc.


Secondary metabolites in fruit crops:

1. Avocado: The flesh is very nutritious containing vitamins B, C and E, chlorophyll and the carotenoids lutein, zeaxanthin, α-carotene and β-carotene.

2. Apples and pears: Apples are a good source of flavonoids and phenolic compounds. The main contributors to the antioxidant capacity of apples are 5-O-caffeoylquinicacid and caffeic acid. The red colour of some cultivars of apples is due to the presence of the anthocyanin, cyanidin-3-O-galactoside. The phenolic composition of pears is very similar to that of apples containing 5-O-caffeoylquinic acid, 4-O-p-coumaroylquinic acid, procyanidins and quercetin glycosides. The main difference in the phenolic content of apples and pears is the presence of hydroquinone glucoside (arbutin) in pears and the hydroxychalcones in apples.

3. Apricots, Nectarines and Peaches: Peaches and nectarines contain cyanidin-3-O-glucoside, cyanidin-3-Orutinoside, quercetin-3-O-glucoside and quercetin-3-O-rutinoside. 3-O-Caffeoylquinic acid also occurs in stone fruits and in larger amounts than 5-O-caffeoylquinic acid. Apricots and peaches both contain carotenoids principally in the form of β-carotene.

4. Cherries: contain anthocyanins, mainly cyanidin-3-Orutinoside, with lower levels of other anthocyanins, including cyanidin-3-O-glucoside and peonidin-3-rutinoside.

5. Plums: Plums are a rich source of anthocyanins in the form of cyanidin-3-O-glucoside and cyanidin-3-O-rutinoside which are also found in peaches.

6. Citrus fruits: Citrus fruits are significant sources of flavonoids, principally flavanones, which are present in both the juice and the tissues that are ingested when fruit segments are consumed. Citrus peel, and to a lesser extent the segments, also contain the conjugated flavanone naringenin-7-O-rutinoside (narirutin) as well as hesperetin-7-O-rutinoside (hesperidin) which is included in dietary supplements and is reputed to prevent capillary bleeding. Naringenin-7-O-neohesperidoside (naringin) from grapefruit peel and hesperetin-7-O-neohesperidoside (neohesperidin) from bitter orange are intensely bitter flavanone glycosides. Citrus fruits also contain significant amounts of terpenoids. Citrus fruits also contain the more complex limonoids which are modified triterpenoids. The bitterness due to limonoids is an important economic problem in commercial citrus juice production. Among the more than 30 limonoids that have been isolated from citrus species, limonin is the major cause of limonoid bitterness in citrus juices. Nomilin is also a bitter limonoid that is present in grapefruit juice and other citrus juices, but its concentration is generally very low so its contribution to limonoid bitterness is minor.

7. Pineapple: The fruit is notable for the presence of a proteolytic enzyme, bromelain, which is used to prevent a proteinaceous haze in chill-proof beer when refrigerated.

8. Dates: contain unspecified carotenoids and anthocyanins together with protocatechuic acid, vanillic acid, syringic acid and ferulic acid. Green dates contain three isomeric caffeoylshikimic acids which appear to form glucosides.

9. Mango: The peel of the fruit contains higher levels of total phenols than the pulp and both peeled and unpeeled fruits are used to prepare purées. The red colour of ripe mango peel is due to cyanidin-3-O-galactoside. Mango latex also contains the contact allergen, 5-(12-heptadecenyl)-resorcinol which may contaminate the peel but not normally the fruit itself. Mango extracts are used widely in traditional medicines for treating a number of conditions including diarrhoea, diabetes and skin infections.

10. Grapes: The flavonols in red grapes are conjugates of myricetin, quercetin, kaempferol and isorhamnetin. The anthocyanin content is quite complex with the main components in Cabernet Sauvignon grapes being malvidin-3-O-glucoside, malvidin-3-O-glucoside, malvidin-3-O-glucoside and delphinidin-3-O-glucoside while the presence of significant amounts malvidin-3, 5-Odiglucoside is an indication of a hybrid grape. The grapes also contain gallic acid, severalp-coumaroyl derivatives and caftaric acid. The phytoalexintrans-resveratrol-3-O-glucoside (trans-piceid) also occurs but in low and variable amounts that are probably dictated by cultivar and disease pressure.

Health benefits of secondary metabolites:

Many secondary metabolites have positive effects on human health. Some of them are even essential to life, as are vitamins (such as tocopherols and tocotrienols, alias vitamin E). However, all vitamins are not considered to be secondary metabolites, like ascorbate, the major antioxidant in plants and for humans. A few other secondary metabolites are provitamins, that is, compounds that are converted into vitamins in animal bodies (such as β-cryptoxanthin, a carotenoid found in Citrus fruits, or β-carotene, found in several fruits and vegetables endowed with provitamin A properties).

A. Vitamins: Vitamin C, also known as ascorbate, is a vital micronutrient for humans. A lack of vitamin C hampers the activity of a range of enzymes and may lead to scurvy in humans. In most cellular functions, ascorbate acts as an electron donor, but it may also act directly to scavenge reactive oxygen species (ROS) generated by cellular metabolism. Due to the role of ascorbate in protecting cells against oxidative stress and the involvement of ROS in neurodegenerative disorders (Alzheimer’s and Parkinson’s diseases) or inflammatory response (arteriosclerosis), it is strongly suggested that vitamin C could prevent heart, chronic inflammatory, and neurodegenerative diseases. Vitamin E (tocopherols) is present in all cell membranes and plasma lipoproteins, especially in red blood cells of the human body. As the major lipid-soluble chain-breaking antioxidants in humans, vitamin E protects DNA, low-density lipoproteins, and polyunsaturated fatty acids from oxidative damage. It moreover plays a role in hemoglobin biosynthesis, modulation of immune response, and stabilization of the structure of membranes. Vitamin K1 is a liposoluble vitamin, synthesized from phylloquinone by bacteria in the intestinal tract. It plays a positive role in the control of blood clotting, bone formation, and repair. Deficiency of vitamin K1may result in hemorrhagic disease in newborn babies, as well as postoperative bleeding, muscle hematomas, and intercranial hemorrhages in adults. Vitamin K3 menadione was shown to exhibit cytotoxic activity and inhibit growth of tumors in humans.

B. Phenolic Compounds: Many dietary phenolics are antioxidants capable of quenching ROS and toxic free radicals formed from the peroxidation of lipids and, therefore, have anti-inflammatory and antioxidant properties at the body level. Several hydroxycinnamic acid derivatives, for instance, caffeic acid, chlorogenic acid, ferulic acid,p-coumaric acid, and sinapic acid, present strong antioxidant activities by inhibiting lipid oxidation and scavenging ROS. Flavonoids are known to prevent production of free radicals by chelating iron and copper ions to directly scavenge ROS and toxic free radicals and to inhibit lipid peroxidation. Production of peroxides and free radicals, which may damage DNA, lipids, and proteins, has been linked to aging, atherosclerosis, cancer, inflammation, and neurodegenerative diseases such as Alzheimer’s and Parkinson’s. Flavonoids were also demonstrated to protect low-density lipoprotein (LDL) cholesterol from being oxidized, thus preventing the formation of atherosclerotic plaques at the level of the arterial wall. Many dietary flavonoids and hydroxycinnamic acids bind to human serum albumin, the main protein involved in lipid transportation within blood. This cotransport provides an efficient antioxidant protection to lipids.

C. Carotenoids: Carotenoids endowed with provitamin A activity are vital components of the human diet. Vitamin A is implicated in hormone synthesis, immune responses, and the regulation of cell growth and differentiation. It can be produced within certain tissues from carotenoids such as β-cryptoxanthin present in Citrus fruits. A carotenoid-deficient diet can lead to night blindness and premature death. Carotenoid-rich diets are correlated with a significant reduction in the risk for certain cancers, coronary heart disease, and several degenerative diseases. Carotenoids have demonstrated anticancer and antimutagenic properties.

D. Phytosterols: They regulate the fluidity and permeability of the phospholipid bilayers of plant membranes. Certain phytosterols are precursors of brassinosteroids, plant hormones involved in cell division, embryonic development, fertility, and plant growth. Plant sterols possess, morevoer, cholesterol-lowering properties and play a positive role by decreasing the incidence of cardiovascular diseases. Being structurally similar to cholesterol, they can compete with cholesterol, thus limiting its absorption from fat matrices into the intestinal tract. Plant sterols have been hypothesized to have anticancer, antiatherosclerosis, anti-inflammation, and antioxidant activities.

E. Saponins: Saponins are attributed with cardioprotective, immunomodulatory, antifatigue, and hepatoprotective physiological and pharmacological properties. Antifungal activity is generally ascribed to the ability of saponins to complex with sterols in fungal membranes, thus causing pore formation and loss of membrane integrity. They also affect membrane fluidity. Dietary saponins have been observed to reduce blood cholesterol, stimulate the immune system, and inhibit the growth of cancer cells. Saponins inhibit active transport by increasing the general permeability of the enterocytes. Saponins can also form insoluble complexes with minerals such as zinc and iron.

F. Glucosinolates: A reduction in the prevalence of certain forms of cancer has been attributed to the anticarcinogenic properties of certain glucosinolates and their breakdown products. Glucosinolates act by activating enzymes involved in the detoxification of carcinogens and by providing protection against oxidative damage. Certain glucosinolates have been observed to inhibit enzymes involved in the metabolism of steroid hormones.

References:

Jimenez-Garcia, S. N., Vazquez-Cruz, M. A., Guevara-Gonzalez, R. G., Torres-Pacheco, I., Cruz-Hernandez, A., and Feregrino-Perez, A. A. (2013). Current Approaches for Enhanced Expression of Secondary Metabolites as Bioactive Compounds in Plants for Agronomic and Human Health Purposes-a Review. Polish Journal of Food and Nutrition Sciences, 63(2): 67-78.

Poiroux-Gonord, F., Bidel, L. P., Fanciullino, A. L., Gautier, H., Lauri-Lopez, F., & Urban, L. (2010). Health benefits of vitamins and secondary metabolites of fruits and vegetables and prospects to increase their concentrations by agronomic approaches. Journal of agricultural and food chemistry, 58(23): 12065-12082.


About Author / Additional Info:
Author is a PhD Scholar in Division of Fruits & Horticultural Technology, ICAR- Indian Agricultural research Institute, New Delhi, India