Toxic Factors In Edible Plant Foods Biology Essay

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Plants are the major food source for all living organisms as they are exceptionally nutritious providing vitamins, minerals and proteins( e.g. vitamin C, carotene, iron etc) but few plants can be full of toxicity which could be poisonous to living organism especially to humans (warm blooded) as we humans have different immune system compared to animals and therefore the anti-nutritional factors may negatively influence the metabolic pathway due to the chemical composition from such toxic plants. Many plants use mechanical and morphological protection such as thorns, spikes, and poisonous chemical toxins in their leaves against grazers. These chemical compounds occur naturally in plants acting as a defence mechanism protecting the plants from being attacked by animals, insects, etc. Many studies are done on chemical composition of such plants and are referred to as secondary metabolites and these chemical compounds have been isolated and structures are determined by using various methods such as mass spectrometry, magnetic resonance, or X ray diffraction. There are many anti-nutritional and toxic factors in edible plants, which these plants use to defend themselves from harmful elements and thus limits the use of plants for animal feed. Anti-nutritional factors (ANFs) are produced in plant tissues and organs and most are stored in vacuoles. Biosynthesis of these chemicals takes places in cytoplasm and in few plants its synthesized in the chloroplast, mitochondria, vesicles, roots, idioblasts ( contain tannins) and etc. Anti-nutritional factors and toxins in edible plants (e.g. lectins) can be lethal and can reduce the growth and absorption of nutrients in mammals, also can inactivate the intake of vitamins and minerals, and therefore a lot to study has been done on such factors. This review will focus on five such anti-nutritional and toxic factors i.e.; lectins, saponins, tannin, ricin, and

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Lectins are glycoprotein (carbohydrate binding protein) mainly found in legumes; peas, beans, and lentils, which are capable of agglutinating erythrocytes (red blood cells) by interacting with specific carbohydrate residues on cell membrane structures . Lectins are resistant to attack of microorganism due to toxic compounds produced by these pathogens once digested by humans and animals. After digestion, lectins are able to bind to the glycosyl cells that lines the digestive tract causing various harmful reactions and production of toxins, damaging epithelial cells, cell membrane, could affect the whole digestive immune system and or disrupt lipids, internal organs and tissues which could affect the growth of the animal. Molecular, biochemical, cellular, physiological and evolutionary arguments indicate that lectins have a role in plant defence and are most stable if conditions are favourable such as right pH, heat, predators, etc. Therefore lectins are very important to the plant as it defends plants against different kinds of plant eating organisms. These have the additional capacity to distinguish between different cell types, including malignant and normal cells. For example, it has been reported that transformed cells are frequently much more sensitive to the cytotoxic effects of lectins than normal cells. Lectins can be lethal if directly inject in mammals and can lead to impaired growth and poor absorption of food nutrients if included in diet. On the other hand, the use of lectins has been suggested as an alternative in cancer treatment. Plants recognize presence of lectins chemicals by Rhizobium bacteria. Plant breeding may be used to reduce their absolute concentration in seed but cannot be entirely eliminated from legume seeds because of their importance in biological nitrogen fixation. Lectins are either carbohydrate-binding proteins or glycoprotein. They are capable of recognizing and binding carbohydrates in complex glycoconjugates.

Lectins usually contain two or four subunits, each with a single carbohydrate-binding site. Lectins are the only plant proteins that are able of recognize and bind to simple sugars (such as Glc, Man, or Gal) but lectins favour oligosaccharides which are not common in plants but are common in animals as being the major glycoprotein. Type 2 RIPs are known to be potent cytotoxic agents which are toxic if enter cytoplasm of mammals. The sugar-binding B chain binds to a (glycoconjugate) receptor on the cell surface, promoting the uptake of the A chain. After its entry into the cell the A chain catalytically inactivates eukaryotic ribosomes by cleaving the N-glycosidic bond of a single adenosine residue of the large rRNA.

Lectins are resistant to breakdown in the digestive tract and bind to surface receptors causing changes in the metabolism of the epithelial cells. These changes include cell hypertrophy and hyperplasia. High levels of lectin can cause rapid body weight loss, increase weight of small intestine, changed gut endocrine systems and hormone production, changed gut immune systems and disturbances in gut bacterial ecology, which, combined, can cause a greater sensitivity to bacterial infection. These effects are mediated via reduced growth, diarrhoea, reduced nutrient absorption, and increased incidence of bacterial infections. To minimize the toxic levels heat treatment is suggested as most lectins are heat liable as breeding won't be possible because of their role in nitrogen fixation. Most beans require long soaking and cooking times before they are soft enough to eat, and therefore, for human consumption, most seed is rendered safe before it is consumed. However, if beans are heated above normal temperature required 80 °C, lectin levels may be increased, and thus the beans are more toxic.

Protease inhibitors

Protease inhibitors found in plants particularly legumes (e.g. soybean) act as defence mechanism against predators and are responsible for proteolytic enzyme inhibition and impaird growth in animals. In soybean two kinds of protease inhibitors are present; kunitz and bowman-brik inhibitors. According to numerous studies many protein inhibitors in plants are water soluble with non-glycosylated polypeptide chains. Bowman-birk protease was isolated by Bowman and Birk, having seven disulphide bonds which is the active point. Protease inhibitors are accumulated in seeds of plants such as in soybean which gives beans its characteristic off flavour. lipoxygenase during the process of roasting, fermentation and germination, the lipoxygenase breaks down the oil into a number of volatile compounds of which eight are responsible for beany flavour, and are responsible for presence of protease inhibitors. Lipoxygenase in soybean seeds is present in the form of three isozymes ( Lox-I, Lox-II and Lox-III) (Kumar, et al, 2003). Soybean is an iron containg enzymes which catalyzes the oxidation of polyunsaturated fatty acids with cis,cis-1,4-pentadiene units to produce protease inhibitors and conjugated unsaturated fatty acid peroxides which are responsible for the off flavour in soybeans as well as bitter taste (Li et al, 2008; Wang et al, 2008). It comprises of enzymes that are commonly found in soybean, can arise by either hydrolysis of fatty esters or oxidative fat corrosion. Naturally occurring enzymes such as lipid acyl-hydrolases, directly hydrolyze the fatty ester bonds of triglycerides and phospholipids, producing free fatty acids.

According to many studies peroxides, lipoxygenase, and phospholipids are the main cause of off production of protease inhibitors in soybean and odour in soybean and many studies show that pH and temperature may also have influence on the flavour impact causing undesirable taste. According to Iassonova, removing lipoxygenase (LOX) isozymes can reduces the amounts of protease inhibitors in soybeans and soy products drastically, but are not completely eliminated. The present work presents evidence that lipoxygenase-null (LOX-null) soybeans contain a LOX-like enzyme that is responsible for the off-flavors in LOX-null soybeans. Volatiles production in triple LOX-null soybeans was terminated by heat treatment, which suggests an enzymatic cause to the off-flavors. The source is LOX-like in that the volatile compounds produced are similar to LOX-generated products of polyunsaturated fatty acids. Oxygen was consumed when a LOX-null protein solution was incubated with crude soybean oil suggesting that the enzyme catalyzed oxygen consuming reactions. The generation of flavour compounds was inhibited by the typical LOX inhibitors propyl gallate and nordihydroguaiaretic acid (NDGA). The enzyme appears to be more active with phosphatidylcholine than with other lipid substrates. According to present studies, eliminating LOX in beans may reduce the levels of protease inhibitors which are undesirable anti nutritional factors in soybean.


Castor plant (Ricinus communis L.) seed is an important product for a number of industries because it produces oil. The main drawback is that it contains ricin, a toxic protein which makes oil unfavourable. Although much research has been done to reduce the toxicity (e.g. cold-pressed oil extracted seed meal known to contain ricin was boiled in the presence of 50mM calcium hydroxide (pH 12.5)). However heat and chemical treatments showed denaturation of ricin within whole seed. Boiling and autoclaving showed varying degrees of effectiveness depending on the sample type. Hot pressing of the castor seed produced oil that was less toxic, indicating that the ricin had been denatured during the oil extraction. Therefore, by removing the toxic component of the castor meal, this by-product could create a new commodity from the production of castor oil, making castor oil production more profitable. Around 85% of the total lipids found within a castor seed is ricinoleic acid which gives this oil its unique characteristic of being miscible with methyl and ethyl alcohols. Ricin (RCA60) is a class II ribosome-inactivating protein; a heterodimeric protein consisting of two chains linked by a disulfide bond. The A-chain of the ricin molecule is the effective toxin. It works by depurinating specific residues on the rRNA of the 28 s subunit of the ribosome. The B-chain of the ricin molecule is important as it is responsible for cell entry as chain-A cannot enter the cell without the B- chain which allows lectin to bind to galactosides on the cell membrane.

Ricin is very toxic if directly injected and can be used as a bio terrorism weapon but it less toxic if consumed orally. The oil is toxic which contains ricin and detoxifying or heating it can be useful and as meal is less toxic. The estimated temperature required for ricin protein to unfold is 70 -C. Most studies are done on ricin and ways to eliminate toxicity level by using various methods such as hot-press technology, urea, guanidine, and calcium hydroxide as castor oil is very useful (e.g. used to induce labor). Food-grade castor oil and sodium ricinoleate are prepared in a way as to be free of the castor bean constituents that have been proven to be the source of reported Type I immediate hypersensitivity responses. Plant toxins such as ricin bind to cells glycoproteins and glycolipids over the cell surface by chain-A disulphide bonds whereas and Chain-B that has enzymatic activity enters the cytosol prior to endocytosis and disrupts the cell membrane and protein sysntehis by denaturing 28S ribosomes subunits, and are transported to endosomes through the Golgi apparatus to the endoplasmic reticulum before the enzymatically active part enters the cytosol. Several of the bacterial toxins are still a threat to human health but few are also used as components in targeted drug treatment, such as in cancer therapy. The B-chain consists of a galactose-binding region which is capable of binding to target cell's membrane, without the B-chain , the A-chain cannot enter the cell.


Saponin is found in soybeans, peanuts, sugar and beets and have bitter taste and is thought to affect bioavailability of minerals. Saponins are glycosides containing polycyclic aglycone (C27 steroid or C30) sapo-genes attached. The main characteristic of saponin is bitter taste and foaming properties and is toxic if injected intravenously. Saponins anti-nutritional effect has been studied in animals such as chicks and pigs, which seemed to impede the growth , causing bloat, also is responsible for reduces in food intake. Saponins are a varied group of chemicals, for their ability to form soap-like foams in aqueous solutions. They occur in a significant number of plant species ranging from asparagus. Animals such as pigs and poultry are fed rations containing such products as alfalfa (Medicago sativa) meal; there is a reduction in growth. On the positive side, it has been shown that saponins can form complexes with cholesterol in the gastrointestinal tract. This leads to increased excretion of cholesterol and a reduction in blood cholesterol level. With the high level of coronary artery disease in most developed countries, this is seen to have potential medical benefits. The most common form of ingestion by humans is through alfalfa sprouts. sprouts are more dangerous because of bacterial contamination, leading to gastroenteritis, than because of their saponin content.

Aglycone is present in all saponins which is linked to one or more sugars or oligosaccharides. Saponin's have a complex structure which depends on aglycone structure and its position and the way glycosides are attached to the molecule. Aglycones can be linked to Image-glactose, Image-arabinose, Image-rhamnose, Image-glucose, Image-xylose, Image-mannose, and Image-glucuronic acid. The chain lengths are generally linear and comprise two to five saccharide units. The major physiological effect of saponins is on cell membrane permeability, combing with the memberance cholesterol to for permeable micelles in the membrane. Saponin level can be reduced by soaking, washing, and rubbing in grain. Industries process the seed to eliminate saponin off odour, by milling, or re-washing.


High tannin levels in crops such as faba beans, can act as a defence mechanic for plants from birds, herbivores, and insect damage .There are two types of tannin, hydrolysable and condensed tannin (fig 1.1 and 1.2). Once ingested tannin can bind to cell membrane and irritate the gut lining and stimulate the secretion of mucus thus increases endogenous protein secretion and therefore increases protein demand and is the main concern for reduction of mineral absorption leading to weight loss in animals and humans. According to many studies leaves rich in tannin, could be fed to animals without any undesirable effect, because animals have an ability to consume proteins in excess of their requirement from the concentrate and therefore, the anti-nutritional effects of tannins are veiled. Tannin is a generic name for polyphenolic compounds from plants and is divided into two classes, hydrolyzable and condensed. The former can be broken down by acid, alkali, and some hydrolytic enzymes. The latter are resistant to being broken down. Faba bean (Vicia faba) tannins are involved with the human disease favism. The inclusion of high levels of tannin in rations for a range of monogastric animals reduces weight gain and feed-conversion efficiency. When fed to hens, tannins reduce egg production. However, the consumption of rummage with high tannin levels can be

Fig.1.1 ( A) Condensed tannin( tannic acid) Fig.1.2 (B) Hydrolysed tannin

advantageous in ruminant animal production. Tannins prevent the formation of the stabilized foams that produce bloat. Tannis may have long chains of gallic acid coming from the central glucose core. When hydrolysed with acid or enzymes, tannins break down into phenolic acids and carbohydrates components.

If beans containing tannis are consumed by humans than it can cause hemolytic anemia, stomach pains, headache, vomiting, nausea, yawning, and a raised temperature and hemoglobinuria in severe cases and rarely death may occur which is due to the lack of an enzyme, glucose-6-phosphate dehydrogenase in red blood cells. Tannins are water soluble phenolic compounds with the ability to precipitate proteins from aqueous solution and occur almost in all vascular plants. Tannins inhibit digestive enzymes forming less digestible complex with dietary proteins. Tannin protein complexes involve hydrogen-bonding and hydrophobic interactions which depends upon pH, ionic strength and molecular size of tannins. To reduce levels of tannin in seed, heating may not be the best option but tannins are unstable under alkaline conditions and therefore soaking in an alkaline solution followed by washing may reduce the level of tannins. Also the use of urea may be beneficial but more studies are still underway.


As everyone is aware of harmful toxins and anti-nutritional factors in plants, there have been much study done and many methods developed to reduce the toxicity in plants involved. Such processes include; of soaking, testa removal, heating, fermenting, germinating, or protein extraction to produce isolates. To produce castor oil the heat that is produced during crushing and milling may be sufficient to provide safe food to humans. Many plant breeders have come up with ways to eliminate chemical compounds (anti-nutritional factors) in plants that are harmful to humans but the downside is that these chemical are important to defend plants from birds, fungi and insects white they are growing.

Anti-nutritional factors are heat liable and therefore heating or autoclaving has been found useful in removing the effects of harmful chemicals in edible plants. Many methods such as detannification, physical treatment, chemical treatment, protease inhibitors and lectins can be treated by heat but excess heating can lead to loss of important nutrients. Although, this may be very expensive due to the energy consumption, treatment and transport cost. The negative point of this is that during this process to eliminate toxicity from edible plants the nutrient and minerals may also be lost or reduced, as few of these factors are beneficial such as protease inhibitors are used as drugs for therapies for treating patients also used for life expectancy in HIV patients according to new studies.