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LECTINS (HAEMAGGLUTININS)

The lectins or haemagglutinins are proteins that possess a specific affinity for certain sugar molecules. Since carbon hydrate moieties exist in most animal cell membranes, they may attach themselves to these so-called receptor groups if the specific structure of the latter is suitable. As indicated by their names, haemagglutinins or lectins can be characterized and detected by their action on red blood cells: they have the ability to agglutinate the blood cells.

The most active substances in agglutination of blood cells, and the most toxic, were found in Phaseolus vulgaris (phasin) end glycine max (soybean agglutinin) by De Muelenaere [19], Toms and Turner [65], Reiner [59] and Jaffé and Hannig [35] .

The Vicia faba lectin consists of two apparently identical subunits [5] and is heat-labile. The observation that Vicia faba lectins react differently towards the erythrocytes of different animal species aroused the interest of immunologists with regard to their potential use as blood typing reagents. The interaction of lectin components with glycoproteins on the cell surface is manifested in vitro by an agglutination of the cells. Studies with rat, mouse, and human erythrocytes have demonstrated that faba bean lectins specifically interact with D-mannose and D-glucoseamine residues on the surface of erythrocytes, whereas soybean lectins react with D-galactose amine residues [46] . Further, the relative haemagglutiniactic activities of faba beans differ from those of raw soybeans when tested with red blood cells from different animals [45]. Mouse, turkey, and rat erythrocytes were precipitated in the presence of faba bean but not soybean extracts. Both soybean and faba bean extracts precipitated erythrocytes from pigs and rabbits [46].

TABLE 8. The levels of vicine, convicine and DOPA of different Egyptian faba bean varieties

Source: Saxena and Stewart [60].

Marquardt et al. [46] screened eight faba bean cultivars in Canada for the presence and level of haemagglutinating activity; this ranged from 3,800 to 5,600 haemagglutinin units/g whole seed. In Egypt, a lower level of only 640 haemagglutinin units/g raw faba bean was found [31]. Jaffé [34] reported that the presence of 0.5 per cent haemagglutinin in the rat diet is enough to introduce impairment of the intestinal absorption, resulting in death. The simultaneous existence of protease inhibitors and their stimulation of the excretion of pancreatic juice may also lead to an excessive loss of endogenous protein. This loss obscures the effect of lectins on intestinal absorption, since both result in increased faecal nitrogen excretion.

Most Vicia faba haemagglutinins are localized in the cytoplasm of the cotyledon and the embryo; they appear during ripening and disappear during germination [53, 64]. The detoxification of lectins is usually achieved by the traditional methods of household cooking. Nevertheless, under special conditions, complete detoxification may not always be achieved, especially if ground beans are used or industrial processes (such as dry heat) for quick-cooking products are applied. The use of bean flour for baking should be viewed with caution [19, 18] .

TANNINS

Faba beans contain a growth-depressing factor (for chicks) other than trypsin inhibitor, haemagglutinin or the favism-inducing factor. It was identified as a condensed tannin, which is located primarily in the hull, relatively heat-labile and water-soluble [46, 14,49, 68] . Tannins may be classified as polyphenolic substances. The active substance was identified as condensed proanthocyanidin [14, 50]. Faba bean tannins contain molecules of flavon-3-olene (catechin, gallacatechin) and flavon-3, 4-dioline (leucocyanidin, leucodelphinidine). These substances are present in different concentration in faba bean seeds according to variety, maturity, location and growth conditions; there is a faba bean variety free of such tannins [48]. The addition of 4 per cent of such tannins to the chicken's diet reduced its growth [49] .

There appears to be little doubt that the growth depression caused by faba bean tannins is due to an adverse effect on protein and dry matter digestibility [50] . This effect may be related to the fact that tannins interfere with the digestive action of trypsin and alpha-amylase either by binding the enzymes themselves or by binding dietary protein into an indigestible form. Tannins can also complex with vitamin B₁₂, causing a decrease in the absorption of this vitamin in rats. Faba bean tannins are also believed to be responsible for decreased laying rate, decreased efficiency of food utilization, and increased mortality of laying hens and ducklings [14, 50].

Elias et al. [21] found that tannin concentration was high in coloured seed coats and low in white-coated beans. Moreover, there is a correlation between tannin concentration in the seed coat and trypsin inhibitor activity; the hulls have much greater amounts of trypsin inhibitor than the cotyledon. Probably most of the trypsin inhibitor activity of the hulls is attributable to tannins. Dehulling the faba bean seeds, soaking the beans before cooking, and autoclaving the beans at 130°C for 30 minutes reduces and/or destroys the tannin substances [49].

PHYTATE (PHYTINS)

Phytins, the mixed Ca and Mg salts of myo-inositol-1, 2, 3, 4, 5, 6-hexakis (dihydrogen phosphate), also known as physic acid, are common constituents of plant tissue, especially of cereals and legumes. They are the principal form of phosphorus in many seeds; 60 to 90 per cent of the phosphorus in these seeds is present as physic acid [9] . Beans generally have a high physic acid content ranging between 150 to 1,800 mg/100g [40, 9] and faba beans are relatively rich in this substance (about 250 to 350 mg/ 100 g). The phytate phosphorus content of seeds taken from single plants of the variety Dacre indicated a large variation (193 to 403 mg/100 g) and was not significantly correlated with protein content [28].

The nutritional importance of physic acid lies in its ability to chelate several mineral elements, especially calcium, magnesium, zinc, copper, and iron, and if thereby reduces the availability of these elements in the intestinal tract. Phytic acid has been held responsible for the commonly observed interference by plant sources of protein on the availability of dietary minerals [55, 22,43]. In the case of the white-flowered faba bean varieties, phytate content and its associated iron-binding capacity appeared to be restricted to the cotyledons while seed coat extracts prepared from coloured flowered varieties had almost twice the iron-binding capacity of similarly prepared cotyledon extracts. It was concluded that the active constituents in the seed coats of coloured, flowered varieties were the condensed tannins [27].

Phytic acid is present in seeds in an almost water-soluble form (as sodium or potassium salt). During processing, it becomes insoluble (as calcium, magnesium, or ferric phytate). Phytate also forms complexes with proteins, making them less soluble. There is evidence that phytate-protein complexes are less subject to proteolytic digestion than the same protein alone, depending on pH. Phytate has an inhibitory effect on the peptic digestion of ovalbumin and elastin This effect is believed to be related to its ability to form insoluble combinations with proteins in an acid medium and in a range of pH which corresponds precisely to the optimum for the action of pepsin. Phytate itself is generally considered unavailable to humans due to the lack of an endogenous enzyme system[6] .

TABLE 9. Level of vicine, convicine, and DOPA in seven Egyptian dishes based on faba beans (g/100g fresh weight, as eaten)

Source: Saxona and Stewart [60].

Excess phytate in the diet—for example in the predominantly bread-and-bean diets eaten in many less developed countries—decreases the availability of some minerals, including iron, causing deficiencies. The effect of physic acid on mineral availability is influenced by many factors, such as the mineral composition of the food as well as its association with dietary protein, heat treatment, processing history of the diet, pH, and the presence of other components reducing the mineral bioavailability such as fibre, oxalates, phenolics, tannins, saponins, and histidine, which are capable of binding or interacting with minerals or physic acid to varying extents [7] .

A variety of methods for removing physic acid from seeds and vegetables were reviewed by Askar et al. [6], including mechanical processes such as milling and grinding operations. Reducing wheat flour extraction or dehulling of beans decreases the content of physic acid. Water extraction, during soaking and regermination, removes the natively water-soluble phytate. Enzyme action during the manufacture of bread or during the germination of the seeds, dialysis and ultrafiltration to separate physic acid and protein, using the molecular size differences, are suitable ways for reducing the physic acid content.

Dehulling of faba beans decreased the content of physic acid by about 30 per cent of the original values, while prolonged soaking decreased it even further. Blanching the beans after soaking was also beneficial. These data led El-Samahy et al. [22] to recommend soaking of dry faba beans before cooking, especially in the countries where people use them in great amounts, as in the developing countries. Moreover, when such legumes are used for baby foods, soaking or germinating them helps prevent mineral deficiency.

FAVISM-INDUCING FACTORS

Favism is another form of anaemia common in the Mediterranean area. A haemolytic anaemia associated with genetic deficiency of erythrocyte glucose-6-phosphate-dehydrogenase (G6PD), it causes excessive blood destruction. Haemolysis occurs in individuals with G6PD deficiency when they are exposed to faba beans (Vicia faba), and all persons with a history of favism have G6PD deficiency in erythrocytes. This deficiency, which affects more than 100 million people, is one of the most prevalent inherited enzymatic defects of clinical significance [11] .

The haemolysis is of variable intensity and gravity, the more severe cases being attended by haemoglobinuria and jaundice. In contrast to the usually low number of cases suffered by adults, several fatality cases have been noted in young children. The age distribution shows that favism is most common in young children, and in Egypt 50 per cent of the cases are in children under one year [8].

Favism is most prevalent in the Mediterranean area, particularly among oriental Jewish communities in Israel, Sardinians, Cyriots, Greeks, Egyptians, and certain African populations; American negroes are also affected. The highest incidence of favism has been reported in Sardinia, with five cases per year per 1,000 population. Outside the Mediterranean basin, favism has been reported in other Middle Eastern countries such as Iraq, Iran, and Bulgaria, and also in China. In contrast, the disease is observed only sporadically in other European countries, the United States and Canada. In most countries, the problem is most common at the time of harvesting the fresh beans. Cooking does not seem to destroy the anti-nutritional substances.

It has been established that the active substances, which have been tentatively identified as divicine, isouramil, and L-dopa, can be transferred by breast-milk. Diviane and isouramil can be obtained from the respective glycosides (vicine and convicine) by mild hydrolysis or enzymatic splitting with B-glucosidase. Both divicine and isouramil are highly active reducing reagents and highly unstable in oxygen. In contrast, the glycosides show none of the reducing properties of their aglycones and are remarkably heat-stable [8].

Considerable variation was found in the vicine and convicine content of field bean varieties and locations. The concentrations of vicine and convicine, on a per cent dry basis, were approximately: 1.94 and 0.83 per cent in Canadian varieties [57]; 0.58 and 0.36 per cent in Italian varieties [36]; and. 0.70 and 0.28 per cent in Egyptian varieties [60]. The vicine and convicine content in faba beans was highest in young seeds and decreased rapidly with maturity [57, 60] .

ACKNOWLEDGEMENT

The author thanks the Fulbright Commission in Cairo, Egypt, and the Council for International Exchange of Scholars in Washington, D.C., for the Fulbright Award.

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