Food toxins have been a serious issue throughout mankind’s love of food. It is a particular issue for the raw foodists who enjoy uncooked raw foods but sometimes fall foul of the toxins that are found inside some plants. These are the lectins and haemaglutinins.
One of the most damaging lectinis the phytohaemagglutinin of raw red kidney beans. Kidney beans can only be eaten if they are cooked long enough. There have been serious instances of raw red kidney bean poisoning where consumers have suffered vomiting and extreme diarrhea eating kidney beans in salads for example. There was a case in early 2020 in Denmark where 45 people suffered from this type of poisoning.
What Are Lectins?
Lectins are proteins that bind specifically to carbohydrates and in many cases specific sugar groups (moieties). The toxin known as Ricin in Ricinus communis (Castor Oil plant) was the first to be identified as a lectin due to its specific property of agglutination in 1888 by Peter Hermann Stillmark (Stillmark, 1889). It gained particular notoriety following the poisoning and death of the Bulgarian dissident Gyorgi Markov in London in the 1970s.
Lectins are found in plants and animals. They are especially prevalent and abundant in legume seeds. One of the richest sources is the red kidney bean (Phaseolus vulgaris). As we’ve already mentioned the lectins in legumes have significant economic importance because they are so toxic.
One of the most comprehensive studies was by Allen and Brilliantine in 1969 who looked at lectins in 2663 species of plants. They found lectins in 800 of them.
The lectins have also been found in fish, in venom and in seafood such as clams (Kornfeld et al., 1972).
Agglutination is the binding of cells together causing clumping. It happens to red blood cells with different blood groups (Boyd and Shapleigh, 1954; Jaffe and Brucher, 1972). White blood cells can also be agglutinated together (Brandt et al., 1962).
In cancer cell studies, these lectins also cause the clumping of cancerous cells which may be of some clinical benefit if the mechanisms can be managed properly (Nungester and Van Halsema, 1953).
The agglutinating activity comes from binding to particular glycoproteins in the cell membrane. The lectins are proteins and are able to recognise specific polysaccharides on these glycoproteins.
The Structure of Lectins
Lectins are tetramers of 4 nearly identical protein subunits. These sub-units are each capable of binding to a specific carbohydrate moiety of other proteins. The binding is reversible. It has been shown to be a noncovalent interaction with carbohydrate conjugates.
Lectins Can Be Acute Cytotoxins
We’ve already mentioned one and that was ricin. Ricin which comes from the Castor Oil plant (Ricinus communis) is a well-known protein that inactivates the type-2 ribosome. That is why it is such a cytotoxic and lethal agent. It may be one of the most potent toxins found in nature. The others in this category include abrin, volkensin, and the mistletoe lectin. These three come from rosary peas (Abrus precatorius), the roots of Adenia volkensii, and Viscum album respectively.
Ricin is especially poisonous at very low doses. The LD50 is only 22 µg/kg bw usually by inhalation or injection. If it is ingested, about 1 or 2 mg of ricin in a typical adult will produce the equivalent significant response after a number of hours. A castor oil seed is lethal to a child on that basis.
The lectin found in red kidney beans (Phaseolus vulgaris) is a phytohaemagglutinin which causes gastric troubles but is destroyed by heating as in cooking. About 4 or 5 uncooked kidney beans when eaten will cause vomiting etc. in a few hours.
Injection of toxic phytohaemagglutinins is reported to cause pathological changes such as faulty infiltration of liver, zonal necrosis, oedema and haemorrhage in animal tissues (Kumar et al., 2022).
Processes to Destroy Lectins
Conventional cooking methods use high temperatures as in boiling, simmering and pressure cooking. It’s important to recognise that to avoid being poisoned by consuming untreated foods that contain lectin is a public safety measure. Generally, kitchen cooking methods are good enough to destroy lectins and in a relatively short time as well. A well-known study by Thompson et al., (1983) in the Department of Nutritional Sciences, University of Toronto in Canada established some key processing parameters for heat processing. They found cooking presoaked beans at 100ºC for 15 minutes or 80ºC at 2 hours was good enough to reduce the lectin content to a safe level. They also found pressure cooking at 15 psi for 45 minutes without presoaking was effective. Reassuringly, commercially canned beans have little or no lectin present and their levels were similar to beans cooked in a pressure cooker for 30 minutes.
The temperature is critical in cooking. There is no benefit in cooking at 65ºC and even after a long time such as 12 hours – it has no effect on destroying (denaturing the lectin).
Industrial Uses Of Lectins
The lectins are also useful biochemical tools. They form good affinity ligands in affinity chromatography for binding glycoproteins. Concanavalin A is particularly useful and was the first to be used in this type of chromatography (Agarwal & Goldstein, 1972).
Lectin have also proven to be useful accelerators of enzyme activity in such systems as enzyme-based reactors. The addition of Concanavalin A (Con A) to a reactor system helps to accelerate the hydrolysis of carbohydrates like sucrose by a mixed enzyme system using E. coli beta-galactosidase and yeast invertase. The enzyme system produces glucose. In this case the lectin binds to the product and prevents feedback inhibition of the enzyme beta-galactosidase (Agarwal & Gupta, 1994). As a result the enzyme maintains a high catalysis rate. Fresh Con A needs to be added to replenish the Con A which has become saturated. In this study, 38% of the sucrose was hydrolysed with the presence of Con A compared to just 22% being hydrolysed.
The use of lectins on an industrial scale has not proven to be worthwhile but these may prove more useful if high value metabolites are needed from what might be an uneconomic fermentation without such a lectin inhibitor. One of the issues would be the need to recover the inhibitor from the final product mix.
Leave a Reply