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.
One of the most damaging is 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 found in plants and animals. They are especially prevalent and abundant in legume seeds. As we’ve already mentioned the lectins in legumes ahve 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).
The toxin known as Ricin in Ricinus communis was one of the first to be identified as a lectin due to its specific properlty of agglutination (Stillmark, 1889). It gained particular notoriety following the poisoning and death of the Bulgarian dissident Gyorgi Markov in London in the 1970s.
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 lectins are also useful biochemical tools. They form good affinity ligands in affinity chromatography for binding glycoproteins. Concanavalin A is particularly useful and was one for the first to be used in this type of chromatography (Agarwal & Goldstein, 1972).
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.
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 kidney beans when ingested will cause vomiting etc. in a few hours.