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Did you know that peach seeds contain a toxic substance? The chemistry of amygdalin

After a dinner in Tunisia, some retired former Italian secret service agents were hospitalized for poisoning peach seeds. For one of the agents, the ingestion of these seeds, present in a peach liqueur served at the end of the meal, proved fatal. Investigations are currently underway to understand how it was possible, but beyond the legal matter, let’s try to understand what happened from a chemical point of view. Peach pits contain a substance, amygdalincapable of release hydrogen cyanide when it comes into contact with some enzymes in our digestive tract. Let’s see how it works and what risks are involved.

Cyanogenic glucosides: where they are found and what they are used for

Amygdalin is part of a group of substances called cyanogenic glycosidesi.e. molecules that can release hydrogen cyanide when they come into contact with enzymes of the hydrolase family (in particular, β-glucosidase). For lovers of detective stories and crime dramas, let’s talk about the infamous cyanide with the characteristic smell of bitter almonds.

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They are present in over 3000 species of plants which, unable to escape when attacked by predators, use them as defense mechanism from bacterial, fungal and above all animal attacks. The animal eats the stone, feels bad and will no longer try to bother the plant again. They are also present in some fungi and bacteria mainly as a source and nitrogen reserve when they cannot obtain it from the surrounding environment.

Amygdalin is present mainly in the stones and seeds of plants that belong to the family of Rosaceaesuch as peaches, apricots, but also apples, cherries and, the most famous case, bitter almonds (which actually contain between 2.5% and 3.5%). After bitter almonds, the peach seed, called armellina and located inside the kernel, contains the highest percentage of amygdalin, between 1.2-2.4%.

A little chemistry

Chemically, amygdalin is made up of two parts: a sugary part (2 glucose molecules, red circle in the photo) and a non-sugar part (blue circle in the photo), called aglycone.

Chemical structure of amygdalin Yikrazuul, Public domain, via Wikimedia Commons Image modified
Chemical structure of amygdalin
Yikrazuul, Public domain, via Wikimedia Commons
Edited image

By themselves, amygdalin and cyanogenic glycosides are harmless, they only become toxic when ingestedbecause in the stomach they encounter β-glucosidases, which break the bond between the sugar part and the aglycone. This reaction is called hydrolysis and causes the breaking of the β-glycosidic bond and the release of glucose, benzaldehyde and hydrogen cyanide (HCN).

Amygdalin hydrolysis reaction Yikrazuul, Public domain, via Wikimedia Commons Image modified
Amygdalin hydrolysis reaction
Yikrazuul, Public domain, via Wikimedia Commons
Edited image

In humans, β-glucosidases are generally conserved in lysosomesorganelles present in the cell in which various enzymes are stored and then released when macromolecules need to be degraded. In the stomach, however, it is the microbiota to produce β-glucosidases which will break down amygdalin.

Sometimes, for example in bitter almonds, the seed already contains within it a group of β-glucosidases (collectively called emulsin), but it stores them in cells different from those in which it stores amygdalin. The simple one chewing and breaking the seed brings the amygdalin into contact with the enzymes and triggers the hydrolysis reaction.

Toxicity of hydrogen cyanide

In the bloodstream, hydrogen cyanide dissociates, releasing ion cyanide (CN-), capable of forming stable complexes with metalloproteins (proteins that use a metal as a “helper”), fundamental for life. One of the best known metalloproteins ishemoglobinwhich incorporates an iron atom, but the target of the cyanide ion is located in the mitochondria.

At the level of complex IV of the electron transport chain, cyanide binds to ferric ions of the enzyme cytochrome oxidase a3 and prevents the use of oxygen in the last step of cellular respiration, fundamental for the production of ATP.

Simply put, the cyanide ion suffocates literally the cell, preventing it from using oxygen and causing the cell death due to hypoxia (oxygen deficiency).

Symptoms of cyanide poisoning and toxic doses

The first symptoms of cyanide poisoning are both neurological (nausea, nervousness, heachacheconfusion) and cardiac and respiratory conditions with tachycardia and tachypnea, leading to a drop in blood pressure and apnea, with sometimes lethal consequences.

It is curious to note that victims of cyanide poisoning do not have the cyanotic, greyish skin typical of suffocation with lack of oxygen, but the complexion may appear normal or even redder. This happens because cellular hypoxia caused by cyanide leads to a buildup of oxygen in the blood.

EFSA estimates that the lethal dose of hydrogen cyanide is between 0.5 and 3 mg/kg body weight. Therefore, based on the average content in kernels (peach and apricot seeds), the toxic effects occur when eating more than 3 seeds (about 350 mg) for an adult and more than half a seed (about 60 mg) for a child.

Sources:

GL Biagi, E. Speroni, Pharmacognosy, III ed. Patron publisher (2010)

Díaz-Rueda, P., Morales de Los Ríos, L., Romero, L. C., & García, I. (2023). Old poisons, new signaling molecules: the case of hydrogen cyanide. Journal of experimental botany, 74(19), 6040–6051 Graham J, Traylor J. Cyanide Toxicity. (Updated 2023 Feb 13). In: StatPearls (Internet). Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Chaouali, N., Gana, I., Dorra, A., Khelifi, F., Nouioui, A., Masri, W., Belwaer, I., Ghorbel, H., & Hedhili, A. (2013). Potential Toxic Levels of Cyanide in Almonds (Prunus amygdalus), Apricot Kernels (Prunus armeniaca), and Almond Syrup. ISRN toxicology, 2013, 610648. Jaszczak-Wilke, E., Polkowska, Ż., Koprowski, M., Owsianik, K., Mitchell, A.E., & Bałczewski, P. (2021). Amygdalin: Toxicity, Anticancer Activity and Analytical Procedures for Its Determination in Plant Seeds. Molecules (Basel, Switzerland), 26(8), 2253. EFSA Apricot seeds pose risks of cyanide poisoning