Solanine, the toxic compound in potatoes

Memories of a summer evening (yeah, I still cannot get over the idea that summer is gone), preparing dinner. A question about solanine in tomatoes comes up. A long discussion about solanine, tomatine and other glycoalkaloids produced by Solanaceae plants starts.
It seems a nerdy conversation, and maybe it is, but it is actually also a very important topic. The Solanaceae family includes important food plants like tomato (Solanum lycopersicum), potato (Solanum tuberosum), eggplant (Solanum melongena), and sweet and chili pepper (Capsicum species). Interestingly, the same family includes plants like deadly nightshade (Atropa belladonna), mandrakes (Mandragora species), jimsonweed (Datura stramonium), and henbane (Hyoscyamus niger), which are poisonous plants. Just to name the most popular examples of the two groups. Noteworthy, also some food plants of this family produce toxic chemicals (although structurally different from the ones synthesized by the second group of plants), and that is the case for solanine and related compounds.

Solanine

Solanine is a saponin, made up by a steroidal alkaloid moiety (solanidine) bound to a chain of three sugars. It was first isolated in 1820 from the berries of the European black nightshade (Solanum nigrum), after which it was named (1). The main source of solanine in the diet are potatoes, which also contain other less abundant structurally similar compounds (i.e. glycoalkaloids). The glycoalkaloid content of commercial varieties of potatoes needs to be within certain limits, because these chemicals can cause symptoms of poisoning in humans. Currently, there are approximately 5000 potato varieties worldwide (2) and the control of toxins is based on careful selection and breeding, as well as attention to handling and processing in order to reduce mechanical damage and exposure to light (3), which are some of the factors that cause an increase in the production of solanine.

You are surely aware that potatoes need to be stored in a dark, dry, and fresh place. I guess you have also been warned not to eat potatoes that turned green or started sprouting. Well, the colour is not a problem at all since it is given by chlorophylls. However, together with chlorophylls, solanine is synthesized and accumulated (4). Although it has been shown that the synthesis of solanine is independent of greening (i.e. greened tubers do not always have high levels and vice versa) (5), potatoes showing signs of greening, sprouting, rotting, or physical damage should not be eaten because of the likely high concentration of solanine. Indeed, not only exposure to light, but also other stresses and physical damage, aging, and the sprouting process induce the synthesis of glycoalkaloids (5).

Solanine and the other glycoalkaloids are produced by potato tubers, sprouts, and other plant organs already in “normal” conditions. Their levels in most commercial crops range from 2 to 10 mg/100 g fresh weight, while the generally accepted safe limit is 20 mg/100 g fresh weight (5,6). Different potato varieties produce different amounts of glycoalkaloids, which are usually more abundant in and under the skin. This latter part contains 30–80% of the total glycoalkaloid content, while constituting about 2–3% of the tuber (5). Solanine is also abundant in developing shoots. Besides the plant variety, also growth conditions (including nutrient availability and stress exposure) have an influence on the solanine content (5).

Excess of solanine causes a bitter taste and poisoning upon ingestion at certain doses (6), as we will see in the next paragraph. Unfortunately, the glycoalkaloids are not broken-down during cooking and frying, although deep frying seems to reduce their levels (5) (I know what you are thinking here, but deep fried food is still not the healthiest choice). Peeling definitely substantially reduces solanine levels (5).

Solanine toxicity

Why is solanine such a big concern? According to a legend, when Sir Walter Raleigh introduced potatoes for the first time in Ireland, the cooks of Queen Elizabeth I threw out the edible tubers and cooked the stems and leaves, which contained higher levels of solanine, making everyone feeling extremely ill (8). Besides the legends, there have been well documented cases and outbreaks of solanine poisoning  (2,4,7,8). According to official statistics, solanine in potatoes has killed at least 30 people and made over 2000 very sick over the years (8), although the real numbers might be higher.
Doses of 2 to 5 mg/kgbodyweight can cause poisoning symptoms, while doses of 6 mg/kgbodyweight seem to be the fatal threshold (8). Differently said, symptoms can be caused by doses of 200–400 mg for adult humans (20–40 mg for children) (6). Well, if you consider that the regular amount of solanine in commercial potato varieties is in the range of 2 to 10 mg/100 g, it is clear that you would need to eat a huge amount of potatoes to be at risk. However, potatoes that have been exposed to one of the factors listed above inducing the accumulation of solanine (2) should never be eaten in order to avoid unwanted effects, because the solanine content can be really high in these cases.
Symptoms of solanine poisoning occur mainly in the gastrointestinal tract, and include dry mouth, burning of the throat, nausea, diarrhoea, vomiting, stomach cramps, abdominal pain, but it also can lead to problems at neurological level, like headaches, dizziness, seizures and neurological impairment (2,9). In more severe cases also hallucinations, loss of sensation, paralysis, fever, jaundice, dilated pupils, and hypothermia can occur (6).

What is the role of solanine in potatoes?

The production of solanine in potatoes exposed to light is a defence mechanism to prevent the uncovered tubers from being eaten (6). It is indeed reported that solanine is a defence compound against insects and predators, but also against diseases (2). Solanine and chaconine are known antifeedants acting towards snails. They seem to act synergistically, and it is very likely that other glycoalkaloids contribute to this effect (10).
Solanine has also fungicide and pesticide properties. Its mode of action is not completely clear, but as other saponins it might be acting disrupting the cell membrane of the target organisms (11).

It is also interesting that the other two Solanum species that we named before, tomato and eggplant, both produce glycoalkaloids, but the major compounds are different, namely tomatine in tomatoes and solamargine in eggplants (12). However, also these metabolites have a role in the plant defence system.