An Isolated Plant Family

There is a family within the angiosperms (flowering plants) that, from an evolutionary viewpoint, is relatively isolated. The only family that’s somewhat closely related to it is that of the morning glory, Convolvulacea. The family in question has about 1700 species, which is not much compared to the diversity within the pea, orchid and sunflower families, which all together account for over 60 000 species or roughly 20% of all flowering plants.

Its flowers are very often star-shaped with 5 sepals, 5 petals and 5 stamens, and its stems and leaves host a variety of both toxic and useful alkaloids. If you haven’t guessed it yet we are referring to the Solanaceae family, whose most familiar members include tomatoes, eggplants, tobacco, potatoes, petunias, jimson weed and deadly nightshade (belladonna). Not many people realize that the tomato, has a close relative known as the tamarillo or tree tomato, also a native of the Andes. This shrub produces fruits of various colors, with the orange one being sweeter than the red. Currently, the tomato and tamarillo are classified in different genera, but that grouping is probably mistaken according to DNA studies.

On the left are representatives of a couple of tomato varieties from my garden. On the right is a tree tomato, which like its relative is also originally from the Andes.

What kind of alkaloids are found in Solanaceae plants? Potatoes and tomatoes both contain solanine, which has fungicidal and insect-repellent properties. Tomatine in tomatoes serves the same purpose and is also present in unripe tomatoes. If you touch the stems or leaves of tomatoes while picking them, you will cause their tiny hairs (trichomes) to release alkaloids along with oils, which give plants their distinctive scent. Unfortunately, some people are very sensitive to some of the compounds made by the trichomes, and they develop eczema: itchy skin with small, fluid-filled bumps leading to scaly skin from the release of histamines by the immune system.

Other Solanaceae alkaloids go far back in history and have proven to be beneficial. Over 3000 years ago, Egyptians added extracts of a Solanaceae family-member known as mandrake to their beer. Mandrake contains atropine, scopolamine and hyoscyamine, which block the action of the neurotransmitter acetylcholine. The trio of alkaloids, which are also found in jimson weed, belladonna and henbane, can cause hallucinations, but individually, at the right dose and for the right condition, each can serve as a useful drug. Their similar structures can be seen below. Atropine and hyoscyamine (left and top right) are mirror images of one another but not identical in the way that a right handed glove is not the same as a left handed one. Scopolamine differs in having an epoxide ring (triangular arrangement of two carbons and an oxygen) adjacent to the nitrogen ring.

Atropine can be used to dilate eyes, to treat a specific type of heart block and to treat certain cases of organophosphate poisoning. Scopolamine, in the form of a transdermal patch, is used to prevent nausea and vomiting after anesthesia or from narcotic pain medicines, and it’s also good for motion sickness. Hyoscyamine works by decreasing acid-production in the stomach, slowing down the natural movements of the gut and relaxing muscles in many organs, which is why it is used for irritable bowel syndrome and cramps. In Australia, indigenous people have exploited their continent’s Solanaceae’s Duboisia plants (corkwood) for centuries. These plants are also a source of scopolamine and hyoscyamine. The structure of the natural alkaloids has also inspired synthetic derivatives such as benzatropine, which is used to counteract the side effects of antipsychotic drugs and also for Parkinson’s disease.

Benzatropine is a synthetic analogue of atropine. It’s used to counteract dystonia, a side effect of prolonged use of many antipsychotic drugs.

Since atropine, scopolamine and hyoscyamine can easily be absorbed through the skin, it has been told that the flying ointment of underwear-less witches was rubbed on broomsticks, allowing their exposed rectums and vaginas to serve as absorption sites for the drugs. It’s the basis for the notion that witches “flew”— in reality hallucinating—on their broomsticks. When I first heard that story in my youth, I became forever interested in the combination of botany and chemistry!

Sources:

The Botanical Garden. Roger Phillips and Martin Ryx. 2002

Tomato Glycoalkaloids:  Role in the Plant and in the Diet. J. Agric. Food Chem. 2002, 50, 21, 5751–5780

Tropane Alkaloids: Chemistry, Pharmacology, Biosynthesis and Production. Molecules. 2019 Feb; 24(4): 796

Structures from Chemspider.com

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