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Chemistry

Celery (Apium graveolens)

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Main image: Stalks of celery with calculated molecular structures of NBP, Sedanenolide and Sedanolide (Fig. 2) – can you assign them?

Celery (Apium graveolens) is a vegetable commonly used in salads and soups, which for some reason appears to cause more food-crazes than any other. As far back as 1877, an American writer wrote of

Celery as a relish before soup; celery as a salad after meat; celery and cheese as dessert; celery- luncheons; celery salad as a light late supper; celery plain or dressed for gentlemen whose nerves are ante breakfast cocktails; celery chewing parties by young and gentlemen; celery in a hundred ways, and on every possible occasion, is now sure to be the fashion.

And, 140 years later we have a host of celebrities telling the world about their celery juice habit, led by a wellness advisor who is guided by spirit voices. It’s commonly held that celery is a negative calorie food – i.e., that it takes more energy to eat it than we get from it. Sadly, it’s not true – a recent study which involved feeding celery to bearded dragon lizards (chosen because of their ‘docile temperament and willingness to consume celery’) showed this not to be the case.

1) Celery and celeriac are different varieties of the same plant; celery has been bred for its stalks, which are properly called petioles and are the leaf stalks (much like Rhubarb), whereas celeriac has been bred for its bulb, which is properly called the hypocotl. Celery seems to have originally been a marsh plant found throughout Europe and used by the ancient Greeks as a medicine, and cultivation as a food seems to have begun in the 17th and 18th centuries (The American Naturalist, 1886, 20, 599-906). Celery is commonly grown in trenches, with the growing stalks covered in earth in order to keep the light off of them. This keeps them white rather than green and reduces the bitterness, but traditionally red celery was also grown and eaten.

A 1902 advertisement for pink ceery
Figure 1: A 1902 advert for Pink Plume Celery, ‘the most beautiful Celery that ever graced a table’.

2) Just like for fennel, the essential oil can be isolated from celery by the process of steam distillation, and analysis reveals two major classes of chemicals: terpenes (see our post on geraniol) and phthalides. The former are present in the larger quantities, with limonene the major species. However, despite being present in lower quantities, it is the phthalides shown in Figure 2 that determine the taste and smell of celery (HortScience, 1990, 25, 556-559).

The smell and taste of celery are largely due to three particular compounds called sedanolide, sedanenolide and butylphthalide (aka 3-n-butylphthalide or NBP) (Figure 2), which belong to a class of molecules called phthalides (J. Food. Sci., 1970, 35, 766-768). Celery has long been used in Chinese medicine, and the past decade has seen NBP extensively investigated as a treatment for cerebral ischaemia, a type of stroke caused by lack of blood to the brain, as well as other conditions (J. Med. Chem., 2020, 63, 12485-12510).

NBP is a chiral compound, and it is the S-enantiomer that is found in celery (Figure 2). This enantiomer is labelled S because when the groups attached to it are prioritised according the CIP rules, they go 1-2-3 in an anticlockwise direction – S stands for Sinister, the Latin word for left. In the lab it’s much easier to make a mixture of both R and S forms; this too can be used medically, but is less potent because the R form is less active than the S form.

Figure 2: Sedanolide, Sedanenolide and 3-n-butylphthalide (NBP). Spot the difference!

3) Somewhat bizarrely, eating celery can result in the skin becoming light-sensitive. There are several well documented cases of grocery store workers in the US developing blisters and skin darkening after selling celery (Arch. Dermatol., 1987, 123, 1478-1482), and a case of a lady whose visit to the suntan parlor after consuming a large celery root resulted in severe blistering and oedema (Arch. Dermatol., 1990, 126, 1334-1336).

The chemicals responsible for this belong to a class known as psoralens (Figure 3). These are another class of chemical produced naturally by celery; generally the concentration is minimal and does not cause any problems, but it can be greatly increased if the celery has been infected with a fungus called ‘pink rot’ (Biochem., 1963, 2, 1127-1131) .

Figure 3: Psoralen

The sensitivity occurs because psoralens interfere with the DNA in cells. DNA is a double helix, joined together like the two sides of a zip. Because psoralen is a flat molecule, it can slip between the base pairs of DNA in a process known as intercalation, effectively like slipping between the teeth of the zip. Psoralen is also very good at absorbing UV radiation, and when an intercalated psoralen molecule does this, it forms a bond to the thymine moieties within the DNA, joining the two strands together (Figure 4).  Unfortunately, the process of replicating or repairing DNA requires the two strands to be unzipped (Figure 5), and by joining them together psoralen thus causes cell death.

Figure 4: UV light cause psoralen to cross-link DNA strands
Figure 5: Cartoon of a DNA double helix (left hand side) being unzipped in order to replicate (right hand side).

This is deliberately exploited in the medical field of photochemotherapy, where patients are given psoralen and then exposed to UV radiation. It is used in the treatment of several skin conditions such as vitiligo, eczema and psoriasis (J. Phochem. Photobiol. B, 1992, 14, 3-22). And, to take us full circle, it appears that the ancients knew about this too. A famous book from the 13th century, Kitab Al-jami li-mufradat al-adwiya wa al-aghdhiya (The book of medicinal and nutritional terms), describes the treatment of vitilgo using the seed Aatrillal (Ammi majus) and sunlight. Aatrillal is still sold by Egyptian herbalists to cure spots of white skin pigmentation, and its active ingredients are psoralen derivatives.

So, once again we discover that there was valid reasoning behind an ancient medication, but not behind modern-day snake-oil.

Contributors: Chris Adams (Research and Writing), Natalie Fey (main image)

Picture credits: Figure 1 is from the 1902 catalogue of Peter Henderson & Co., and is via the Biodiversity Library. Figure 4 is from An Improved 4’-Aminomethyltroxsalen-Based DNA Crosslinker for Biotinylation of DNA by Wielenberg et al, published in RSC Advances and used under CC BY-NC 3.0 license. Figure 5 is by ChristineImiller via Wikimedia Commons.

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