Furanocoumarin

This article is about the class of plant compounds. For the burn they cause, see Phytophotodermatitis.

Furanocoumarins, or furocoumarins, are a class of plant compounds made of a furan ring fused to coumarin. They are harmless in the dark. In ultraviolet light they bind to DNA, and in skin that has touched them the result is a burn, phytophotodermatitis. They are the reason giant hogweed is dangerous.¹

Furanocoumarins are made chiefly by four unrelated families of plants: the carrot family (Apiaceae), the citrus family (Rutaceae), the legumes (Fabaceae) and the figs (Moraceae).² At least some of them invented the compounds independently of the others. The plants use them as a defence against insects and fungi, and the insects have answered with enzymes of their own, in a coevolutionary arms race that has been studied for half a century.³ People have used them for some 4,000 years to treat skin disease with sunlight,⁴ and meet them unknowingly in grapefruit juice, where they interfere with the action of more than 85 medicines.⁵

Structure

A furanocoumarin has three rings in a flat row: a benzene ring, a pyrone ring (together forming coumarin) and a five-membered furan ring. Where the furan is attached decides the shape.³

• In linear furanocoumarins the furan is fused at the 6 and 7 positions of the coumarin, and the molecule is straight. The parent is psoralen; its best-known derivatives are bergapten (5-methoxypsoralen, 5-MOP) and xanthotoxin (8-methoxypsoralen, 8-MOP, methoxsalen).
• In angular furanocoumarins the furan is fused at 7 and 8, and the molecule is bent. The parent is angelicin; its derivatives include isobergapten, sphondin and pimpinellin.

Two numbering systems are in use. In the traditional one, bergapten is 5-methoxypsoralen; in the systematic name of the fused ring system, the same carbon is number 4. The confusion is general in the literature.⁶³ Estimates of how many natural furanocoumarins are known range from more than fifty to over two hundred, depending on who is counting.⁷²

In hogweed

Every giant hogweed makes both kinds. As early as 1976, a report of five people burned by giant hogweed on a clear day noted that it and 28 other species of Heracleum yield linear furanocoumarins;¹ the genus as a whole has since yielded them from more than a hundred species.⁸

The amounts are large. In Sosnowsky’s hogweed collected in July 2021, four furanocoumarins together made up between 3.5 and 14.4 mg in every gram of fresh tissue, depending on the organ and the population. Flowers from the Moscow region were the richest, with 7.5 mg of xanthotoxin per gram, and reproductive organs generally held more than leaves.⁸ In the leaves of giant hogweed in Slovakia, angelicin rose from about 5.7 mg per gram of dry mass in April to 10–12 mg in June and July.⁹ An earlier study of giant hogweed found the most in the fruit, less in the leaves and least in the stem.¹⁰ Hot weather raises the dose: two days at 35 °C increased the xanthotoxin in the leaves of Sosnowsky’s hogweed by about 80 per cent.¹¹

The hogweeds break a rule of the chemistry. Angular furanocoumarins were long thought to be minor compounds, usually less than a tenth of a plant’s total.³ In both giant hogweeds angelicin is often the largest single component: in the leaves of giant hogweed, and in the fruit coats of Sosnowsky’s hogweed, where it is also the main agent by which the seed suppresses the growth of other plants.⁹¹²

The compounds do not wait to be released. In Sosnowsky’s hogweed they are found in the living hairs of the stem and leaf, and as crystals on the surface of the hairs and other cells, where the lightest contact picks them up.¹³ In the related H. persicum, studied under the name H. laciniatum, the roots of older plants were strongly phototoxic even when dug from frozen ground in December.¹⁴

The small native H. sibiricum, by contrast, contains only traces of angelicin: it lacks a working copy of the gene for the first step of the angular branch.⁸

Biosynthesis

                   phenylalanine
                         |
                   cinnamic acid
                         |   C4H
                  p-coumaric acid
                         |
                  p-coumaroyl-CoA
                         |   C2'H
                   UMBELLIFERONE
                  /              \
       prenyl on C6              prenyl on C8
          (U6DT)                    (U8DT)
            |                          |
    demethylsuberosin              osthenol
            |                          |
       (+)-marmesin            (+)-columbianetin
            |  psoralen                |  angelicin
            |  synthase                |  synthase
            |  -> acetone              |
        PSORALEN                   ANGELICIN
         linear                     angular
        /       \                      |
  bergaptol   xanthotoxol      methoxyangelicins:
      |            |           sphondin, pimpinellin
  BERGAPTEN   XANTHOTOXIN      (enzymes not yet known)
   (5-MOP)      (8-MOP)

Furanocoumarins are built from the amino acid phenylalanine by way of umbelliferone, a simple coumarin.³ The decisive step is the attachment of a five-carbon prenyl group to umbelliferone, carried out by prenyltransferases of the plastid. Attached at carbon 6, it leads to the linear compounds; at carbon 8, to the angular ones.¹⁵¹⁶

On the linear branch, the prenylated intermediate is closed into (+)-marmesin, from which psoralen synthase cuts off the side chain in a single step, releasing it as acetone.¹⁷ Psoralen is then hydroxylated and methylated into bergapten and xanthotoxin.¹⁸ On the angular branch, angelicin synthase converts (+)-columbianetin into angelicin by the same kind of reaction.¹⁹ In giant hogweed this step was followed in living leaves.²⁰ The enzymes that make the angular derivatives sphondin and pimpinellin had not been isolated as of 2019.⁷

Beyond umbelliferone a plant needs only three enzymes to make psoralen, which may explain why the pathway has evolved several times.² The product is dangerous to its maker: psoralen inactivates one of the plant’s own enzymes early in the pathway. Rue and parsley, which make furanocoumarins, carry versions of the enzyme that are less easily disabled.²¹

The genome of Sosnowsky’s hogweed, sequenced in 2023, contains 55,106 genes, against the 25,000–35,000 typical of plants. The excess comes not from a doubling of the whole genome but from repeated duplication of individual genes.²² The genome carries candidate genes for every step of the furanocoumarin pathway, several of them in two or three copies.⁸

Photochemistry

   in the dark               one photon                 two photons
                             (monoadduct)               (cross-link)

   5'       3'               5'       3'                5'       3'
    |       |                 |       |                  |       |
    A ::::: T                 A ::::: T                  A ::::: T
    T ::::: A                 T*::::: A                  T*::::: A
     =======                   *=======                   *=======*
    A ::::: T                 A ::::: T                  A :::::*T
    C ::::: G                 C ::::: G                  C ::::: G
    |       |                 |       |                  |       |
   3'       5'               3'       5'                3'       5'

   the flat molecule         its furan end fuses        its pyrone end fuses
   slips between the         with a thymine             with the thymine on
   base pairs                                           the other strand

A furanocoumarin in the dark does nothing. Being flat, it slips between the stacked base pairs of DNA. When it absorbs a photon of ultraviolet A, it reacts, through its excited triplet state, with a neighbouring thymine, forming a four-membered ring between one of its double bonds and the thymine’s.²³ The reaction is fast: in one psoralen studied in detail it is complete within about 50 microseconds.²⁴

A linear molecule that has bound by its furan end can absorb a second photon and bind by its pyrone end to a thymine on the opposite strand, joining the two strands of the helix together.²⁵²³ Such interstrand cross-links block the copying of DNA. The favoured site is a thymine followed by an adenine, 5′-TpA, and runs of alternating A and T are hot spots.²⁶²⁷

Angular furanocoumarins bind once and stop: their bent shape does not allow a second bond, and they form monoadducts only.²⁸ An influential 1983 review held that the angular compounds could not bind DNA at all.³ Photochemical work showed that they do, if only once,²⁸ and skin tests of the same years confirmed that angelicin, sphondin and pimpinellin are phototoxic.²⁹ Furanocoumarins also produce reactive oxygen in light, which damages membranes; how much this contributes to the burn is disputed.³⁰³¹

The wavelengths that matter lie in the ultraviolet A. For the furanocoumarins of hogweed the skin reaction peaks at 330–335 nm, in the ultraviolet A.³² The injury appears 24 to 72 hours later, followed by darkening of the skin.³³ Among the compounds isolated from H. persicum, bergapten was the most phototoxic.³²harmless in the dark

Medicine

Photochemotherapy

More than 3,500 years ago, healers in Egypt and India treated patches of lost skin colour with the seeds of Ammi majus and Psoralea corylifolia, taken by mouth and followed by sunlight.³⁴³⁵ In 1948 A. M. El Mofty in Egypt reported treating vitiligo with purified xanthotoxin and sunlight.³⁵ In 1974 a group at Harvard gave methoxsalen by mouth, followed by long-wave ultraviolet light, to patients with severe psoriasis; in 21 patients the disease cleared completely. They called the method photochemotherapy, and it became known as PUVA, psoralen plus UVA.³⁶

PUVA was used in thousands of patients for psoriasis, cutaneous lymphoma and more than sixteen other skin diseases, and gave rise to extracorporeal photopheresis, in which blood cells are treated outside the body.⁴³⁴

Cancer

The damage that clears the skin also causes cancer. Methoxsalen with ultraviolet A is classified by the International Agency for Research on Cancer as carcinogenic to humans (Group 1); 5-methoxypsoralen is probably carcinogenic (Group 2A); angelicin with UVA, which forms no cross-links, cannot be classified (Group 3).³⁷ In an American cohort of 1,380 psoriasis patients first treated in 1975–76, a quarter developed squamous-cell carcinomas of the skin, 2,973 in all, and the risk rose steeply with the number of treatments.³⁸ The risk of melanoma rose about fifteen years after the first treatment, especially in patients given 250 treatments or more.³⁹

Grapefruit

The grapefruit effect was found by accident. In 1989 a study of alcohol and the blood-pressure drug felodipine, which gave the alcohol in grapefruit juice, found far more felodipine in the blood than expected.⁴⁰ With grapefruit juice, felodipine reached the blood at 284 per cent of the level reached with water; orange juice had no such effect.⁴¹

The cause is two furanocoumarins of the grapefruit, bergamottin and 6′,7′-dihydroxybergamottin. They destroy the enzyme CYP3A4 in the wall of the gut, which would otherwise break down part of each dose before it reached the blood.⁴²⁴³ The enzyme is not switched off but destroyed; the gut must make it anew.⁴⁴ A grapefruit juice from which the furanocoumarins had been removed did not affect felodipine at all.⁴⁵ More than 85 drugs are known or expected to interact with grapefruit.⁵deregulation

Ecology and evolution

Defence

Furanocoumarins protect the plants that make them. In 1978 May Berenbaum showed that xanthotoxin was toxic to a caterpillar that feeds on many kinds of plant, and less toxic without ultraviolet light, while its precursor umbelliferone was harmless. She called the plant’s conversion of the one into the other biosynthetic escape.⁴⁶ The compounds are also made in response to fungal attack, as phytoalexins.⁴⁷ Defence has a price: in wild parsnip, an induced chemical defence costs the plant photosynthesis and growth.⁴⁸

Arms race

Specialists caught up. The black swallowtail, a butterfly of the carrot family, is hardly harmed by xanthotoxin, but angelicin reduces its growth and fecundity. Berenbaum and Paul Feeny proposed in 1981 that the angular compounds, found in only a few advanced groups of the family, were a further step, an escalation in a coevolutionary arms race.⁴⁹ The swallowtail’s enzyme for breaking down furanocoumarins, CYP6B1, is switched on by xanthotoxin in its food,⁵⁰ and was later found to break down angelicin as well.⁵¹

The parsnip webworm, which feeds only on parsnips and hogweeds, keeps up its detoxifying enzymes even when starved of protein, at the expense of its growth.⁵² In wild parsnip, resistance to the webworm is inherited, but genetic constraints limit the plant’s response and hold the two in what the authors called an evolutionary stalemate.⁵³ When the webworm reached North America in the nineteenth century, the parsnips there, which had grown up without it, became more toxic.⁵⁴ The webworm also feeds on giant hogweed; in the Netherlands, moths raised on it were smaller than those raised on the native common hogweed.⁵⁵both sides pay for the arms

Several inventions

The fig makes furanocoumarins with enzymes unrelated to those of the carrot family. Its first committed enzyme evolved from a different ancestor, and its marmesin synthase is a recent invention of its own lineage. The pathway, it appears, has been acquired independently several times.²⁵⁶ Within the carrot family, the angular compounds appear to have evolved after the linear ones.¹⁶

See also

• Phytophotodermatitis
• Giant hogweed § Health hazards
• Enemy release hypothesis § Defence
• Heracleum § Chemistry