MODUS OPERANDI of FUROCOUMARINS
COUMARINS and FUROCOUMARINS
First of all, a furocoumarin (aka furanocoumarin) is a coumarin, such as Umbelliferone shown, but with the addition of a fused
furan ring. This addition makes furocoumarins far more toxic than are coumarins. The furocoumarin shown is that of the simplest, Psoralen. There are many others with side groups attached, or with the furan ring attached at the lower angle as shown for Angelicin which are known as angular furocoumarins (as opposed to linear furocoumarins like Psoralen). With this in mind, the next paragraphs will show (by example with Psoralen, but in reality any furocoumarin will behave similarly) how furocoumarins can interact with DNA and cause havoc in the body.
THE FOUR NUCLEOBASES
Thymine, (not to be confused with Thiamine, the sulfur-containing Vitamin B1) is also known as 5-methyluracil is one of the four Nucleobases found in DNA. The four bases are Guanine, Cytosine, Adenine and Thymine, abbreviated to G, C, A and T in genomic representations of the DNA. Thymine, 5-MethylUracil, is shown below in red. Both Cytosine and Thymine are Pyrimidines with but a 6-membered ring, whilst both Guanine and Adenine are Purines (with fused pyrimidine and imidazole rings). In the coding part of DNA specific Purines pair up with specific Pyrimidines. Thus Cytosine is paired up (by three hydrogen bonds) with Guanine, and Adenine (with but two hydrogen bonds) with Thymine. [In RNA, another pyrimidine,
Uracil, takes the place of Thymine, and pairs with Adenine].
THYMINE and THYMINE DIMERS
Of those four, Thymine readily dimerizes under the action of ultraviolet light, either with a bridging
cyclobutane linkage, or with just a single bond between the two Thymine units. Two molecules of Thymine sited in close proximity on the DNA will even dimerize when in-situ when on the DNA, and when it does so, can cause havoc by preventing the proper reading (when constructing proteins for example) or copying of the DNA.
DNA ADDUCTS WITH FUROCOUMARINS
Furocoumarins have highly reactive double bonds on both the Furan ring and the Lactone ring which are excited by UV light. When furocoumarins enter skin cells (such as may occur when touching Hogweed or Giant Hogweed) those furanocoumarins are then activated by UV light in sunlight and they will form similar links with Thymine in DNA. The furocoumarins can bond a thymine molecule in one of two differing positions. Mono-Adduct A shows the thymine bonding to the coumarin part, whilst Mono-Adduct B shows it bonding to the furan ring of the furocoumarin. The R- shows where the Thymine bonds to the DNA molecule.
DNA Adducts are shown here. The Bi-Adduct shows it forming a bridging link between two Thymine molecules on the DNA, completely fouling the DNA tying the two strands (or two parts of the same strand) together. Naturally, this is not good news for the well being of the person exposed to furocoumarins in the environment.
Not only can furancoumarins bind to DNA through alkylation which leads to mutations, but they can also bind to proteins resulting in allergies. Both processes lead to apoptotic cell death, cause skin cancer and tumour of the kidneys in animals. In the liver Furocoumarins are metabolised to highly reactive intermediates called
epoxides which can also bind to both DNA and to proteins. High doses of furocoumarins are therefore also toxic to the inner organs as well as to skin.
FuroCoumarins can also
intercalate between the strands of DNA, meaning that they do not chemically bind to it (except perhaps by very weak hydrogen bonds) but rather they remain intact, where nevertheless they are still capable of causing so-called 'frame-shift' mutations. Other alkaloids which can intercalate between the strands of DNA include
PHARMACEUTICAL USES OF FUROCOUMARINS
Furocoumarins, for example 8-Methoxypsoralen, were used medicinally to treat psoriasis. In applying it to the skin they exploit that very same ability of furocoumarins to kill the proliferating keratocytes present in psoriasis sufferers when irradiating by UV light. However, furanocoumarins can also induce cancer, so this treatment has been superseded by safer alternative therapies.
ERRORS IN DNA and RNA CODING
Thymine seems to be the achilles heel of DNA, its propensity to glue strands of DNA together can lead to both mis-constructed proteins and to mutations. How it ever came to have such an important role taking a part in the coded of our genes is astonishing for a molecule that is apt to put a spanner in the works when the genes require decoding. Your Author thinks it possible that it is at least partly instrumental in the evolution of species and without it evolution may not have proceeded so fast. However, it turns out that much of human evolution is due to mutations in the genome that do not occur in those regions coding for the genes themselves, but rather occur in those much shorter regions encoding for promoters and enhancers of those genes. There are about 44,000 enhancers and 180,000 promoters, short regions in the genome that interact and control gene expression within particular cells the human body. Some of these promoters and enhancers control which type of cell the cell will become. Mutations in the genetic composition of these small promoters and enhancers is responsible for most human diseases that have a genetic origin, including many cancers.
However, Thymine is not the only nucleobase with a personality disorder. Cytosine itself (which pairs with Guanine in DNA - both drawn above) is apt to lose an amine group in exchange for an oxygen atom when it then becomes Uracil, the pyrimidine used in RNA but not DNA. This accident is not uncommon - occurring at a frequency of about 1%. If this occurs in DNA, then the presence of an errant Uracil nucleobase will probably be recognised by an automated repair mechanism effected by the enzyme Uracil Glycolase and corrected back to the appropriate Cytosine. If not repaired, the errant Uracil leads to a point mutation. If, however, Cytosine becomes Uracil in RNA (rather than in DNA as above), then this error cannot be corrected (since Uracil is allowed, being one of the four coding nucleobases in RNA) but it will not matter as much: RNA is comparatively short-lived and doesn't store the genome, it is the blueprint for constructing proteins.
Cytosine within DNA can also be methylated by the enzyme DNA Methyltransferase, when it then becomes 5-methylcytosine. Adenine within DNA can also be methylated (to 6-methyladenine). These methylations (and acetylations which are implemented by other mechanisms) are instrumental in epigenetic changes to the genome, affecting evolution (they can be inherited by cell division). However, most of those effects have nothing to do with furocoumarins nor Hogweed.