Botany and Phytochemistry of Silene alba
Silene alba (Miller) E.H.L. Krause, also known as Lychnis dioica L. or Melandrium album (Miller) Garcke, of the family Caryophyllaceae () has many common names: compagnon blanc, robinet, floquet, saponaire blanche, oeil de Dieu, bourbonnaise (in French); white campion, white bachelor’s buttons, white bottle, white cuckoo flower (in English); weipe Lichtnelke, Sommetrose, Wiederstock, Junggesellenknopfe, Je-langer-je-freundlicher (in German); fischi da fischiare, lichnide, gittone bianco, erba nocca, violina di macchia (in Italian) (Bonnier 1911-1935). It is a dioecious herb of varied habit, usually growing as a weed on plowed or cultivated ground throughout the whole of Europe.
A short-lived perennial (sometimes annual) growing up to 80 cm in height, Silene alba usually has rather thick and soft hairs, usually glandular on the upper epidermis. The leaves are ovate or ovate-lanceolate, the cauline is sessile. The inflorescence consists of loosely bound dichasium of large flowers, which open in the evening and are slightly scented. The flowers are pentamerous. The limb of the petal is distinct from the claw, with ten stamens. The calyx of the male flowers is 15-22 mm in length, with ten veins, while that of the female is 20-30 mm, 20-veined, usually inflated and acrescent in the fruit. The calyx teeth are narrowly triangular. Petals are usually white with five styles. The capsule is 10-15 mm in length, usually ovoid, and dehiscing, with ten teeth.
The plants produce anthocyanins but not betalains, and are not tanniferous, lacking both proanthocyanins and ellagic acid, although commonly accumulating triterpenoid saponins.
Silene alba, one of the most widespread of the European species, belongs to a family that is unique in its chemical composition: it contains triterpenic saponins, but no biosynthesizing alkaloids.
We studied the bio transformations of papaverine and other 1-benzylisoquino-lines in Silene alba cell cultures. The choice of this species was justified by the fact that Silene alba, like other Caryophyllaceae, does not biosynthesize alkaloids. Consequently, it is possible without great difficulty to characterize and isolate all the molecules of the basic structure, such as substrates and their biotransformation products. It was also of interest to ascertain the reaction of this type of plant cell toward alkaloid xenobiotics, and as a result, if biotransformation takes place, to determine the type of reactions which occur. Finally, the choice of benzylisoquino-line molecules as substrates was interesting due to the diversity and number that were expected in this type of structures: oxidations (benzyl group, phenolic ethers, aromatic carbons), N-oxidation, reductions, glycosylations after O-dealkylation or O-oxidation, etc. It was also of interest in 1-benzyl and 4-benzyl derivatives to compare the importance of the isomerism of benzyl group, and also the influence of the stericity of this same group with the respective accessibility of the 6- and 7-methoxy groups if the O-dealkylation reaction occurs.
Summary and Conclusions
We have noted that for Silene alba cells, there are great differences in the toxicity of the alkaloid substrates, although they differ only minimally in their structure: isomerization of the benzyl group of position 1 to position 4, and replacement of a dimethoxy group by a chlorine atom. In this respect, it should be noted that in animals, the toxicity of chlorobenzylisoquinoline 3 is less than papaverine 1 and isopapaverine 2 (). The lower toxicity of isopapaverine than its chloro-derivative analog in Silene alba cells shows that a detoxification process intervenes at the cellular level (). This is also valid for the N-oxides and benzoylisoquinolines metabolites ().
For oxidation reactions in biotransformation we observed that with papaverine and isopapaverine the major metabolites formed are O-demethylation derivatives. They are accompanied by N-oxides and benzoyl substrate derivatives. The benzyl secondary alcohol, papaverinol or isopapaverinol, was never evident as it was formed as a metabolite product in only one strain (Thevetia nereifolia) of the 19 strains that were chemically studied (), as was also the case with Glycyrrhiza glabra cells (). Other strains of various species produced N-oxide and benzoyl derivatives of papaverine (), but to our knowledge, only Silene alba cells give O-demethylation metabolites with papaverine (and isopapaverine). These last reactions are analogous to those observed for papaverine and also to certain microorganisms (), hepatic microsomia (), and in animal and human metabolism ().
For the monohydroxylated metabolites, it can be stated that the accessibility of the 3′- and 4′-methoxy groups to enzymes is similar in papaverine 1 and isopapaverine 2. In contrast, demethylation of the C-7 methoxy group of isopapaverine occurs predominantly, whereas in the case of papaverine it is the C-6 methoxy group. This fact can be explained by the conformation of the benzyl group in these two alkaloid molecules (). Thus, the possibility of rotation of a benzyl group around the isoquinoleic skeleton allows one to consider two conformers for papaverine and isopapaverine (). In the one, the benzyl group is thrown to the nitrogen nucleus and the two methoxy in 6 and 7 are accessible to oxidative enzymes; in the other, the 7-methoxy in papaverine and the 6-methoxy in isopapaverine are hindered by benzyl at C-l or at C-4 and are less accessible to oxidative enzymes.
The results obtained with papaverine and isopapaverine show that the oxidative enzymatic systems are not specific to a benzylisoquinolic molecular structure.
For 4-(4′-chlorobenzyl)isoquinoline 3 and its biotransformation, a few examples of aromatic nucleus deshalogenation have been reported in animals and plants (), and it appears to be probably the first example of a substrate deshalogenation by plant cells in vitro, this reaction being coupled with ort/io-dimethoxylation of the aromatic nucleus. A mechanism was proposed which involves the formation of a transitory epoxide 14, which leads either to an ort/io-diphenol methylated in isopapaverine, or to a phenol consecutively methylated in isopapaverine 2 ().
The prospects offered by Silene alba cell cultures are interesting for the possible O-dealkylation of various alkaloid substrates, this reaction being possibly regio-and/or stereo-specific, according to the nature and configuration of the molecules studied.
Selections from the book: “Medicinal and Aromatic Plants IV”, 1993.