Verbascoside, or β-(3′,4′-dihydroxy-phenyl)-ethyl-O-α-L-rhamnopyranosyl (1->3)-β-D-(4-O-caffeoyl)-glucopyranoside (), also called “Labiateae tannin”, belongs to the phenylpropanoid glycosides. These secondary metabolites, along with other compounds like flavonoids are known to enter as substrates for the enzymes causing browning of damaged plant cells or cells of some species when cultivated in vitro ().
Verbascoside is the most widespread of the disaccharide caffeoyl esters (). It is also known as acteoside () and kusaginin (). Moreover, as was shown by Andary and Ibrahim () with labeling experiments of heterosidic caffeoyl esters biosynthesized by plantlets of Stachys albens (Lamiaceae), trisac-charide caffeoyl esters are formed from verbascoside, constituting a further or final step in the biosynthetic pathway of the heterosidic caffeoyl esters. These compounds, as well as verbascoside, can be employed as taxonomic markers in Plantaginaceae and Lamiaceae ().
Verbascoside was originally isolated from Verbascum sinuatum (), but its chemical structure was exactly defined by Andary et al. in 1982. Actually, verbascoside was said to be present in several families of three orders of the Asteridae (): Verbenaceae () and Lamiaceae of Lamiales (); Plantaginaceae of Plantaginales (); and Oleaceae (); Scrophulariaceae (), Myoporaceae (), Orobanchaceae (), Gesneriaceae (), and Acan-thaceae (), Pedaliaceae () of Scrophulariales.
Verbascoside and derivatives (forsythiaside, suspensaside, etc.) showed antimicrobial properties () and were described to be in vitro inhibitors of 3′,5’AMP phosphodiesterase and 5-lipoxygenase ().
Distribution and Importance of the Plant
Hygrophila erecta (Burm. fil.) Hochr. () is a widespread plant in Indo-malaysia belonging to the family Acanthaceae. The genus Hygrophila comprises about 100 tropical species found in wet areas. Some of them are cultivated as an ornamental aquatic plant in aquaria. Others are weeds found in the rice fields from India to Thailand as Hygrophila difformis (L. f.) Blume ().
Hygrophila erecta is a perennial herb whose seeds are used by the cosmetic and pharmaceutical industry in central Europe as a skin-regenerating product. As the cells of the central nervous system have the same embryological origin as the skin cells, one of us (M.H.) requested the collaboration of a laboratory specialized in neuronal cultures (Centre de Neurochimie of Strasbourg) to research any effect on neuronal cells. Lipidic extracts showed a striking effect on the morphological differentiation of cerebral neurons of 13-day-old rat embryos in in vitro cultures (). A long-chain n-alcohol (with 26 carbon atoms), n-hexacosanol was the substance responsible for the growth effect of the glyal and neuronal cells in cultures. It seemed surprising that such a simple long-chain n-alcohol might display such activity, but previously another simple, long-chain fatty alcohol containing 30 carbon atoms, the triacontanol, was described to be a growth promotor in the plant kingdom (). We therefore became interested in determining whether Hygrophila erecta tissue cultures could be induced to produce n-hexacosanol. Unfortunately, at present we have not succeeded in producing in vitro n-hexacosanol. During the chemical and pharmacological studies on the crude extracts of the plant and the tissue cultures, verbascoside was detected as a major extractable metabolite; we report here its production by callus and cell suspension cultures.
The demand of Hygrophila erecta in the world market is not possible to estimate because of the confusion in the industrial cosmetic market between the seeds of Hygrophila erecta and seeds of other species such as Centella asiatica employed for the same biological effect.
Botanical confusion in the identification of the seeds of different species used by the cosmetic industry promoted research on Hygrophila erecta. Pharmacological studies on lipid extracts in the plant showed a growth factor of neuronal cells in cultures, n-hexacosanol. Up to the present, in vitro cell cultures of Hygrophila erecta have failed to produce n-hexacosanol. Despite this failure, verbascoside, a disaccharide caffeoyl ester, was detected in great amounts in the biomass of the cell suspension. Such results gave rise to the increased interest in the in vitro cultures of this plant which can be considered a substantial source of verbascoside, as well as in other species of the three orders of Asteridae where this compound is mainly encountered.
Selections from the book: “Medicinal and Aromatic Plants IV”, 1993.