Distribution and Importance of the Plant
Cornus officinalis Sieb. et Zucc. (Japanese: San-shu-yu; Cornaceae) is a deciduous medium-sized tree, brought to Japan as a medicinal plant in the eighteenth century from China via Korea, and is cultivated as a garden tree because of its yellow flower () and red fruit (). The fruit (common name: cornus fruit) has also been used as an astringent, a tonic, and a hemostatic in these countries, particularly often as one of the ingredients in traditional prescriptions such as Hachimi-gan and Rokumi-gan in Japanese (the Chinese terms are ba-wei-wan and liu-wei-wan) which are popular tonics for preventing and improving symptoms of aging, including pollakiuria and cataract.
Medicinal Components and Conventional Practices
Several noteworthy biological activities of tannins have been recently reported (), and the fruit of Cornus officinalis is one of the rich sources of hydrolyzable tannins (). Generally, the majority of tannins are classified into two large groups of polyphenolic compounds: hydrolyzable tannins, which are metabolites of gallic acid; and condensed tannins, which are also called proanthocyanidins (). The former group of compounds is again classified into several groups: gallotannins, which are galloyl esters of sugar (mostly glucose) or polyalcohol; ellagitannins, which are hexahydroxydiphenoyl esters; and oxidation products from ellagitannins, such as dehydroellagitannins with dehydrohexahydroxydiphenoyl (DHHDP) groups; and others.
“Tannic acid”, among the commercially available tannins, is generally a gallo-tannin mixture of variable composition and purity, produced from the insect gall of Rhus, Quercus, and other species of plants. Tannic acid is listed in the pharmacopoeia of many countries, based on its medicinal applicability. Berberine tannate, arbumin tannate, diphenhydramine tannate, etc. are examples of medicines prepared from tannic acid, and there are various industrial applications of tannic acid.
There are also a variety of drugs of plant origin, in which tannins are regarded as the active principles. Myrobalans (the fruit of Terminalia chebula) in Ayurveda medicine, and Geranium thunbergii in Japanese pharmacopoeia, which are both rich in ellagitannins, are examples of popular medicinal plants of this kind. Cornus officinalis also can be considered as belonging to this group.
The propagation of Cornus officinalis in Japanese horticulture is conventionally done by layering, although the most efficient way of the propagation may be sowing. Whatever the method, it takes a long time to raise adult trees which are capable of producing fruits for medicinal use, and the fruit production is reduced as the tree ages.
Gallotannin mixtures composed of tri-, tetra-, and pentagalloylglucose were produced by callus and suspension cultures of Cornus officinalis Sieb. et Zucc. The amount of 1,2,3,6-tetragalloylglucose, the main component thus produced in the cell suspension cultures, was 36 times its content in the intact fruits. 1,2,6-Tri-galloylglucose, 1,2,3,4,6-pentagalloylglucose, and 6-digalloyl-l,2,3-trigalloyl-glucose were also produced in these cultures. The ratios of the amount among these tannins did not vary much during the culture period, and were not much affected by differences in the combination of plant growth regulators in the medium. The tannin production was stimulated by BA, whereas cell growth required 2,4-D or NAA.
It is notable that almost no ellagitannin or its metabolites, which are the main constituents in the intact plant cells, were produced in the cell cultures. Biogenetically, the HHDP group in ellagitannins is regarded as being produced by the oxidative coupling between two galloyl groups in a molecule of gallotannin (). The absence of ellagitannin, in spite of the production of a large amount of gallotannins in the cultures, is therefore attributed to the lack of enzymatic activity catalyzing this oxidative coupling. These results may support the view that the galloylation of glucose is a biosynthetic step preceding the formation of the HHDP group in ellagitannin biosynthesis.
The substitution patterns in the isolated galloylglucoses are also of interest, because most of them lack the galloyl group on 0-4 of glucose. The galloyllation at 0-4, therefore, may be a step occurring at a later stage of gallotannin formation. This view supports the proposal concerning gallotannin biosynthesis in oak leaves by Cammann et al. (), based on an enzymatic study. The cell cultures established by the present work are expected to offer an enzyme source useful in the study of the biosynthetic pathways of gallotannins and ellagitannins. The Cornus cell cultures may also be useful in the practical production of gallotannins because of their high yield.
Selections from the book: “Medicinal and Aromatic Plants IV”, 1993.