Rhubarb, the rhizome and root of Rheum spp. (Polygonaceae), has been used since ancient times as an important drug in the East and West. It was described in Dioscorides’ De Materia Medica as Ra (pα), designating its native place, the Volga (Ra River) basin. It was said to be effective for disorders of stomach and intestine, as well as for pains in spleen, liver, kidney, abdomen, bladder, and chest (). In traditional Chinese medicine, rhubarb (Da-Huang in Chinese) has been used as a major component of some prescriptions for the treatment of blood stasis, in which it produces mildly purgative, antiinflammatory, and sedative effects. In western countries, rhubarb is mostly employed as a purgative drug in folk medicine.
Several Rheum species are recognized as being the original rhubarb plants, from which Rheum palmatum L., Rheum tanguticum Maxim., and Rheum officinale Bail, are recommended for medicinal use. All these species with palmately or elliptically lobed large leaves are native in cool, high-altitude districts in Tibet, Chianhai, Yunnan, and Si-Chuang Provinces, China. A Korean species, Rheum coreanum Nakai, possesses similarly shaped leaves to those of Chinese origin. The rhizome and root of Rheum sp. growing in lower lands, Rheum undulatum L., Rheum rhaponticum L., and Rh. emodi Wall., which have round-edged leaves, are inferior for medicinal use, while the stem of Rheum rhaponticum is edible ().
Rheum plants of superior quality are not native to Japan. The author introduced the seeds of these species from abroad in 1949 to cultivate at an experimental farm attached to the University of Tokyo and Shinshu University in the Nobeyama Highland (1300 m a.s.l.) in Nagano Pref, Japan. The cultivation of Rheum palmatum, Rheum tanguticum, Rheum officinale, and Rheum coreanum was successful in adapting them to the climate and soil conditions there ().
A hybrid of Rheum palmatum and Rheum coreanum was produced from the strains grown on the Nobeyama farm by the research laboratory of Takeda Chemical Industry Co., Ltd. to cultivate on a large scale at Ikeda-machi, Hokkaido, the northern most island of Japan, where the temperature in mid-summer is cool enough to suit the practical cultivation of the plant ().
Purgative Principles of Rhubarb
Rhubarb is commonly used as a purgative, and the anthraquinones and their glycosides, which are usually contained in various species of rhubarb, have been considered as the active principles. Sennosides which were considered by Stoll et al. () as the purgative principles of Senna leaves were also isolated from rhubarb by Miyamoto et al. () and Oshio et al. (). Sennosides A-F are bianthrone glucosides consisting of 2 mol of rheinanthrone or rhein- and aloe-emodin-anthrone. Recently, rheinosides A-D, which are monomeric rheinanthrone glucosyl derivatives, have also been isolated as the purgative principles ().
Other Constituents of Rhubarb
Several other types of compounds have been isolated from several species of rhubarb. Until recently rhaponticin-type stilbenes had been believed to be the constituents of inferior types of rhubarb, but now they have been found more or less in every type of rhubarb, and perhaps will prove to be the treatment for stasis of blood in Chinese medicine ().
The naphthalene derivatives, torachrysone 8-O-glucoside and 8-O(6′-oxalyl) glucoside, and 6-hydroxymusizin 8-O-glucoside were found in several types of rhubarb. Torachrysone was first isolated from Cassia tora seeds (Leguminosae). Eight kinds of chromone derivatives have been isolated from Mati-dahuang (Rheum officinale ?), but a very minute amount in Ya-huang (another type of Rheum officinale).
Phenylbutanone derivatives have recently been found in rhubarb. Lindleyin, which was first isolated from Aconium lindleyi (Crassulaceae) growing in the Canary Islands, has been isolated from rhubarb with a maximum yield of 3%. It shows fairly strong antiinflammatory activity like aspirin and phenylbutazone ().
Rhubarb contains a large amount of nonhydrolyzable catechin-tannins as well as hydrolyzable tannins. The structures of these tannins have been described by Nonaka and Nishioka () and Nonaka et al. (), and highly polymerized rhatannin I and II were characterized as suppressing the increase on blood urea nitrogen (BUN), which appears in chronic renal disorders. Rhatannins are polymerized galloylepicatechins as formulated below:
Mechanism of Purgative Action of Rhubarb
The multiple functions of rhubarb have thus been explained by the presence of various types of compounds, but the purgative effect is still recognized as the major function of rhubarb of higher quality. The oral administration of sennosides A and C is effective in producing a purgative action, whereas the intravenous injection of sennosides is ineffective. Therefore, it is obvious that sennosides have no direct purgative action, but are made effective by microbial conversion into rheinan-throne in the large intestine. However, the oral administration of rheinanthrone is not effective due to its instability in the digestive organ.
Pretreatment with chloramphenicol decreased the purgative effect of sennosides A and C by the suppression of intestinal microbe activity. The purgative action of rheinanthrone is related to the promotion of biosynthesis and release of prosta-glandin E2, because the inhibitors of prostaglandin biosynthesis, indomethacin, SCI9220, and PPP (polyphloretin phosphate), suppress the purgative activity of rheinanthrone (). The anthra-quinones coexisting in rhubarb which have antimicrobial action may control the activity of intestinal microbes, resulting in a mild purgative effect.
Callus cultivation of rhubarb has been investigated to find the optimal conditions for the production of purgative principles, i.e., sennosides.
A stable hybrid of Rheum palmatum and Rheum coreanum was employed as the explant which induced callus of the best quality. The callus formed in 100% of the cut seedlings on the Murashige-Skoog (MS) medium containing sucrose, 2,4-D, and kinetin in the dark at 21 °C.
The cultivation of the selected callus in the MS medium containing maltose (20 g/l), IAA (1 ppm), and 4PU-30 (1 ppm) in the dark resulted in the best production of sennosides A and B.
Selections from the book: “Medicinal and Aromatic Plants IV”, 1993.