Herbal Research

Da Huang

RADIX ET RHIZOMA RHEI (RHUBARB)

Da Huang is the dried root and rhizome of Rheumpalmatum L., Rheum tanguticum Maxim. cx Baif. or Rheum officinale Baill. (Polygonaceae). The herb from R. Palmatum and R. tanguticum is mainly produced in Qinghai, Gansu and Tibet; and that from R. officinale is produced in Sichuan. The herb is collected only from plants of over 3 years old in late autumn when the plant is withering or in the spring before sprouting. It is then freed from rootlet, decorticated, cut and dried.

CHEMISTRY

The main constituents of the herb are anthraquinone derivatives, which can be further divided into free anthraquinones, anthraquinone glycosides and bianthrones (Table 1)12.

Table 1 The anthraquinone derivatives of rhubarb.

Classification	Compound
Free anthraquinones	Chrysophanol Aloe emodin Emodin Physcion Rhein Citreorosein Alizarin Laccaic acid D
Anthraquinone glycosides	Physcion 1-O-b-D-glucopyranoside Aloe emodin 1-O-b-D-glucopyranoside Emodin 8-O-b-D-glucopyranoside Emodin 1-O-b-D-glucopyranoside Chrysophanol 1 -O-b-D-glucopyranoside (Chrysophanein) Chrysophanol 8-O-b-D-glucopyranoside Rhein l-O-b-D-glucopyranoside Aloe emodin 1'-O-b-D-glucopyranoside Emodin 3-O-b-D-glucopyranoside (Glucoemodin) Physcion 8-O-b-D-gentiohioside 1,8-dihydroxy-3-methylanthraquinone- 1-O-b-D-glucoside (Palmatin) Physcionin
Bianthrones	Chrysophanol hianthrone Aloe emodin hianthrone Sennidins A, B and C Palmidins A- C Rheidins A- C Sennosides A- F

In addition to above anthraquinones, four anthrone C,O-diglycosides named rheinosides A, B, C and D were also isolated from the herb3. Tannins are also the main constituents of rhubarb, with 11% in R. palmatum and 4-7% in R. tanguticum and R. officinale4. The tannins obtained from rhubarb represent both hydrolysable and condensed tannins1.
Besides, a number of stilbene compounds, including a free stilbene piceid as well as stilbene glycosides 4'-O-methylpiceid and rhapontin5, and stilbene gallates 3,5,4'-trihydroxystilbene 4'-O-b-D-glucopyranoside, 3,5,4'-trihydroxystilbene 4'-O-b-D-(6"-O-galloyl)-glucopyranoside and 3,5,4'-trihydroxystilbene 4'-O-b-D-(2"-O-galloyl)-gluco-pyranoside have also been found in the herb1. Chromone and chromanone derivatives including 2,5-dimethyl-7-hydroxychromone, 2-methyl-5-carboxymethyl-7-hydroxychro-mone, 2-(2-hydroxy-propyl)-5-methyl-7-hydroxy-chromone, 2-(2-hydroxypropyl)-methyl-7-hydroxy-chromanone and 3 naphthalenes were also isolated1'6.

PHARMACOLOGY

Purgative Effect

Da Huang is a well known purgative agent. The active constituents have been found to be the combined anthraquinones, especially sennosides (Table 2)7. The content of sennosides correlates with the purgative activity of rhubarb1.

Table 2 The oral purgative ED50 values of the anthraquinones in mice.

Anthraquinones ED50 (mg/kg) Anthraquinones ED50 (mg/kg)

Sennoside A 13.5 Aloe-emodin-8-glyeoside 71.6
Sennoside B 13.9 Emodin monoglucoside 103.6
Sennoside C 13.3 Aloe-emodin 59.6
Sennoside D 13.8 Rhein 97.5
Sennoside E 13.5 Emodin >500
Sennoside F 16.1 Physcion >500
Rhein-8-glucoside 20.0 Chrysophanol >500

Studies on the mechanisms of action found that sennosides act predominantly on the large intestine. The most potent purgative activity was obtained from the rhubarb extract or sennoside A by gastrical administration and from sennidin by intravenous route. Inhibition of the intestinal flora in mice with chloramphenicol significantly decreased the activity of sennoside A but did not affect the activity of sennidin. It is therefore believed that the prosthetic sugar group of the anthraquinone glycosides prevented the anthrone from being oxidized before they are transported into the large intestine and hydrolysed by the bacterial enzyme into free sennidins8. It was found that sennosides are hydrolysed by microbial b-glycosidase in a stepwise fashion to the corresponding sennidins via 8-monoglycosides. The resulting metabolites sennidins were further reduced, possibly by a reductase bound to cell membranes of intestinal bacteria, to rheinanthrone as the purgative principle9'10.
Ligation of the junction of the large and small intestines failed to prevent anthraquinone glycoside producing a purgative effect in the large intestine. Oral administration of rhubarb started to produce effect 6-8 h later. These results suggest that there is also a large part of anthraquinone glycosides absorbed in the small intestine and transformed in the liver before they act on the pelvic plexus and produce peristalsis and purgation8.
On the other hand, small doses (0.05-0.3 g by oral administration) rhubarb caused constipation because of its high content of tannins. The constipation effect can be prevented by decocting rhubarb together with Huang Lian (Rhizoma Coptidis). This is because the tannins and berberine, the main constituent of Huang Lian, form gelatinous precipitates during decoction8.

Antimicrobial Effect

Tested by mixing virus with dilutions of aloe emodin for 15 mm at 370C, herpes simplex virus type 2 and type 3, varicella-zoster virus, pseudorabies virus, influenza virus were inactivated. Electron microscopic examination of the virus demonstrated that the envelopes were partially disrupted, indicating that it is directly virucidal to enveloped viruses ~ Emodin and rhein showed antiviral activity against human cytomegalovirus (HCMV) strain AD-169. When tested against a ganciclovir-resistant strain of HCMV, the EC50 value for rhein was superior to the value obtained for the AD-169 strain12. The aqueous extract of R. palmatum inhibited hepatitis B virus (HBV) polymerase activity and to bind hepatitis B virus surface antigen (HBsAg). Intravenous dose of 50 mg/kg of the extract to duck hepatitis B virus (DHBV) carrier ducklings showed antiviral activity against DHBV using serum DHBV DNA level and DHBV DNA polymerase activity as antiviral indicators13.
Rhubarb exhibited inhibition against staphylococci, Streptococcus hemolyticus, Corynebacterium diphtheriae, Bacillus subtilis, B. brucellosis, B. mycoides, B. smegatis, Mycobacterium graminis, Yersenia pestis, Salmonella tophi, S. paratyphi, Shigella dysenteriae and Neisseria gonorrhea. Staphylococci and Neisseria gonorrhea were most sensitive to the herb. The main antibacterial components were the anthraquinone derivatives with the structure of 1,9-dihydroxyanthraquinone. 3-Carboxyrhein, hydroxyaloe-emodin and hydroxyemodin showed the most potent antibacterial activity. The bacteriostatic concentrations of rhein, emodin and aloe-emodin against staphylococci, streptomyci, Corynebacterium diphtheria, Bacillus subtilis, B. anthracis and Salmonella tophi were 1.5-25 lg/ml. The antibacterial action is believed to be the result of inhibition of the mitochondrial respiratory chain of microorganisms. Respiration of Staphylococcus aureus was strongly inhibited by emodin, aloe-emodin and rhein'4. Rhein, emodin and rhein specifically interfered with the redox function of NADH dehydrogenase15,16.
The aqueous, ethanolic and ether extracts of rhubarb are also antifungal against many pathogenic fungi, including Achorion schoenleini, Trichopphyton concentricum, T violaceum, T gypsum, Nocardia astero ides, Epidermophytonflocosum and Sporotrichum schenckii. The decoction of rhubarb exhibited inhibition against the influenza virus. The minimal effective dose in chicken embryo in vitro and semi in vivo was 5 mg per embryo8.

Antineoplastic and Antimutagenic Effects

Intraperitoneal administration of 75 mg/kg of emodin produced a 45% inhibition against the mammary carcinoma of mice. The inhibition rates of 5 mg/kg of rhein and emodin against murine melanoma were 76% and 73%. Rhein, emodin and aloe-emodin inhibited murine leukemia P388 in vivo, increasing the survival time and decreasing the ascites volume17. They also inhibited Ehrlich ascites carcinoma. Emodin was a strong inhibitor of respiration in Ehrlich ascites carcinoma cells, with an ED50 of 20 lg/ml18. Cellular respiration in leukemia L1210 cells was also inhibited19. Palmatin, crysophanein and physcionin also exhibited moderate cytotoxic activity against several types of carcinoma cells2.
The extract of the herb from R. palmatum and emodin induced a dose-dependent decrease in the mutagenicity of benzo(a)pyrene [B(a)P], 2-amino-3-methylimidazo(4,5-f) quinoline (IQ) and 3-amino-l-methyl-5H-pyrido(4,3-b)indole (trp-P-2) in Salmonella typhimurium TA98. It was further found that emodin reduced the mutagencity of IQ by direct inhibition of the hepatic microsomal activation and not by interaction with proximate metabolites of IQ and/or by modification of DNA repair processes in the bacterial cell20. Emodin also markedly decreased the mutagenicity of 1-nitropyrene (1-NP) in a dose-dependent manner inAmes/microsomal test with S. typhimurium TA98 and the genotoxicity in SOS chromotest with E. coli PQ37. Furthermore, emodin significantly inhibited the formation of 1-NP DNA adducts in S. typhimurium TA98. The results suggest that emodin acts as a blocking and/or suppressing agent to reduce the direct-acting mutagenicity of I-NP21.

Hemostatic Effect

Rhubarb is also used in TCM as a hemastatic agent. The hemastatic activity has been proved experimentally and clinically22'23. Rhubarb is effective for both external and internal hemorrhage. It was effective in the treatment and prevention of experimental gastric bleeding and ulcer formation in rats24. Significant therapeutic effects of the powdered rhizome of R. palmatum in the treatment of gastrointestinal bleeding were also reported25. It reduces coagulation time and the permeability and fragility of capillaries. It increases fibrinogen and promotes bone marrow to produce platelets7'8'26.

Immunosuppressive Effect

Emodin at 3 x 10-7-3 x 10-4 M dose-dependently suppressed the responses of human mononuclear cells to phytohemagglutinin and mixed lymphocyte reaction. It was further found that after exposure to emodin (10-6 M) the production of interleukin- 1 (IL-1) and interleukin-2 (IL-2) and the expression of 11-2 receptor were all decreased. Emodin may be a new template for the development of better immunosuppressive agents for use against transplantation and autoimmune disease27'28.

Choleretic Effect

Rhubarb can stimulate construction of the gallbladder and relax Oddi's sphinctercan, thus promoting bile secretion. It also increases the contents of bilirubin and bile acid7'8.

Other Effects

Oral administration of emodin and rhein provoked marked diuretic, natriuretic and kaliuretic effects in rabbits29. Oral administration of rhubarb increased urinary excretion of sodium and potassium, alkalizing urine to a pH value as high as 8.48. Rhubarb also inhibits the activities of pepsase, trypsase, pancreatic arnylase and pancreatic lipase. It lowers blood pressure and blood cholesterol levels25. Rabbits with fever induced by subcutaneous injection of pneumococci responded with reduced temperature after oral administration of the decoction of rhubarb30.
Intraperitoneal administration of 15 mg/kg of emodin exhibited antiinflamatory activity against carrageenin-induced pedal inflammation in rats. In the same dosage it also showed antiulcerative activity against pylorus-ligated, aspirin and immobilization stress-induced gastric ulcers in rats. It decreased acid and pepsin output and augmented mucus secretion in terms of total carbohydrate:protein ratio in the gastric juice of aspirin treated pylorusligated rats, indicating that the antiulcerative effect of emodin may be due to this effect on gastric secretion31.

FUNCTIONS AND APPLICATIONS

Traditional Description

Da Huang has a bitter taste and a cold property, acting on the spleen, stomach, large intestine, liver and heart channels. It has the functions of:

(i) purging heat and loosening the bowels, used for retention of the feces and abdominal pain, fever with constipation and dysentery with inadequate discharge of the bowels;
(ii) reducing heat in the blood and counteracting toxicity, used for hematemesis and epistaxis, inflammation of the eyes and swelling of the throat and gum due to heat in the blood; jaundice caused by damp-heat. Externally used for scalds and burns; and
(iii) eliminating blood stasis and stimulating menstrual discharge, used for amenorrhea due to blood stasis, traumatic injuries, hemorrhage from the upper gastrointestinal tract, appendicitis with abdominal pain, boils, sores and abscess.

Applications
Hemorrhage and thrombocytopenia

The herb had a significant hemostatic effect in external and internal bleeding. Two to 4 g of the pills of the herb were effective in profuse hemoptysis and chronic mild hemoptysis. Used in 1000 cases of acute hemorrhage of upper digestive tract, it was effective in 97% of the cases. Bleeding stopped in 2 days on average. Chrysophanol was used in the treatment of menorrhagia, bleeding following abortion, epistaxis, functional uterine bleeding and thrombocytopenia.

Skin diseases

The total anthraquinones were used in neurodermatitis, seborrheic dermatitis and contact dermatitis. The herb was also effective in impetigo contagiosa, eczema and leg ulcer.

Hyperlipemia

The capsules of the powdered herb was effective in 78% of the 30 cases tested.

Infectious diseases

The herb can be used in acute bacillary dysentery and acute infectious hepatitis.

SIDE EFFECTS AND TOXICITY

Da Huang has low toxicity at therapeutic doses. However, overdosage may cause nausea, vomiting, vertigo or stomach ache. Chronic application (3-9 months in rats) resulted in pathological changes in the liver, thyroid and stomach with hypertrophy of the tissue cells. Anthraquinones can be excreted through lactation and can cause diarrhea in lactants6'7.
The oral LD50 values of the anthraquinones in mice were 0.56 glkg for emodin, 1.15 g/kg for physcion and 10 glkg for chrysophanol7.

DOSAGE

3-30g.

REFERENCES

1. Tang, W., Eisenbrand, U. (1992) Chinese Drugs of Plant Origin - Chemistry, pharmacology and use in traditional and modern medicine, pp. 855-875. Berlin, Heidelberg: Springer-Verlag.
2. Kubo, I., Murai, Y., Soediro, I., Soetamo, S., Sastrodihardjo, 5. (1992) Cytotoxic anthraquinone from Rheum palmatm. Phytochemistry, 31, 1063-1065.
3. Yamagishi, T., Nishizawa, M., Ikura, M., Hikichi, K., Nonaka, G., Nishioka, I. (1987) Studies on rhubarb (Rhei rhizoma). XI. New laxative constituents of rhubarb, isolation and characterization of rheinosides A, B, C and D. Chemical and Pharmaceutical Bulletin, 35, 3 132-3138.
4. Luo, WY, Zhang, Y.Z. (1986) Studies on the determination of tannin in Dahuang. I. Casein method. Chinese Journal of Pharmaceutical Analysis, 6, 1548.
5. Kuhn, I., Murai, Y. (1991) Efficient isolation of glycosidase inhibitory stilbene glycosides from Rheum palmatum. Journal of Natural Products, 54, 1115-1118.
6. Zhou, R.H. (1993) The Resource Sciences of Chinese Medicinal Materials, pp. 255-264. Beijing: China Medical & Pharmaceutical Sciences Press.
7. Wang, J.M. (1985) Chinese Herbal Pharmacology, pp. 43-45. Shanghai: Shanghai Science & Technology Publisher.
8. Wang, Y.S. (1983) Pharmacology andApplications of Chinese Materia Medico, pp. 67-68. Beijing: People's Health Publisher.
9. Kobashi, K., Nishimura, T., Kusaka, M., Hattori, M., Namba, T. (1980) Metabolism of sennosides by human intestinal bacteria. Planta Medica, 40, 225-236.
10. Honori, M., Kim, G., Motoike, S., Kobashi, K., Namba, T. (1982) Metabolism of sennosides by intestinal flora. Chemical and Pharmaceutical Bulletin, 30, 1338-1346.
11. Sydiskis, R.J., Owen, DO., Lohr, IL., Rosier, K.H., Blomster, RN. (1991) Inactivation of enveloped viruses by anthraquinones extracted from plants. Antimicrobial Agents and Chemotherapy, 35, 2463-2466.
12. Barnard, DL., Huffman, J.H., Morris, J.L.B., Wood, 5G., Hughes, B.G., Sidwell, R.W. (1992) Evaluation of the antiviral activity of anthraquinones, anthrones and anthraquinone derivatives against human cysomegalovirus. Antiviral Research, 17, 63-78.
13. Chuang, T.H., Kim, J.C., Lee, CY., Moon, M.K., Chac, S.C., Lee, IS., et al. (1997) Potential antiviral effects of Terminalis chebula, Sanguisorba officinalis, Rubus coreanus and Rheum palmatum against duck hepatitis B virus (DHBV). Phytotherapy Research, 11, 179-182.
14. Chen, C.H., Li, T.T., Su, H.L., Wang, CI. (1963) Chinese rhubarb. VII. Mechanism of antibiotic action of anthraquinone derivatives. Effects on the respiration of Staphylococcus aureus. Acta Biochimica et Biophysica Sinica, 3, 426-433.
15. Zhang,Y., Chen, Q.H. (1986) Biochemical study of Chinese rhubarb. XIV. Inhibitory effects of anthraquinone derivatives on some NAD linked dehydrogenase. Acta Biochimica et Biophysica Sinica, 18, 239-245.
16. Chen, C.L., Chen, Q.H. (1987) Biochemical study of Dahuang. XIX. Localization of inhibition of anthraquinone derivatives on the mitochondrial respiratory chain. Acta Pharmaceutica Sinica, 22, 12-18.