Isolation, Antihypertensive Activity and Structure—Activity Relationship of Flavonoids from Three Medicinal Plants.

Abstract

Eight flavonoids, namely naringenin 1, 3-β-hydroxynaringenin designated as isoaromadendrin 2, taxifolin 3, isoaromadendrin-7-O-β-D-glucopyranoside designated as isosinensin 4 have been isolated from Euphorbia cuneata Vahl; 5hydroxy-3, 4′, 7-trimethoxyflavone 5, 5-hydroxy-3, 3′, 4′, 7-tetramethoxyflavone (retusin) 6, 4′, 5-dihydroxyflavone-5-O-βD-glucopyranoside named as verbenacoside 7 are obtained from Salvia verbenaca L; and epicatchin 8 is isolated from Osyris abyssinica. Their structures have been established by spectral and chemical methods, whereupon the compounds 2, 4 and 7 are found to be new flavonoids. Compounds 1-5, 7, 8 and alcoholic extract of Salvia verbenaca have also been screened biologically for antihypertensive activity in normotensive albino rats, which showed a varied degree in the decrease of blood pressure and heart rate. The flavonoids 2, 3 and 5 are found most potent exhibiting the decrease in blood pressure 36.5, 20.0 and 30.0 (mmHg) respectively in comparison to normal values. Other compounds also exhibit a good activity. The structure activity relationship (SAR) has also been discussed. IPC: Int.Cl. 7 C 07 D 311/30 The flavonoids are considered potential for human health as well as constitute an important part of human diet. They are also considered as active principles in various medicinal plants 1 , and have been reported to possess anticancer, antioxidant, antimicrobial, antihypertensive, antiulcer, antipyretic and antidiabetic properties etc. In the present article we report the isolation of some flavonoids, three of them new, from Euphorbia cuneata, Salvia verbenaca and Osyris abyssinica, and evaluation of their antihypertensive activity by measuring the arterial blood pressure and heart rate in normotensive albino rats. Euphorbia cuneata (Euphorbiaceae), locally known as Abu-Laban is reported to be used for the treatment of ringworm, scorpion sting and snake bite by the traditional medicine practitioners of Saudi Arabia 2 . We have isolated naringenin 1, 3-βhydroxynaringenin designated as isoaromadendrin 2, taxifolin 3 and isoaromadendrin-7-O-β-D-glucopyranoside designated as isosinensin 4 from the alcoholic extract of aerial parts of the plant. Salvia verbenaca L. (Labiatae) is distributed in tropical region including Saudi Arabia. The alcoholic extract of the aerial parts has been reported to potentiate smooth muscle contractions induced by acetylcholine, histamine, BaCl2, and serotonin 3 . We have isolated 5-hydroxy-3, 4′, 7-trimethoxyflavone 5, 5-hydroxy-3,3′,4′,7-tetramethoxyflavone (retusin) 6 and 4′,5-dihydroxy flavone-5-O-β-D-glucopyranoside designated as verbenacoside 7 from the alcoholic extract of aerial parts of the plant. The plant Osyris abyssinica var. speciosa (Santalaceae) is distributed in South Africa including Saudi Arabia 4 . We have isolated epicatechin 8 from the aerial parts of the plant. The literature survey revealed that the antihypertensive activity of the above flavonoids has not yet been reported so far. We have, therefore, tested the above flavonoids and ethanolic extract of Salvia verbenaca for their antihypertensive activity, which showed a significant decrease in blood pressure and heart rate in normotensive albino rats, wherein the compounds 2, 3 and 5 were found most potent exhibiting 36.5, 20.0 and 30.0 (mmHg) decrease in blood pressure respectively. Other compounds also displayed a good decrease in blood pressure and heart rate.