Effects of phenylpropanoid and iridoid glycosides on free radical-induced impairment of endothelium-dependent relaxation in rat aortic rings☆
Introduction
The genus Phlomis L. (Labiatae) is represented by 34 species in Turkish flora (Davis, 1982). Some of the Phlomis species are used as tonic and stimulant in Anatolia (Baytop, 1984). We have previously reported the isolation and the structure elucidation of three phenylpropanoid glycosides forsythoside B, alyssonoside, leucosceptoside B and the iridoid glycoside lamiide (Saracoglu et al., 1997a) along with a new phenylpropanoid glycoside, hattushoside (Saracoglu et al., 1998) in the aerial parts of Phlomis pungens var. pungens.
Phenylpropanoids are glycosides of phenylethanol, esterified by a cinnamic acid (e.g. caffeic, ferulic and p-coumaric acid) molecule. Phenylpropanoid glycosides are known to possess antibacterial and antifungal activity. The inhibitory effect of phenylpropanoids on the enzymes 5-lipoxygenase, cyclic AMP phosphodiesterase, aldose reductase and protein kinase C (Çaliş et al., 1988, Andary, 1993) has been reported. The immunosuppressive activity of phenylpropanoid glycosides has also been shown (Sasaki et al., 1989). Furthermore, in our previous studies, we have demonstrated the cytotoxic and cytostatic activities of some phenylpropanoid glycosides (Saracoglu et al., 1995, Saracoglu et al., 1997b) and reported that they may induce apoptosis (Inoue et al., 1998).
Iridoids are glycosides that contain cyclopentane and pyrane rings. Iridoid glycosides possess diverse biological activities including choleretic, purgative, liver protective, vasoconstrictor, antimicrobial, analgesic, antitumor, sedative and anti-inflammatory activities (Sticher, 1977, Hölzl, 1997).
Free radicals such as hydrogen peroxide, superoxide anion and hydroxyl radical, are highly reactive species that can interact with biologically active substances. Thus, they may play an important role in several pathological states and the vascular system, particularly the endothelium is prone to damage by free radicals (Lancaster, 1992). Several active compounds in plants such as carotenoids and flavonoids were found to possess antioxidant/free radical scavenging activity. In the present study, we have investigated the possible free radical scavenging property of phenylpropanoid and iridoid glycosides isolated from the aerial parts of P. pungens var. pungens. For this purpose, the effects of aqueous extract, phenylpropanoid and iridoid fractions and the major components of these two fractions (forsythoside B, alyssonoside and lamiide) (Fig. 1) against free radical-induced impairment of endothelium-dependent relaxation in isolated rat aorta were studied. Free radicals were generated by the electrolysis of the physiological buffer in which the tissues were bathed (Jackson et al., 1986a, Jackson et al., 1986b, Gumusel et al., 1996).
Section snippets
Plant material
P. pungens Willd. var. pungens (Labiatae) was collected from the surroundings of Corum, Bogazkale, Hattusas in Inner Anatolia (Turkey). The plant was identified by Professor Dr Iclal Saracoglu, Department of Pharmacognosy, Hacettepe University. A voucher specimen has been deposited in the Herbarium of Faculty of Pharmacy, Hacettepe University, Ankara, Turkey (HUEF 91020).
Extraction and isolation
Air-dried aerial parts of the plant were extracted twice with MeOH at 40 °C (×2, 3.5). The combined extracts were evaporated
Results
Acetylcholine (10−8–10−5 M) induced concentration-dependent relaxation in aortic rings precontracted by phenylephrine. This relaxation response was abolished by endothelium denudation and significantly inhibited by the electrolysis of the bathing solution (Fig. 2). The maximal relaxation to acetylcholine in aortic rings before and after electrolysis was 77.86±3.52 (n=17) and 11.26±2.66.% (n=6), respectively.
Incubation of the aortic rings with the aqueous extract (200 μg/ml), phenylpropanoid
Discussion and conclusion
The results of the present study indicate that phenylpropanoid glycosides isolated from the aerial parts of P. pungens var. pungens, possess protective activity against electrolysis-induced impairment of endothelium-dependent relaxation in rat isolated aorta. On the other hand, iridoid glycoside lamiide was devoid of this activity.
Electrolysis of a physiological solution has been shown to generate free radicals such as superoxide radical (O2−), hydrogen peroxide (H2O2) and hydroxyl radical (OH)
Acknowledgements
This paper reports the preliminary data for the cell culture studies of natural products, supported by Turkish Planning Organization of Government (No: 97 K 121 360).
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Presented at the 46th Annual Congress of the Society for Medicinal Plant Research, Vienna, Austria, 31st August–4th September 1998.