PHYTOCHEMISTRY Phytochemistry 68 (2007) 1179–1188 www.elsevier.com/locate/phytochem Structure–activity relationships of flavonoids for vascular relaxation in porcine coronary artery Y.C. Xu a, S.W.S. Leung a, D.K.Y. Yeung a, L.H. Hu b, G.H. Chen b, C.M. Che b,c, R.Y.K. Man a,c,* a Department of Pharmacology, Faculty of Medicine, The University of Hong Kong Pokfulam Road, Hong Kong SAR, PR China b Department of Chemistry, Faculty of Science, The University of Hong Kong Pokfulam Road, Hong Kong SAR, PR China c Open Laboratory of Chemical Biology of the Institute of Molecular Technology for Drug Discovery and Synthesis, The University of Hong Kong Pokfulam Road, Hong Kong SAR, PR China Received 11 July 2006; received in revised form 20 November 2006 Available online 28 March 2007 Abstract Flavonoids are polyphenolic compounds that are widespread in the plant kingdom, and structure–activity relationships (SAR) for vascular relaxation effects were examined for 17 of them using porcine coronary arteries. Density functional theory was employed to calculate the chemical parameters of these compounds. The order of potency for vascular relaxation was as follows: flavones (apigenin and luteolin) P flavonols (kaempferol and quercetin) > isoflavones (genistein and daidzein) > flavanon(ol)es (naringenin) > chalcones (phloretin) > anthocyanidins (pelargonidin) > flavan(ol)es ((+)-catechin and (À)-epicatechin). SAR analysis revealed that for good relax- ation activity, the 5-OH, 7-OH, 40-OH, C2@C3 and C4@O functionalities were essential. Comparison of rutin with quercetin, genistin with genistein, and puerarin with daidzein demonstrated that the presence of a glycosylation group greatly reduced relaxation effect. Total energy and molecular volume were also predictive of their relaxation activities. Our findings indicated that the most effective relax- ing agents are apigenin, luteolin, kaempferol and genistein. These flavonoids possess the key chemical structures demonstrated in our SAR analysis. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Flavonoid; Structure–activity relationship; Chemical parameter; Relaxation activity; Porcine coronary artery 1. Introduction focused on their cardiovascular effects (Vitor et al., 2004; Stocker and O’Halloran, 2004). Epidemiological reports Flavonoids are polyphenolic compounds that are wide- have demonstrated that people can have lower incidence spread in the plant kingdom, and they are important of heart diseases if they have a high dietary intake of flavo- components of many traditional Chinese medicine and noids (Knekt et al., 2002; Jenkins et al., 2003) and this may phyto-medicine. They have diverse pharmacological effects, help explain the lower mortality of coronary heart diseases such as anticancer, antioxidant, anti-aging, and antibacterial in some Asian countries (Lock et al., 1988). Furthermore, properties (Terao et al., 1994; Bos et al., 1996; Guo et al., other studies have demonstrated that some flavonoids pro- 1999; Meng and Wang, 2001). Recently many studies have duced concentration-dependent relaxation responses in contracted arterial rings (Guerrero et al., 2002; Fusi et al., 2003). These relaxations are in part mediated by stimulation of nitric oxide release from the endothelium * Corresponding author. Present address: 2/F Laboratory Block, Faculty of Medicine Complex, 21 Sassoon Road, Hong Kong SAR, (Kim et al., 2000; Mishra et al., 2000; Zhang et al., China. Tel.: +852 2819 9250; fax: +852 2817 0859. 2002). However, the majority of the relaxation is attributed E-mail address: [email protected] (R.Y.K. Man). to direct action of the flavonoids on the vascular smooth 0031-9422/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.phytochem.2007.02.013 1180 Y.C. Xu et al. / Phytochemistry 68 (2007) 1179–1188 muscle (Mishra et al., 2000; Zhang et al., 2002; Ajar et al., 1995; Vaya and Tamir, 2004). Certain chemical parameters 2003). may be responsible for their molecular interactions (Cam- Since different flavonoids have different relaxation argo et al., 2002). Therefore, SAR studies using quantum effects, the structure–activity relationships (SAR) of flavo- chemical parameters have become important in qualitative noids have become the subject of many investigations (Hii- and quantitative analyses of three-dimensional molecular pakka et al., 2002; Taubert et al., 2002; Chen et al., 2003; interactions (Dewar et al., 1985; Souza et al., 2003). How- van Zanden et al., 2004). For an understanding of the ever, few investigations have examined the relationships three-dimensional microscopic interaction and binding between chemical parameters and vascular relaxation of between a ligand and a receptor, detailed analysis in SAR flavonoids. The present study focuses on SAR analysis of is important in drug design and synthesis (Costa et al., 17 natural compounds (1–17) using structural parameters OH OH OH HO HO O O (Z) (Z) OH O OH O Apigenin (1) Luteolin (2) OH OH OH HO O HO O (E) (E) OH OH OH O OH O Kaempferol (3) Quercetin (4) OH OH HO O OH (E) OH OH HO O O OH OH O OH HO O CH2 (Z) O H3C HO O OH OH OH O HO Myricetin (5) Rutin (6) OH OH OH HO O HO O OH OH O OH O Naringenin (7) Taxifolin (8) Fig. 1. Molecular structures of the 17 flavonoids* studied. *The numbering system for these flavonoids is as follows: Y.C. Xu et al. / Phytochemistry 68 (2007) 1179–1188 1181 OH OH OH OH H HO H O HO O (R) (R) H (S) (R) OH OH H OH OH (+)-Catechin (9) (-)-Epicatechin (10) OH (Z) OH HO + O HO OH (Z) OH OH OH O Phloretin (11) Pelargonidin (12) HOH2C O HO O HO O O HO OH H2O (Z) (Z) OH O OH O OH OH Genistein (13) Genistin (14) HO HO O OH CH2OH HO O HO O (Z) (Z) O O OH OH Puerarin (15) Daidzein (16) HO O (Z) OH3C O OH Glycetein (17) 3' 2' 4' 8 1 B O 2 1' 5' 7 A C 6' 6 3 5 4 Fig. 1 (continued) 1182 Table 1 Names and classes of natural flavonoids in this study, and relaxation properties Number Class Chemical name Compound name % Relation Relaxation effect 30 lM 100 lM 1 Flavones 40,5,7-trihydroxy flavone Apigenin 114.5 ± 3.5* 118.5 ± 3.9* Good 230,40,5,7-tetrahydroxy flavone Luteolin 107.6 ± 2.7* 114.2 ± 2.6* Good 3 Flavonols 3,40,5,7- tetrahydroxy flavone Kaempferol 108.2 ± 2.7* 116.4 ± 2.3* Good 4 3,30,40,5,7- pentahydroxy flavone Quercetin 20.6 ± 3.8 116.7 ± 4.0* Moderate 5 3,30,40,5,50,7- hexahydroxy flavone Myricetin 22.4 ± 3.1 55.8 ± 6.9* Weak 630,40,5,7-tetrahydroxy-3-O-rutinose flavone Rutin 29.7 ± 3.5 73.6 ± 6.9* Weak 7 Flavanon(ol)es 4,5,7- trihydroxy flavanone Naringenin 75.2 ± 4.4* 108.4 ± 1.2* Moderate 8 3,30,40,5,7- pentahydroxy flavanone Taxifolin 45.5 ± 7.0 80.6 ± 4.4* Weak 9 Flavan(ol)es (+)-trans-3,30,40,5,7- pentahydroxy flavane (+)-catechin 31.9 ± 7.9 56.2 ± 7.4* Weak 10 cis-3,30,40,5,7- pentahydroxy flavane (À)-epicatechin 34.0 ± 8.7 66.8 ± 7.6* Weak 11 Chalcones 20,40,60-trihydroxy-3-(4-hydroxyphenyl)propiophenone Phloretin 68.2 ± 6.9* 105.8 ± 1.8* Moderate 12 Anthocyanidins 3,40,5,7- tetrahydroxy flavylium Pelargonidin 54.8 ± 9.5* 87.9 ± 5.3* Moderate Y.C. Xu et al. / Phytochemistry 68 (2007) 1179–1188 13 Isoflavones 3,40,5,7- tetrahydroxy isoflavone Genistein 95.4 ± 3.2* 106.2 ± 3.7* Good 14 3,40,5,7- tetrahydroxy isoflavone 7-glucoside Genistin 24.4 ± 1.1 25.1 ± 2.4 None 15 40,7-dihydroxy-8-b-D-glucose isoflavone Puerarin 25.3 ± 4.8 45.0 ± 5.5* Weak 16 40,7-dihydroxy isoflavone Daidzein 77.3 ± 3.2* 102.9 ± 0.6* Moderate 17 40,7-dihydroxy-6-methoxy isoflavone Glycetein 46.7 ± 4.3* 93.1 ± 1.7* Moderate Data represent mean ± S.E.M. * P < 0.05 vs the relevant solvent control group. at 30 relaxing agents, whereas those with little or no relaxation ( good relaxing agents,almost and 100% this relaxation(the group at maximal includes 30 (the relaxation). apigenin concentration to The achieveinto 50% three flavonoids relaxation) categories andvasorelaxation, efficacy that taken it into induce account isgroups of possible ( their to potency 100 classify these flavonoids maximum concentration tested, i.e.30 100 noids inducing 50% relaxation at concentration around genistin ( tracted with U46619 (30endothelium-intact nM), porcine all coronary tested arterycontraction that flavonoids were except in con- thewere presence expressed of asbiological the evaluation a tested of percentage flavonoids compound.Table examined relaxation In 1 in of thiscompound the study name initial ofstructures are the showed 17and in flavonoids without are anvasorelaxant intact presented activities endothelium, in inent and porcine their subgroups coronary molecular of arteries flavonoids with weresugar attached. investigated forsubstituent, their the degreeglycosylated of substitutents, unsaturation the androne position and the of differ the types fromFlavonoids benzenoide of one are based another on in themethod terms structure of of is a hydroxyl phenylbenzopy- understood. hampered or The by investigationnism(s) their of for flavonoids vast by theiret structural the al., action diversity. SAR 2002; andvascular Jenkins protection SAR et ( al., haveamount 2003 of not interest with been regard to fully their potential in cardio- 2.1. In vitro vascular relaxation activity 2. Results and discussion investigated. effects of flavonoids incalculated porcine coronary by arteries were density also functional theory. The vascular consistent with thethe findings type of andegories species are of shown thecurves in blood of vessels, some representative our flavonoidsweak results in these are vasodilators three cat- ( lium, flavonoids also induced significant relaxation.