The antimicrobial activity of 3,5,7-trihydroxyflavone isolated from the shoots of Helichrysum aureonitens

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Abstract

Extracts from Helichrysum aureonitens are used topically by the indigenous people of South Africa against infections. The antimicrobial activity-guided fractionation by bioautography of the acetone extract from the aerial parts of H. aureonitens led to the isolation of 3,5,7-trihydroxyflavone (galangin). Evaluation of the antibacterial activity of the compound against ten randomly selected bacteria indicated significant activity against all the Gram-positive bacteria tested with the minimum inhibitory concentration (MIC) ranging from 0.1 to 0.5 mg/ml. The compound was not active on Gram-negative bacteria except for Enterobacter cloacae which was significantly inhibited at an MIC of 0.1 mg/ml. Galangin indicated considerable activity against the fungi tested with the exception of Cladosporium herbarum. Penicillium digitatum and P. italicum appeared to be particularly susceptible at a concentration of 0.01 mg/ml.

Introduction

Helichrysum aureonitens Sch. Bip. (Asteraceae) is a hairy perennial herb which grows mostly in the KwaZulu-Natal province of South Africa. It belongs to a large genus of about 500 species with 246 growing in South Africa. Extracts from Helichrysum aureonitens are used topically by the indigenous people of South Africa against infections.

The antimicrobial activities of extracts from Helichrysum species have been widely reported (Tomas-Barberan et al., 1988, Tomas-Lorente et al., 1989, Cosar and Cubukcu, 1990, Tomas-Barberan et al., 1990, Rios et al., 1991). However, the compounds responsible for these activities have been identified in only a few cases. The antibacterial and antiherpes activities of extracts from H. aureonitens were recently reported from our laboratory (Meyer and Afolayan, 1995, Meyer et al., 1996).

In this paper we report on the isolation and structural elucidation of the main antimicrobial compound found in the epicuticular acetone extract of this herb. We also report our observations on the inhibitory properties of this compound on some bacteria and fungi.

Section snippets

Plant material

Aerial parts of H. aureonitens (excluding flowers) were collected from the KwaZulu-Natal province of South Africa and a voucher specimen (Afol. 2001) of the herb was prepared and deposited at the National Botanical Institute of South Africa, Pretoria.

Extraction and isolation of the antimicrobial compound

The air-dried aerial parts (200 g) were shaken in acetone for 5 min. The resultant extract was concentrated to dryness and gave 2.362 g of dry matter. This was dissolved in acetone and subjected to column chromatography on Sephadex LH-20

Results

The antimicrobial activity-guided fractionation by bioautography of acetone extract from H. aureonitens led to the isolation of the bioactive galangin (3,5,7-trihydroxyflavone). Spectra data were compared with those found in the literature (Markham et al., 1982, Markham and Geiger, 1993). 1H NMR (in acetone-d6): 6.28 (1 H, d, J=2 Hz, H-6), 6.55 (1 H, d, J=2 Hz, H-8), 7.49–7.57 (3 H, m, H-3′, H-4′, H-5′), 8.25 (2 H, dd, J=1.5 and 8.0 Hz, H-2′, H-6′). 13C NMR (in acetone-d6): 94.5 (C-8), 99.3

Discussion

Galangin, was first isolated in 1881 from the root of galangal, Alpinia officinarum (Wollenweber, 1988) and synthesised by Heap and Robinson (1926). Since then, it has been found in extracts of many angiosperms (Ferraro et al., 1981, Wollenweber et al., 1985, McDowell et al., 1988, Sabatier et al., 1992). Galangin together with chrysin, tectochrysin, pinobanksin and pinocembrin are characteristic flavonoids from propolis (bee glue) and bee wax (Tomas-Barberan et al., 1993, Ferreres et al., 1994

Acknowledgements

This study was financed by a grant from the Foundation for Research Development of South Africa.

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