Identification and characterization of a novel triyne and cinnamate 4-hydroxylase in Helichrysum aureonitens Sch. Bip.

Abstract

For centuries people have used plants as medicine or food additives with varying success to cure and prevent diseases. Written records about medicinal plants date back at least 5 000 years to the Sumerians. According to World Health Organization (WHO) around 80 % of the population in developing countries is dependent on herbal medicine for basic healthcare needs. Even at the dawn of the twenty-first century, 11 % of the 252 drugs considered as basic and essential by WHO were exclusively of flowering plant origin. The genus Helichrysum, belongs to the family Asteraceae and is represented by approximately 600 species in Africa, of which 244 species are indigenous in South Africa. In Helichrysum aureonitens, galangin is one of the flavonol compounds with good medicinal properties. H. aureonitens was targeted to be enhanced via cell suspension culture to potentially produce valuable natural products. In ethanol extracts of cell suspension cultures, galangin was not detected even though it was present in the leaves of the intact plants. Some other compounds were induced in higher amounts in the cells of H. aureonitens suspension cultures when compared to that produced in the intact plants. To find out the reasons for the absence of galangin in the cells of H. aureonitens suspension cultures, some of the intermediates of the 4’-OH biosynthetic pathway for production of flavonols were analyzed by GC-MS, including cinnamic acid, p-coumaric acid, naringenin and kaempferol. None of these were detected in the H. aureonitens cell suspension cultures. The major compound from H. aureonitens cell suspension cultures was isolated and identified as a new chlorophenol compound named 4-chloro-2-(hepta-1,3,5-triyne-1-yl)-phenol (a triyne). This triyne has previously been proposed as being an intermediate in the acetylene biosynthetic pathway in Helichrysum spp., however only the methyl ether form had previously been isolated from the roots of H. coriaceum. The triyne isolated from the H. aureonitens cell suspension cultures in the present study was detected in intact plant extracts, but at very low concentrations. Results of the anti-tuberculosis assay of the cell suspension culture extracts and the triyne showed that the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of the ethanol extract of cell suspension cultures against Mycobacterium tuberculosis H37Rv were found to be 1.0 mg/ml and 2.0 mg/ml respectively and the triyne was not active at 200 µg/ml. The ethanol extract of the cell suspension cultures and the triyne were also evaluated for their cytotoxicity against monkey kidney Vero cells and human prostate epithelial carcinoma (DU145) cell lines. The inhibitory concentrations (IC50) of the crude extract and the triyne was found to be 12.11 and 1.51 µg/ml against the Vero cells respectively. The crude extract and the triyne showed similar activity in the prostate cancer cell lines by exhibiting IC50 values of 3.52 and 2.14 µg/ml respectively. The triyne therefore warrants further investigation for its potential as an anticancer drug. Flavonoids represent the major phenolic compounds which are responsible for the medicinal properties in the Helichrysum genus. Some of flavonols, including kaempferol, quercetin, and galangin are also present in H. aureonitens. In this study both galangin and kaempferol (containing a 4’-OH group) were detected in leaf samples of H. aureonitens. But GC-MS analysis of the leaf samples of H. aureonitens did not show the existence of biosynthetic intermediates such as p-coumaric acid and naringenin (compounds having a 4’-OH) while cinnamic acid and some other compounds with no OH at the 4’ position on the B ring, were detected. The chemical structure analysis of the isolated compounds showed that they are pinocembrin chalcone, pinocembrin, pinobanksin and galangin, all containing no OH group at the 4’ position. This indicates that some part of the 4’-OH biosynthetic pathway for 4’-OH flavonoids is not functional in H. aureonitens. Since the only (yet identified) enzyme responsible for hydroxylation at the 4’ position on the B ring is cinnamate 4-hydroxylase (C4H), it can be postulated that C4H might be able to hydroxylate other substrates in H. aureonitens plants. One copy of C4H was isolated and cloned from H. aureonitens. It has 1518-base pairs (including stop codon, TAA) and an open reading frame encoding a 506-amino-acid polypeptide. It showed the highest homologies to Echinacea angustifolia (Asteraceae) C4H with 83.6 % identity on the nucleotide level but 93 % identity on the amino acid level. The genomic DNA sequence of the isolated C4H from H. aureonitens indicates the presence of three introns with a longer size compared to the Arabidopsis thaliana C4H gene structure. The presence of the first intron has not been reported before in the C4H gene from other plants and it is therefore a new finding from the isolated C4H in H. aureonitens. To check the putative isolated C4H, the full length cDNA of C4H was isolated from H. aureonitens and for the first time integrated in a secreted expression vector, pPICZáC, and transformed into Pichia pastoris. After the 48 hrs induction protein was collected, precipitated by ammonium sulphate and finally column purified. The results of SDS-PAGE electrophoresis and Western blot showed the expression of a protein with a size of 50-60 kDa. The calculated mass of C4H with regarding to a polyhistidine tag is about 60.5 kDa. The secreted expression was found as an effective system for the production of a soluble C4H protein with easy purification.