PHYTOCHEMICAL AND ETHNOPHARMACOLOGICAL REVIEW OF HETEROPYXIS NATALENSIS

The bark, leaves, roots, and twigs of Heteropyxis natalensis are widely used as herbal medicines in Southern Africa. The aim of this study was to review the phytochemical and ethnopharmacological properties of H. natalensis so as to provide baseline data required for evaluating the therapeutic potential of the species. Information on the botanical profile, medicinal uses, phytochemistry, and pharmacological properties of H. natalensis was undertaken using databases such as ScienceDirect, SciFinder, PubMed, Google Scholar, Medline, SCOPUS, EThOS, ProQuest, OATD, and open-thesis. Pre-electronic literature of conference papers, scientific articles, books, book chapters, dissertations, and theses were carried out at the university library. Literature search revealed that H. natalensis is used as an aphrodisiac, anti-infection, blood purifier, decongestant, for weaning, ethnoveterinary medicine and as herbal medicine for bleeding disorders, gums, nose, colds, gum infections, impotence, menorrhagia, respiratory disorders, toothache, and wounds. Phytochemical compounds identified from the species include essential oils, 3β-hydroxylup-20(29)-en-28-al, (E)-1-(2’,4’-dihyroxy,5’methoxy,3’-methylphenyl)-3-phenylprop-2-en-1-one, (2E)-2-[(2E)-1-hydroxy-3-phenylprop-2-en-1-ylidene]-5-methoxy-6,6-dimethylcyclohex-4-ene1,3-dione, 2’,4’-dihydroxy-6’-methoxy-3’,5’-dimethylchalcone, 3’,4’,5’-tri-O-methyl-3,4-methylenedioxy ellagic acid, 3,5,7-trihydroxyflavan, 5-hydroxy7-methoxy-6-methylflavanone, aurentiacin A, betulinic acid, cardamomin, lupenone, lupeol, quercetin, and sitost-4-en-3-one. Pharmacological studies revealed that H. natalensis extracts and compounds have antibacterial, antimycobacterial, antifungal, antioxidant, anti-inflammatory, cytotoxicity, and pro-inflammatory activities. Detailed studies are required to establish the efficacy, clinical relevance, safety and mechanisms of action of the plant extracts, and compounds of H. natalensis.


INTRODUCTION
Heteropyxis natalensis Harv. is a tree species which belongs to the Heteropyxidaceae family. Research by Cunningham [1] and Williams et al. [2] revealed that the bark of H. natalensis is widely used as herbal medicine in South Africa and therefore, traded in medicinal informal markets in Gauteng and KwaZulu-Natal Provinces in the country. The species is also included in the monographic treatment of "medicinal plants of South Africa" [3], a publication which provides details of how important medicinal plants in the country are utilized. Traditional, complementary, and alternative medicines are popular and widely used in tropical Africa with about 80% of the people in the continent relying on them for primary health care . Mahomoodally [29] argued that the use of herbal medicines is a fundamental component of the African traditional health-care system with traditional healers prescribing herbal medicines because they are the most easily accessible and affordable health resources available to local communities. The same author also argued that Western or allopathic modern medicine is rooted in traditional, complementary, and alternative medicine, and currently, several new pharmaceutical drugs and health-promoting products are being produced and developed from plants traditionally used as herbal medicines [30,31]. Van Wyk [30,31] argued that the essential oils, leaves, and roots of H. natalensis are important in the development of new pharmaceutical and health products in Southern Africa for colds, weaning, nose bleeding, bleeding gums menorrhagia, aromatherapy, and topical products. This is not surprising since 25% of pharmaceutical drugs and products and 11% of prescription drugs which are regarded by the World Health Organization as essential to human health are prepared from herbal medicines [32]. It is, therefore, within this context that this study was undertaken aimed at summarizing the phytochemical and ethnopharmacological properties of H. natalensis, an important medicinal plant species in Southern Africa.

BOTANICAL PROFILE OF H. NATALENSIS
H. natalensis is a small deciduous to the semi-deciduous tree of not more than 15 m in height [33,35,45]. The tree consists of a branched trunk, dense leafy branches, and highly aromatic foliage [33,35]. The crushed twigs and leaves are strongly aromatic, reminiscent of lavender, hence the English common name of the species, lavender-tree [33]. The bark is pale gray to almost white in color, flaking off on older stems in large pieces. The leaves are simple, alternate, narrowly elliptic, ovate to obovate, shiny dark green above, paler green below, and slightly hairy to hairless when mature, pleasantly aromatic when crushed [33]. The flowers are small, bisexual, occur in branched terminal clusters yellowish green in color. The fruit is a capsule, oval and brownish in color when mature [33]. H. natalensis has been recorded in Bushveld and along forest margins in riverine fringes and often in rocky places, from sea level to an altitude of about 1400 m above sea level [33,35,45].

Antibacterial activities
Gundidza et al. [60] evaluated antibacterial activities of the essential oil isolated from H. natalensis against 25 bacterial species. The essential oil exhibited activities against all tested microbes showing good activities against pathogens Escherichia coli, Klebsiella pneumoniae, Staphylococcus aureus, and Moraxella species with lower sensitivities for Pseudomonas aeruginosa [60]. Van Vuuren and Viljoen [64] evaluated the antibacterial activities of the essential oil of H. natalensis against S. aureus, Staphylococcus epidermidis, Bacillus cereus, Bacillus subtilis, E. coli, P. aeruginosa, Enterococcus faecalis, and K. pneumoniae using the microtiter plate method with ciprofloxacin (0.01 mg/ml) as a positive control. The essential oil exhibited activities with while minimum inhibitory concentration (MIC) values ranging from 4.5 mg/mL to 32.0 mg/mL [64]. Van Vuuren and Viljoen [64] exposed the oil to S. aureus, and the cidal efficacy was plotted over time against the logarithm of viable colonies. The oil showed a rapid cidal effect against the tested pathogen [64]. Van Vuuren et al. [63] evaluated the antibacterial activities of the essential oil of H. natalensis against B. cereus, E. faecalis, E. coli, K. pneumoniae, Moraxella catarrhalis, P. aeruginosa, and S. aureus using the microtiter plate method with ciprofloxacin (0.01 mg/ml) as a positive control. The essential oil exhibited activities with MIC values ranging from 1.0 mg/ml to 16 mg/ml [63]. Braithwaite et al. [52] evaluated the antibacterial activities of methanol and acetone leaf extracts, essential oil and smoke fraction of H. natalensis against S. aureus, B. cereus, and K. pneumoniae using the microtiter plate technique with ciprofloxacin as a positive control. The extracts exhibited activities with MIC values ranging from 0.25 mg/ml to 32.0 mg/ml [52]. Henley-Smith et al. [69] evaluated the synergistic activities of H. natalensis mixed with Melaleuca alternifolia (Maiden and Betche) Cheel, Mentha piperita L., and the green tea extract known as TEAVIGO™ against Streptococcus mutans and Prevotella intermedia. The mixed extract successfully inhibited the growth of the pathogens [69]. Sharma and Lall [70] evaluated antimicrobial activities of leaf ethanol extracts of H. natalensis against pathogenic bacteria, Propionibacterium acnes using the broth dilution method with tetracycline as a positive control (0.2 mg/mL). The extracts showed weak activities with MIC value of 250 µg/mL in comparison to the MIC value of 3.1 µg/mL demonstrated by tetracycline, the positive control [70]. Cock and Van Vuuren [71] evaluated antibacterial activities of aqueous and methanol leaf extracts of H. natalensis against Alcaligenes faecalis, Aeromonas hydrophila, B. cereus, B. subtilis, Citrobacter freundi, E. coli, K. pneumoniae, Proteus mirabilis, Proteus vulgaris, P. aeruginosa, Pseudomonas fluorescens, Salmonella typhimurium, Serratia marcescens, Shigella sonnei, S. aureus, and S. epidermidis using a modified disk diffusion method with ampicillin (2 µg) and chloramphenicol (10 µg) as positive controls. Only methanol extract exhibited activities against A. faecalis, A. hydrophilia, B. cereus, B. subtilis, P. mirabilis, P. vulgaris, and S. epidermidis with a zone of inhibition ranging from 6.0 mm to 8.0 mm, and the MIC values ranged from 525 µg/ml to 6150 µg/ml [71]. Akhalwaya et al. [72] evaluated antibacterial activities of aqueous and dichloromethane:methanol (1:1) leaf and stem extracts of H. natalensis against S. mutans, Streptococcus sanguis, Lactobacillus acidophilus, Lactobacillus casei, Porphyromonas gingivalis, and Fusobacterium nucleatum using the microtiter plate dilution assay with ciprofloxacin (0.1 mg/mL) as a positive control. The extracts exhibited activities with MIC values ranging from 0.21 mg/mL to >8.0 mg/mL [72]. Henley-Smith et al. [54] evaluated antibacterial activities of ethanolic leaf and twig extracts of H. natalensis against pathogenic oral bacterial organisms, Actinomyces israelii, P. intermedia, S. mutans, and Lactobacillus paracasei, a commensal bacterium essential in plaque prevention using the microdilution technique with 5% chlorhexidine gluconate as a positive control. The extracts exhibited activities with MIC values ranging from 0.9 mg/ml to 12.5 mg/ml while minimum bactericidal concentration (MBC) values ranged from 3.3 mg/ml to >12.5 mg/ml [54].

Maroyi
with MIC values ranging from 0.5 mg/mL to 3.5 mg/mL [65]. Henley-Smith et al. [54] evaluated antibacterial activities of the compounds cardamomin, aurentiacin A, 5-hydroxy-7-methoxy-6-methylflavanone, quercetin, and 3,5,7-trihydroxyflavan isolated from H. natalensis against A. israelii using the microdilution technique with 5% chlorhexidine gluconate as a positive control. Only cardamomin, aurentiacin A, quercetin, and 3,5,7-trihydroxyflavan were active against the pathogen with MIC values ranging from 0.06 mg/ml to >1 mg/ml, while MBC values for aurentiacin A and quercetin were 0.06 mg/ml to 1 mg/ml, respectively [54]. An enzymatic bioanalysis of lactic and acetic acid production from S. mutans and L. paracasei was carried out after 24-h incubation with the ethanolic leaf and twig extract of H. natalensis. A reduction in the acid production from each bacterium was observed on exposure to the extract, and this consequently increased the pH, which could possibly reduce the demineralization of enamel which may help prevent the formation of dental caries [54].

Antimycobacterial activities
Dzoyem et al. [73] evaluated the antimycobacterial activities of acetone leaf extracts of H. natalensis against Mycobacterium smegmatis, Mycobacterium aurum, Mycobacterium fortuitum, and Mycobacterium tuberculosis using a tetrazolium violet based broth microdilution method with isoniazid (µg/mL) and rifampicin (µg/mL) as positive controls. The extracts exhibited activities with MIC values ranging from 0.08 mg/mL to 0.62 mg/mL and the total activities ranged from 147.2 mL/g to 1150 mL/g [73].

Antioxidant activities
Muchuweti et al. [74] evaluated the antioxidant activities of ethanol leaf and stem extracts of H. natalensis using the 2,2-diphenyl-1-picrylhydrazyl hydrate (DPPH) radical scavenging assay. The percentage inhibition exhibited by the extracts was 29.7% for the standard β-carotene, and the amount of total phenolics was 0.096 ± 0.02 milligrams tannic acid per 100 mg of plant sample [74]. Mzindle [75] evaluated antioxidant activities of aqueous and methanol extracts of H. natalensis using the DPPH radical scavenging assay with rutin as a positive control. The extracts showed free radical scavenging abilities ranging from 42.9 ± 2.8% to 101.1 ± 2.1%, while rutin exhibited free radical scavenging abilities ranging from 27.4 ± 1.4% to 95.3 ± 0.5% [75].

Anti-inflammatory activities
Frum and Viljoen [76] evaluated anti-inflammatory activities of the essential oils 1.8-cineole, linalool, limonene, and β-pinene isolated from H. natalensis using the 5-lipoxygenase inhibitory assay. All oils tested exhibited promising 5-lipoxygenase inhibitory activities with half maximal IC 50 value of 46.6 ppm [76]. Mzindle [75] evaluated anti-inflammatory activities of aqueous and methanol leaf extracts of H. natalensis using the lipoxygenase inhibitor screening assay with nordihydroguaiaretic acid as a positive control. The aqueous and methanol extracts inhibited lipoxygenase enzyme by 101.6% ± 3.8% and 58.2 ± 12.3, respectively, which was lower than 122% and 129% inhibition demonstrated by nordihydroguaiaretic acid, the control [75].

Pro-inflammatory activities
Henley-Smith et al. [54] evaluated the pro-inflammatory activities of ethanolic leaf and twig extracts of H. natalensis using C. albicans and P. intermedia. A significant reduction of interleukin-8 production by macrophage cells was observed when exposed to the extract. It is possible that H. natalensis can prevent excessive tissue damage in periodontal diseases through its reduction of inflammation [54].

CONCLUSION
H. natalensis is an important herbal medicine in Southern Africa, and significant breakthrough has been made in the past 40 years elucidating the phytochemical and pharmacological properties of the species. However, there are still some research gaps regarding correlating the medicinal uses of H. natalensis with its chemical compounds and associated pharmacological properties of the compounds and extracts of the species. Detailed studies on the pharmacokinetics, in vivo and clinical research involving compounds isolated from the species and its extracts, are required.