作者 author#Jed W. Fahey,corresponding author#1,2,3,4 Mark E. Olson,#5,6 Katherine K. Stephenson,1,3 Kristina L. Wade,1,3 Gwen M. Chodur,1,4,12 David Odee,7 Wasif Nouman,8 Michael Massiah,9 Jesse Alt,10 Patricia A. Egner,11 and Walter C. Hubbard2
Glucosinolates (GS) are metabolized to isothiocyanates that may enhance human healthspan by protecting against a variety of chronic diseases. Moringa oleifera, the drumstick tree, produces unique GS but little is known about GS variation within M. oleifera, and even less in the 12 other Moringa species, some of which are very rare. We assess leaf, seed, stem, and leaf gland exudate GS content of 12 of the 13 known Moringa species. We describe 2 previously unidentified GS as major components of 6 species, reporting on the presence of simple alkyl GS in 4 species, which are dominant in M. longituba. We document potent chemoprotective potential in 11 of 12 species, and measure the cytoprotective activity of 6 purified GS in several cell lines. Some of the unique GS rank with the most powerful known inducers of the phase 2 cytoprotective response. Although extracts of most species induced a robust phase 2 cytoprotective response in cultured cells, one was very low (M. longituba), and by far the highest was M. arborea, a very rare and poorly known species. Our results underscore the importance of Moringa as a chemoprotective resource and the need to survey and conserve its interspecific diversity.
Glucosinolates (GS) account in part for the remarkable medicinal potential of Moringa oleifera (“moringa,” Moringaceae, Brassicales; apparently native to the sub-Himalayan lowlands in NW India)1. Glucosinolates (β-thioglucoside N-hydroxysulfates), mostly restricted to the angiosperm order Brassicales2, are metabolized by the enzyme myrosinase to their biologically active, cognate isothiocyanates (ITC)1,3,4. Isothiocyanates have long been known for their herbivore deterrent, fungicidal, bacteriocidal, nematocidal, and allelopathic properties5–11. Isothiocyanates such as sulforaphane from broccoli have antibiotic activity against numerous human pathogens including Escherichia coli, Salmonella typhimurium, Candida sp., and Helicobacter pylori12–18. These medicinal properties have been ascribed both to temperate cruciferous plants that are well-known sources of glucosinolates, and to Moringa oleifera, the most widely cultivated and economically important species of the monogeneric tropical family Moringaceae19–22. Because moringa is highly drought resistant, it can provide benefits to the large and often underserved human populations in the tropics and sub-tropics worldwide.
Many of the medicinal properties such as cancer treatment, regulation of blood glucose levels, and antibiosis that have long been ascribed to M. oleifera in traditional medicine are likely attributable to its glucosinolates or isothiocyanates22. For example, one of the main uses of M. oleifera in Ayurvedic tradition is cancer treatment23. The biomedical research literature now contains numerous animal studies showing preventive effects against carcinogenesis22 that are plausibly accounted for by known mechanisms of action of non-moringa GS and ITC7–25. Additionally, we showed that 4-α-L-rhamnopyranosyloxy)benzyl isothiocyanate (4RBITC), the isothiocyanate created by hydrolysis of “glucomoringin” (4RBGS or 4-(α-L-rhamnopyranosyloxy)benzyl glucosinolate) from M. oleifera is a potent and selective antibiotic against H. pylori15. Other studies have shown that the antibiotic activity of 4RBITC from M. oleifera is selective and potent against other important human pathogens such as Staphylococcus aureus and Candida albicans26. It also appears to be effective in controlling certain manifestations of both ALS and multiple sclerosis in mouse models27,28. A growing number of epidemiologic, animal, and clinical studies link dietary glucosinolates and their cognate isothiocyanates to protection against chronic diseases including a variety of cancers, diabetes, and autism spectrum disorder via the Keap1-Nrf2-ARE-mediated induction of phase 2 cytoprotective enzymes29–44. The coordinated Nrf2-mediated upregulation of this large group of enzymes is responsible for the very important indirect antioxidant activity of these isothiocyanates31,34,45–50.
It is likely that the chemistry of the uniquely rhamnosylated glucosinolates from Moringa spp. (compared to all 120 or so other glucosinolates) might provide special advantages to mammals consuming them at moderate levels22,37. Outside Moringaceae, rhamnosylated glucosinolates have only been documented in the related Resdaceae (Reseda spp.)51,52 and Brassicaceae (Noccaea caerulescens)53. One reason for particular interest in the rhamnosylated glucosinolates is the strong possibility of biologically significantly different absorption, distribution, metabolism, and excretion compared to other glucosinolates2,22,43,54.
With these considerations in mind, we screened glucosinolate diversity and phase 2 enzyme induction potential across Moringa. We focus primarily on adult leaves, because in M. oleifera and M. stenopetala leaves are the most commonly used parts of the plant, providing nutritious vegetables that are consumed both fresh and cooked. We also examine young leaves and the exudates from leaf glands of some plants. Glands on young leaves secrete a clear, sticky exudate that often attracts ants55. Because glucosinolates, via their cognate isothiocyanates, serve anti-herbivory and other protective functions, these compounds are often present in highest quantities at early, more vulnerable ontogenetic stages56,57. Young leaves, mature leaves, seeds, flowers, and extrafloral nectaries might all be expected to have differing proportions of glucosinolates. Our study thus provides a survey of the diversity and relative amounts of glucosinolates across the leaves, seeds, and exudates across this small but chemically and morphologically diverse family.
To the extent that morphological diversity reflects potentially different ways of interacting with herbivores, it is reasonable to presume that surveying across Moringa species could identify species with compounds that are even more efficacious than those currently known. In addition to the commonly grown M. oleifera, there are 12 other species in this monogeneric family. All of the species are native to the dry tropics of Africa, Asia, and Madagascar, with the center of diversity being the Horn of Africa at the intersection of Kenya, Ethiopia, and Somalia. The most commonly cultivated species M. oleifera is apparently native to northwestern Indian lowlands, and seems likely to have been domesticated in India thousands of years ago, with the domesticate differing markedly from the wild plants in its much faster growth rate, shorter maturation time, and softer leaflets. Its close relative M. concanensis is also native to the Indian subcontinent, where it is relatively widespread in dry tropical woodlands. The closest relative to the Indian species is M. peregrina, which is found from the Dead Sea south to the northern Horn of Africa. Four species with massive, water-storing trunks are found in Madagascar (M. drouhardii and M. hildebrandtii), Namibia and Angola (M. ovalifolia), and Kenya and Ethiopia (M. stenopetala). A group of closely-related species is restricted to the Horn of Africa, and is made up of medium sized to small trees with tuberous roots (M. arborea, M. rivae, M. ruspoliana), to dwarf shrubs to tiny herbs with massive underground tubers (M. borziana, M. longituba, M. pygmaea). Sampling across the diversity from massive trees to tiny herbs, we explore the diversity of glucosinolates in leaves, seeds, bark, inflorescences, and glandular exudates from both field and cultivated specimens of 12 of the 13 species.
We assess the glucosinolate contents of 12 of the 13 known species of Moringa, mostly in leaves (dried and fresh), but also when available seeds, stems, and leaf gland exudates. We document the occurrence of 2 heretofore unidentified glucosinolates as major components of 6 Moringa species, though they are not abundant in the 2 most common species (domestic M. oleifera and M. stenopetala). We report on the occurrence of simple alkyl glucosinolates in four species (M. peregrina, M. ruspoliana, M. rivae, and M. longituba), in one of which (M. longituba) they are the dominant glucosinolates. Finally, we document potent chemoprotective potential in most of the species, show that the activity comes from their glucosinolates, and measure the cytoprotective activity of 6 of these glucosinolates in purified form in a variety of cell lines. We could find no obvious relationships between growth habit, geography, phylogenetic relatedness or other obvious variables, and glucosinolate content or spectrum, underscoring the importance of conserving and studying all the species in the genus.