An investigation of the phytochemistry and biological activity of Asparagus laricinus

Abstract

Medicinal plants are part of indigenous people‟s cultural heritage, thus since ancient times treatment of various diseases using medicinal plants has been part of human culture. The value of medicinal plants to mankind has been very well proven. It is estimated that 70% to 80% of people worldwide rely mainly on traditional health care systems, especially on herbal medicines (Stanley and Luz, 2003). In many societies the medicinal properties of plants were discovered mostly through trial and error, but use was also influenced by the belief systems of the people involved and often became entangled with religious and mythical practices (Mathias et al., 1996). Besides that, medicinal plants are proving to be rich resources of constituents that can be used in drug development and synthesis. Medicinal plants have been a source of a wide variety of biologically active compounds for many centuries and have been used extensively as crude material or as pure compounds for treating various disease conditions. Between 1% and 10% of plants out of an estimated 250 000 to 500 000 species of plants on earth are used by humans (Boris, 1996). 2 Plants used for medicinal purposes contribute significantly to the development of major medical drugs that are used today. Most common medicines have compounds extracted from plants as their primary active ingredients and many have provided blueprints for synthetic or partially synthesized drugs (Simpson and Ogorzaly, 2001). There has been a major resurgence of interest in traditionally used medicinal plants, with a number of international and local initiatives actively exploring the botanical resources of southern Africa with the intention to screen indigenous plants for pharmacologically active compounds (Gurib-Fakim et al., 2010; Rybicki et al., 2012). South Africa is considered a “hot spot” for biodiversity and more than 22 000 plant species occur within its boundaries. This represents 10% of the world‟s species, although the land surface of South Africa is less than 1% of the earth‟s surface (Coetzee et al., 1999). Plants have also been used by man for various purposes, among others as arrow and dart poisons for hunting, poisons for murder, hallucinogens used for ritualistic purposes, stimulants for endurance and hunger suppression, as well as medicine (Duke et al., 2008; Cragg and Newman, 2005). A derivative of the polyhydroxy diterpenoid ingenol isolated from the sap of Euphorbia peplus (known as “petty spurge” in England or “radium weed” in Australia), which is a potential chemotherapeutic agent for skin cancer, is currently under clinical development by Peplin Biotech for the topical treatment of certain skin cancers (Kedei et al., 2004; Ogbourne et al., 2004). Combretastatin A-4 phosphate, 3 a stilbene derivative from the South African bush willow, Combretum caffrum, acts as an anti-angiogenic agent causing vascular shutdowns in tumors (Newman et al., 2005; Holwell et al., 2002). Further reliance on plants for drug development is demonstrated by the use of galantamine hydrobromide, an alkaloid obtained from the plant Galanthus nivalis used traditionally in Turkey and Bulgaria for the treatment of Alzheimer‟s disease (Howes et al., 2003; Heinrich and Teoh, 2004). The plant chemicals used for the above-mentioned purposes are secondary metabolites, which are derived biosynthetically from plant primary metabolites (e.g. carbohydrates, amino acids and lipids). Secondary metabolites are organic compounds that are exclusively produced by plants and that are not directly involved in the normal growth, development and reproduction of a plant (Firn and Jones, 2003). Yet, they have many functions that are important for the plant‟s long-term health and appearance. Plants, being stationary, have to cope with a number of challenges, including engineering their own pollination and seed dispersal, local variation in the supply of the simple nutrients that they require to synthesize their food and the coexistence of herbivores and pathogens in their immediate environment. Plants have therefore evolved secondary biochemical pathways that allow them to synthesize a spectrum of organic molecules, often in response to specific environmental stimuli, such as herbivore-induced damage, pathogen attacks, or nutrient deprivation (Reymond et al., 2000; Hermsmeier et al., 2001). 4 The biosynthesis of secondary metabolites is derived from the fundamental processes of photosynthesis, glycolysis and the Krebs cycle to afford biosynthetic intermediates which, ultimately, result in the formation of secondary metabolites also known as natural products (Dewick, 2002). It is hypothesized that secondary metabolism utilizes amino acids and the acetate and shikimate pathways to produce “shunt metabolites” (intermediates) that have adopted an alternate biosynthetic route, leading to the biosynthesis of secondary metabolites (Sarker et al., 2006). Modifications in the biosynthetic pathways that produce secondary metabolites are probably due to natural causes (e.g. viruses or environmental changes) or unnatural causes (e.g. chemical or radiation processes) in an effort to adapt or provide longevity for the plant (Sarker et al., 2006). Plants‟ secondary metabolites can be classified into several groups according to their chemical classes, such alkaloids, terpenoids and phenolics (Harbone, 1984; Wink, 2003).