Phytochemical evaluation of Curtisia dentata (Burm.f.) C.A.Sm. stem bark and seasonal and geographical region variability

Abstract

The stem bark of the protected tree species, Curtisia dentata (Burm. f.)C.A.Sm., is one of the most popular plant species harvested and traded at traditional medicine markets in South Africa. The overexploitation of C. dentata trees lead to a “Near Threatened” conservation status and the population trend is portrayed as “declining”. In the KwaZulu-Natal Province of South Africa, C. dentata is completely conservation dependent. This study is not based on drug discovery or toxicological studies, but on the concern that the stem bark of C. dentata trees are harvested, prepared into remedies and consumed as traditional medicine without knowledge regarding the chemical compounds in the stem bark, particularly since the chemical composition of C. dentata stem bark was unknown to date. Phytochemical analyses were firstly conducted to determine the chemical composition of C. dentata stem bark using various solvents and various analytical methods, and secondly, to determine how seasons and regional separation of C. dentata trees affect the chemical profiles of C. dentata stem bark from an environmental and nature conservation perspective. Plants are known to contain numerous chemical compounds. Compounds isolated from a particular plant species are therefore not the only compounds present in that species, and although a plant has proven pharmacological properties, they can still cause harm. Previous studies on C. dentata aimed at validating the plant species as a medicinal plant by examining extracts of the leaves, twigs and stem bark’s potentials against known pathogens and selected cancer cells in vitro and in vivo, and its anti-inflammatory, antioxidant and antiverotoxic properties. Four pentacyclic triterpenoids and one steroidal compound were also previously isolated from C. dentata leaves, however, the leaves are not used in traditional medicines, but were suggested as alternative for stem bark as the harvesting of leaves is less destructive. The efficacy of these compounds as therapeutic agents is, however, compromised by their low solubility in water and thus their potential to penetrate permeating biological membranes. Moreover, in vitro toxicity studies distort the picture of its actual potentials on human health as the whole human metabolome and all its processes, including uptake and phase I and phase II biotransformation are not included. In vivo toxicity studies on mammalian animal species may also not present a true picture of a chemical or extract’s toxic effects on humans as animal metabolisms differ from those of humans. The chemical composition of leaves and stem bark may furthermore also be in contrast to some extent, and therefore chemical compounds were also isolated from C. dentata stem bark in this study. Scientific studies on plant-based medicines generally involve the discovery or identification of compounds that may be beneficial, and which can be exploited in future. Chemical compounds in traditional medicines or other plant-based health products which may cause adverse effects are generally ignored. Moreover, scientific studies that consider that some compounds present in plant extracts may derive from contaminants are equally limited. Traditional plant-based medicines are neither standardized nor regulated in South Africa. Users of traditional plant-based traditional medicines therefore consume uncertain dosages of both beneficial and hazardous substances, as well as contaminants simultaneously. Certain chemical compounds are carcinogens or mutagens or have the ability to accumulate in human tissues. Adverse effects may therefore only manifest after several years of use and will subsequently not be connected to the use of a particular traditional plant-based medicine. The goal of the thesis is therefore to provide a holistic portrayal of the full spectrum of chemical compounds in extracts of C. dentata stem bark and to discuss, where literature is available, the effect(s) each chemical compound may have on human health. Moreover, this thesis investigates variations in chemical composition and concentration in individual trees, seasonal variations and variations in composition and concentrations in the stem bark of C. dentata trees from geographically distinct regions. Most unexpected was that not all C. dentata stem bark samples contained chemical compounds with known beneficial potentials at each sampling date, and that chemical compounds may be region-specific and also tree-specific, which confirms that plants produce secondary metabolites according to the needs of each individual plant. Additional insight into the chemical composition and concentration of C. dentata trees is provided by the distribution profiles of amino acids in C. dentata stem bark. Extreme variations within populations and between geographical areas support the need for the cultivation of C. dentata trees to ensure sustainable production of homogenous material for chemical homogeneity.