Immunomodulating polysaccharides from Lessertia frutescens leaves: isolation, characterization and structure activity relationship

Abstract

Ethnopharmacological relevance: Sutherlandia frutescens (syn. Lessertia frutescens) is an indigenous plant in Southern Africa and has been extensively studied from the ethnobotanical point of view. Amongst the various traditional uses, several illnesses involving the immune system have been reported. Due to some of the therapeutic effects observed, in relation to the traditional uses reported by the "khoi san" and "nama" people on cancer related illnesses, the plant has been given the local name kankerbos (cancerbush). Recently the plant has also been used amongst HIV/AIDS patients to stimulate the immune system.

Materials and methods: Leaves of Sutherlandia frutescens were extracted sequentially with ethanol, 50% ethanol/water, and water at 50 and 100°C. The polysaccharides were extracted with water and fractionated by ion exchange chromatography and gel filtration to obtain enriched polysaccharide fractions. The bioactivities of the fractions were tested in the complement assay. Some of the fractions were treated with the enzyme pectinase, and the fragments thus produced were separated by gel filtration and their activities tested. Monosaccharide compositions and linkage analyses were determined for the relevant fractions.

Results: The leaves of Sutherlandia frutescens contain polysaccharides of the pectin type. Fractions from both the water extracts of 50 and 100°C were bioactive. Fractions chosen for further studies showed that the fragment with the highest M(W) after the pectinase treatment had a substantially higher biological effect than the parent molecules. Based on a comparison of the different fractions it was concluded that galactose-rich regions were important for the bioactivity, these being of the AGII and AGI type, with the latter probably being more important than the former. Fragments rich in xylose also gave higher activity than those without it.

Conclusions: Our theory that the polysaccharides present in the leaves of Sutherlandia frutescens could be of importance as immunomodulating agents was confirmed. It was also shown that certain types of polysaccharides had a higher effect in the complement system than others. Thus both the water extracts obtained at 50 and 100°C contain interesting biologically active polysaccharides.

Keywords: Complement system; Lessertia frutescens; Pectins; Polysaccharides; Sutherlandia frutescens.