The presence of oligosaccharides in higher plants is common. Early studies confirmed their presence in Echinacea species but, other than assigning molecular weights of approximately 50 kDa to these, structural information was limited (3). Pharmacological studies suggested that they possessed weak antihyaluronidase activity (84, as synopsized in Ref. 3).
Studies of particular relevance to immunostimulation were reported by the Wagner group starting in 1981 (85-87). Two immunostimulatory polysaccharides were isolated from the aboveground parts of E. purpurea, one with an apparent molecular weight of 35 kDa and the other of 50 kDa. The first, named PS 1, was a methylated polymer of glucuronic acid and arabinose and the second, named PS 2, was an acidic polymer of galactose, arabinose, and xylose. Partial structures of the repeating units of these materials were assigned. A third polysaccharide of apparent molecular weight of about 80,000 kDa was shown to contain xylose and glucose. The expressed juice contained a pectinoid that, however, had very weak immunostimulatory power. The primary assay used in these studies was stimulation of macrophage ingestion of carbon particles. This method primarily measures the speed and efficiency of macrophages in engulfing carbon particles. It is convenient but does not allow easy determination of all participating cells in the immune system nor does it allow quantitative examination of the maturation process and intercellular signaling between participating cells. The carbon clearance tests present convincing evidence that the solvent-soluble constituents, including cichoric acid, of echinacea, stimulate nonspecific immune capacity (3,67,69,73,88,89). Echinacoside is not immunostimulatory (56,90-92). These constituents would be present in hydroalcoholic extracts and in juice preparations and indeed are routinely analyzed in marketed products. (Formula Chart 9).
In later work, successful large-scale tissue culture work using E. purpurea resulted in isolation of three homogeneous polysaccharides whose structures differ from those of the whole plants. Two of these are neutral polymers of fucose, galactose, and glucose (of 10 kDa and 25 kDa, respectively) and the third an acidic arabinogalactan of molecular weight about 75 kDa. Partial structures have been assigned to these as well (74,93,94). The arabinogalactan was subsequently shown to activate macrophages against
Formula Chart No. 9 Echinacea polysaccharides.
tumor cells and microorganisms. The material did slightly increase T-cell but not B-cell proliferation. The macrophages were shown to produce IL-2, IFN-h, and TNF-a (95). (Formula Charts 10 and 11).
D. Analysis of High-Molecular-Weight Constituents of Echinacea
From the immediately preceding account, it can be seen that characteristic water-soluble polysaccharides with immunostimulatory properties (a 4-O-methyl-glucuronylarabinoxylan of average MW 35,000, an acidic arabi-norhamnogalactan of MW 45,000 from the aerial parts of E. purpurea, xyloglucan, MW 79,500, from the leaves and stems, and a pectin-like polysaccharide from the expressed juice) are present in echinacea but these are more difficult to analyze chemically than the low-molecular-weight organics because of their complexity and their lack of a suitable chromophore. On the other hand, bioassay can be performed readily (4,85,88,92). The bioassays have the virtue of reflecting the intended end product use of echinacea preparations. The carbon clearance assays covered briefly above have greatly
clarified the situation. The three glycoproteins (MW 17,000, 21,000, and 30,000; approximately 3% protein content) that have been isolated from E. angustifolia and E. purpurea roots contain arabinose (64-84%), galactose (1.9-5.3%), and glucosamine (6%) as well as large amounts of aspartate, glycine, glutamate, and alanine (93) and an ELISA assay has been developed for the glycoproteins (94,95). An arabinogalactan protein was isolated from pressed juice of E. purpurea and shown to have a molecular weight of about 1,200,000 and to contain about 83% polysaccharide and about 7% protein. The core saccharides are highly branched (92). Polysaccharides have also been isolated and characterized from cell culture material but these are chemically significantly different from those isolated from the field-grown plants because of the intrinsic characteristics of plants grown in tissue culture (74,89).
Capillary electrophoresis has been used for the analysis of Echinacea species (96,97). The technique is particularly valuable for the analysis of high-molecular-weight polysaccharides and glycoproteins (98,99).
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