Introduction Nitrogen Compounds Alkamides Alkaloids Glycoproteins Polysaccharides Hydrocarbons Essential Oils Flavonoids Cinnamic Acids Acknowledgments References
Echinacea is a small genus of the Compositae, tribe Heliantheae, containing six species (Shetler and Skog, 1978) that are all endemic to North America. Only three of the species — E. angustifolia DC., E. purpurea (L.) Moench, and E. pallida Nutt. — are used medicinally for their immuno-stimulatory properties (Bauer, 1998; Brevoort, 1996). The plant parts used include the rhizomes (usually termed roots in most references) of E. angustifolia and E. pallida, and less frequently of E. purpurea; the aerial parts of E. purpurea; and whole-plant homeopathic tinctures of E. angustifolia and E. pallida.
Phytochemical reports are restricted to these three medicinally important taxa. The identity of the commercially available plants of E. angustifolia from the botanic gardens of Europe that were used in analyses before ca. 1985 is suspect. Thus, more recent taxonomic determinations have revealed that the majority of these plants were in fact the taller more abundant species, E. pallida. Hence, most of the studies included here are post-1985. Four classes of compound are known to contribute to the immunostimulatory activity of Echinacea extracts: alkamides, glycoproteins, polysaccharides, and cinnamic acids. Alkamides (fatty acid amides) are characteristic rhizome components of E. angustifolia and the rhizomes and aerial parts of E. purpurea. Their absence from and the presence of polyacetylenes in rhizomes of E. pallida serve to distinguish this tissue from rhizome preparations of E. angustifolia. As in other members of the Heliantheae, the alkamides are mostly of the acetylenic type.
Glycoproteins, which induce cytokine production and show mutagenic activity, are other important rhizome constituents of both E. angustifolia and E. purpurea. Immunostimulatory polysaccharides present in the aerial parts and produced in tissue cultures of E. purpurea are of special interest contents introduction
* Deceased since one of the tissue culture products, an arabinogalactan, can now be produced on an industrial scale and is being considered for clinical trials. Hydrocarbons, mainly ketoalkenes and ketoalkynes (polyacetylenes), are further characteristic rhizome components of E. pallida with only small amounts of simple alkenes and esters present in rhizomes of the other two species.
Caffeoyl quinic and caffeoyl tartaric acid esters are the major cinnamic acid derivatives of all three species with each species having its own distinctive profile. For example, chicoric acid (2,3-dicaffeoyl tartaric acid), a major immunostimulatory rhizome component of E. pallida and of rhizomes and aerial parts of E. purpurea, is absent from rhizomes of E. angustifolia.
Alkamides are fatty acid amides containing one or more double bonds that may be accompanied by up to three acetylenic linkages. Alkamides are characteristic constituents of the roots of E. angustifolia and the roots and aerial parts of E. purpurea. There is one old report of an alkamide named echinacein (12E, 4Z, 8E, 10E)-N-isobutyldodeca-2,4,8,10-tetraenamide) from roots of E. angustifolia (Jacobson, 1954, 1967), which has never been confirmed by others (Greger, 1988). The plant material used by Jacobson was most probably misidentified E. pallida, but even so Greger (1988) doubts that echinacein occurs in any Echinacea species. Alkamides had not been reported from the rhizomes of E. pallida until a recent study comparing the chemical components of different Echinacea species (Sloley et al., 2001), when they were found in small amounts in this tissue. Even so, the accumulation of alkamides as major rhizome constituents of E. angustifolia and E. purpurea can still be used to distinguish root powders of these plants from those of E. pallida. Polyacetylenes, on the other hand, have been found only in the roots of E. pallida (see section on polyacetylenes below), and so their presence or absence is another valuable diagnostic character for determining the composition of commercial root preparations.
As in other members of the Compositae, tribe Heliantheae, the alkamides reported from E. angustifolia and E. purpurea are mainly of the acetylenic type, together with a small number of purely olefinic structures (Table 5.1). Some 15 alkamides (1 to 15) (Figure 5.1) have been characterized in roots of E. angustifolia (Bauer et al., 1989a) (Table 5.1). These are derived mostly from undecanoic and dodecanoic acids and isobutylamide but also include trideca- (10), pentadeca- (6) and hexadecanoic acid (7) derivatives and two 2-methylbutylamides (4 and 5). The main constituents, the isomeric (2E, 4E, 8Z, 10E/Z)-N-isobutyldodeca-2,4,8,10-tetraenamides (14 and 15), were shown to display marked inhibitory activity in vitro in the 5-lipoxygenase and cyclo-oxygenase assays (Muller-Jakic et al., 1994), but showed only weak stimulation of phagocytosis (Bauer et al., 1989b). Four other 2,4-diene type alkamides (8,9,11,12) were detected but the remaining compounds (1 to 7 and 10) were all of the 2-monoene type, such as (E/Z)-N-isobutylundec-2-ene-8,10-diynamides (1 and 2).
In the roots of E. purpurea, ten alkamides (11 to 20) (Figure 5.1 and Figure 5.2) have been characterized, all of which are of the 2,4-diene type except for compound 19, trideca-2E,7Z-dien-10,12-diynic acid isobutylamide (Table 5.1). The major constituents were again compounds 14 and 15 (Bohlmann and Grenz, 1966; Bauer et al., 1988b) and three other E. angustifolia root constituents (11 to 13) were detected also in roots of E. purpurea (Bauer et al., 1988b).
The alkamides found in the aerial parts of E. purpurea are mainly of the same 2,4-diene type as in the roots with compounds 14 and 15 as the major components and five other root constituents (11, 13, 16, 17, and 19) (Figure 5.1 and Figure 5.2) as minor components. Three of five non-2,4-diene type alkamides, previously reported by Bohlmann and Hoffmann (1983) were not recorded in the more recent analysis of this tissue by Bauer et al. (1988c), who gave the reference but did not comment on the data. These compounds are (2E,7Z)-N-(2-methylbutyl)trideca-2,7-diene-10,12-
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