At present the literature is divided as to whether the low-molecular-weight alcohol-extractable materials or the high-molecular-weight oligosaccharides of echinacea are responsible for the species' immunostimulatory properties.
There is evidence to support both views and it is quite possible that both classes contribute. It is not clear, however, which may contribute the most or whether they are additive or even synergistic.
The lower-molecular-weight secondary constituents of echinacea belong to several classes: caffeic acid analogs, phenolics, phenolic glycosides, terpenes, polyunsaturated amides, polyunsaturated acetylenes, and alkaloids.
Chemical work on echinacea and its constituents began in the 1890s (53) and the results reflect the comparatively crude methods available at that time. For example, sugars, volatile oils, alkaloids, and such general plant constituents were quantified but not identified at the molecular level. Much of the early work is somewhat doubtful because of confusion about the identity of the plant material employed.
Solid modern work that connects specific echinacea compounds with biological activity can be dated from the efforts of Stoll, who identified echinacoside as a weak antibiotic (54), but the chemical structure of this compound required three decades more to be settled (55). Echinacoside is present in many species of echinacea but apparently not in E. angustifolia, so it is not a reliable genus marker. Verbascoside is an echinacoside analog lacking a glucosyl residue (56). Descaffeoylechinacoside and desrhamnosylverbasco-side have also been isolated from E. pallida (57). One also finds in echinacea species a variety of caffeoyl cyclitol analogs including chlorogenic acid and cynarin (58). Of the various caffeoyl-substituted tartrates, cichoric acid is characteristic of E. purpurea but less so of E. angustifolia (59). Cichoric acid occurs in several different plant species in which the optical isomer of the tartaric acid component varies. That in Echinacea species appears to be (2R,3R)-( + )-tartrate. Caftaric acid and a variety of other analogs of cichoric acid have also been found in various echinacea species (56,57). (Formula Charts 1 and 2).
Echinacea species are rich in flavonoids and flavonoid glycosides. Luteolin, kaempferol, quercetin, quercetagetin-7-glucoside, luteolin-7-gluco-side, kaempferol-3-glucoside, quercetin-3-arabinoside, quercetin-3-galacto-side, quercetin-3-xyloside, quercetin-3-glucoside, kaempferol-3-rutinoside, rutoside, isorhamnetic-3-rutinoside, and others have been found in the leaves of E. angustifolia (3,60,61). Likewise, the leaves of E. purpurea were found to contain quercetin, quercetin-7-glucoside, kaempferol-3-rutinoside, quercetin-3-glucoside, rutin, quercetin-3-robinobioside, quercetin-3-xylosylgalactoside (or an isomer thereof), and others.
Clearly quercetin and its glycosides are plentiful in these leaves. In addition to these, Echinacea species were also found to contain cyanidin-3-O-(h-D-glucopyranoside) and cyanidin-3-O-(6-O-malonyl-h-D-glucopyranoside (62).
Echinacea contains a variety of essential oils in quantities ranging from 0.1 to 4% depending on the plant part and the season. Notable among these
are pentadeca-(1,8Z)-diene, 1-pentadecene diene (63) and echinolone ((E)-10-hydroxy-4,10-dimethyl-4,11-dodecadien-2-one) (64). Among the nonvolatile sesquiterpenes, four cinnamoyl esters of the germacrane and guaiane skeletal types have been isolated from E. purpurea roots (65). Many other terpenes have been found in echinacea oils (3). (Formula Chart 3).
Polyacetylenes are frequently found in the Compositae and the Aster-aceae are not exceptional in this. These compounds are not stable, readily undergoing air oxidation on standing, so it is doubtful that commercial echinacea dosage forms contain much of them in native form. Schulte et al. (66) determined the total and partial structures of 15 such compounds. The first two of these were among those present in significant quantity and these
showed mild antibacterial and antifungal activity. Many of the remaining analogs were present in very small quantities allowing only partial characterization. Later Bauer et al. (67) investigated the roots of E. pallida and determined the structures of eight ketones and three hydroxylated analogs that did not occur in E. angustifolia. In a similar work (68), two additional hydroxylated acetylenes were characterized. Subsequently, many papers have appeared that describe qualitative and quantitative investigation of these analogs. (Formula Charts 4 and 5).
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