HypericumSt Johns Wort

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The dried flowering tops of St John's Wort {Hypericum perforatum; Guttiferae/Hypericaceae) have been used as a herbal remedy for many years, an extract in vegetable oil being used for its antiseptic and wound healing properties. St John's Wort is now a major crop marketed as an antidepressant, that is claimed to be as effective in its action as the widely prescribed antidepressants of the selective serotonin re-uptake inhibitor (SSRI) class such as fluoxetine {Prozac ®), and with fewer side-effects. There is considerable clinical evidence that extracts of St John's Wort are effective in treating mild to moderate depression and improving mood. However, to avoid potentially dangerous side-effects, St John's Wort should not be used at the same time as prescription antidepressants. St John's Wort is a small to medium height herbaceous perennial plant with numerous yellow flowers characteristic of this genus. It is widespread throughout Europe, where it is generally considered a weed, and has also become naturalized in North America. The tops, including flowers at varying stages of development, which contain considerable amounts of the active principles, are harvested and dried in late summer.

The dried herb contains significant amounts of phenolic derivatives, including 4-5% of flavonoids, though the antidepressant activity is considered to derive principally from naphthodianthrone structures such as hypericin {about 0.1%) and pseudohypericin {about 0.2%), and a prenylated phloroglucinol derivative hyperforin {Figure 3.35). The fresh plant also contains significant levels of protohypericin and protopseudohypericin, which are converted into hypericin and pseudophypericin during drying and processing, as a result of irradiation with visible light. Hyperforin is a major lipophilic constituent in the leaves and flowers {2-3%),

OH O

HO HO

OH O

HO HO

OH O OH R = H, hypericin R = OH, pseudohypericin

OH O

HO HO

OH O

HO HO

OH O OH R = H, protohypericin R = OH, protopseudohypericin

OH O OH R = H, hypericin R = OH, pseudohypericin

OH O OH R = H, protohypericin R = OH, protopseudohypericin

hyperforin

Note: hyperforin is a mixture of tautomeric forms

Note: hyperforin is a mixture of tautomeric forms hyperforin

and is now thought to be the major contributor to the antidepressant activity, as well as to the antibacterial properties of the oil extract. Studies show clinical effects of St John's Wort on depression correlate well with hyperforin content. Standardized aqueous ethanolic extracts containing 0.15% hypericin and 5% hyperforin are usually employed. The aqueous solubility of hypericin and pseudohypericin is markedly increased by the presence of flavonoid derivatives in the crude extract, particularly procyanidin B2, a dimer of epicatechin (see page 151). Hypericum extracts have been demonstrated to increase levels of serotonin, noradrenaline, and dopamine, which may be responsible for the antidepressant activity.

Hypericin also possesses extremely high toxicity towards certain viruses, a property that requires light and may arise via photo-excitation of the polycyclic quinone system. It is currently under investigation as an antiviral agent against HIV and hepatitis C. Antiviral activity appears to arise from an inhibition of various protein kinases, including those of the protein kinase C family. Hypericin and pseudohypericin are potent photosensitizers initiating photochemical reactions, and are held responsible for hypericism, a photodermatosis seen in cattle after consuming Hypericum plants present in pasture. Patients using St John's Wort as an antidepressant should also be warned to avoid overexposure to sunlight. There is also considerable evidence that St John's Wort interacts with a number of prescription drugs including the anticoagulant warfarin, the cardiac glycoside digoxin, the bronchodilator theophylline, the HIV protease inhibitor indinavir, the immunosuppressive drug cyclosporin, and oral contraceptives. In some cases, it is known to promote the cytochrome P-450-dependent metabolism of the co-administered drugs.

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