Figure 29.6 • Chemical components of St. John's wort.
and pseudohypericin. (Fig. 29.6) Flavonoids present are quercetin, hyperoside, quercitrin, isoquercitrin, and rutin. (Fig. 29.6) Two C-C-linked biflavins, amentoflavone and biapigenin, are present, as is the acylphloroglucinol derivative hyperforin. Procyanidin, a chiral flavone dimer, adds to the list of flavones. Some terpenes and n-alkanes are present as minor components. The primary active ingredients have traditionally been held to be the hypericins and the flavone/flavonols, especially the hypericins. In fact, the German Commission E Monograph specifies that the herb should be standardized to hypericin content.
Despite the evidence of efficacy, the mechanism of action of St. John's wort remains unclear. Several possibilities have been put forth. Probably, the most popular one is the MAOI/COMT hypothesis. According to this hypothesis, St. John's wort increases the levels of catecholamines at the brain synapses by inhibiting their inactivation by oxidative deamination (MAOI) and by catechol functional-ization (catechol-O-methyltransferase [COMT]).47 Recent studies have shown that hypericins possess such activities only at pharmacologically excessive concentrations. If true, these effects at normal doses are small and do nothing to alleviate depression. Other hypotheses suggest hormonal effects or effects on the dopaminergic system. Hyperforin has become a candidate for the major antidepressant constituent of St. John's wort, supposedly inhibiting serotonin reuptake by elevating free intracellular sodium.
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