The hyperin and tannin content of H. perforatum is higher at growth temperatures above 14 degrees C. (tannin, 15.06% of dry weight) than below (13.42%). Both hyperin and rutin content is higher in dry conditions (1.25% and 2.32% respectively) than wet conditions (no figure given and 1.89%, respectively). Hyperin content is highest at 7pm (84). Total tannin content is highest when the buds are forming, just prior to flowering, in June (85,86).

Higher amounts of flavonoids, including rutin, quercetin, and hyperin occur in plants of northern slopes with few generative shoots (87). Flavonoid content (rutin, hyperin, quercetin, and quercitrin) is highest in the leaves of St. John's wort, and is at maximum concentration during full bloom. In the flowers, the content of flavonoids is highest at the start of flowering, falling sharply during flowering (88).

St. John's wort (flowers) had the highest content of flavonoids (11.71%) of any of 223 species tested (89).

Pharmacology
Extracts of the flowering tops of Hypericum perforatum have shown a variety of effects in the laboratory, including psychotropic activity, wound and burn-healing activity, bactericidal effect against pathogens in pyelitis and cystitis, anti-viral effects, sunscreen activity (disputed), antidepressive activity, and diuretic, anthelmintic, and mildly uterotonic activity (90, 91, 92). Although much more work needs to be done to validate the use of St. John's wort for the many uses it finds in clinical and common practice, there are a few laboratory studies which corroborate its use for some of these conditions and point the way for further research.

Following is a summary of the laboratory work that has been conducted on the pharmacological effects of St. John's wort extract or oil.

Anti-depressive and Psychotropic Activity
Among the most common psychiatric illnesses today are depression, mania, (abnormal elation with irritability), bipolar affective disorder, characterized by swings between depression and mania, and schizophrenia. One of the best-known (but contoversial) theories hypothesizes that depression is caused by deficiency or decreased effectivness of norepinephrine and serotonin, acting as nerve-impulse transmitting substances (neurotransmitters), in particular nerve pathways. One method for treating depression uses the monoamine oxidase (MAO) inhibitors which retard one of the enzymes responsible for monoamine (a precursor) breakdown, increasing the concentration of neurotransmitters in the central nervous system (93, 94).

Because of St. John's wort's history of use for psychiatric conditions, it was tested for MAO inhibiting activity. Suzuki, et al. (1984), in an international effort, first demonstrated that xanthones, common in the Guttiferae (the family of St. John's wort) and the Gentianaceae (Gentian family), inhibit both type A and B monoamine oxidase. Among these is the compounds isogentisin, which has been found in some species of Hypericum, but not H. perforatum (95, 96). A further study by the same group found that hypericin from H. perforatum irreversibly inhibits type A and B MAO in vivo. The authors stress, however, that although this study is suggestive, no definite conclusion can be drawn yet regarding St. John's wort's antidepressant activity (97).

A standardized (hypericin) extract of H. perforatum has been tested in various animal models generally used for determining antidepressant activity, and has been found to enhance the exploratory activity of mice in a foreign environment, extended the narcotic sleeping time dose-dependently, and has shown reserpine antagonism and decreased aggressive behavior in socially isolated male mice (98).