Biochemical activities of extracts from Hypericum perforatum L. 5th Communication: Dopamine-beta-hydroxylase-product quantification by HPLC and inhibition by hypericins and flavonoids

Extracts from the herb "St. John's wort" (Hypericum perforatum L.) exhibit beneficial effects on patients suffering from mental depressions. Lack of c atecholamine neurotransmitters may be one biochemical mechanism for this pr oblem under discussion. It has been recently reported that alcoholic extrac ts from Hypericum perforatum inhibit dopamine-beta-hydroxylase (D-beta-H) w ith an I-50 of 0.1 mu mol/l on the basis of total hypericin content and wit h an I-50 Of 21 mu mol/l with pure commercial hypericin. As test system pol arographic determination of oxygen uptake with tyramine as a substrate anal ogue was used. In the present paper the quantification of the enzymatic act ivity and the potential influence of inhibitors are reported using dopamine as substrate and product (noradrenaline) quantification by HPLC. With this test system it could be shown that D-beta-H is strongly inhibited by pseud ohypericin (I-50 approx. 3 mu mol/l) and hypericin (I-50 approx. 5 mu mol/l ), whereas the Iso-values of various flavonoids (quercitrin, isoquercitrin, hyperoside, rutin, quercetin, amentoflavone, kaempferol) are in the range of 50 mu mol/l or higher.