Ellagitannins as active constituents of medicinal plants.

Isolation and structure determination, accompanied by measurement of various biological activities of each isolated tannin, particularly of ellagitannins, have brought about a marked change in the concept of tannins as active constituents of medicinal plants. Their biological activities should now be discussed on the basis of the structural differences among each tannin, in a way similar to that of the other types of natural organic compounds. The anti-tumor activity exclusively exhibited by several oligomeric ellagitannins, and their anti-HIV activities are examples of such biological activities. The inhibitory activity against lipid peroxidation, which is different in the strength among tannins of various structures, is exhibited in general more strongly by ellagitannins than by the other types of tannins of similar structures. The radical-scavenging activities of tannins as the mechanism of their inhibition, which is regarded to participate in several biological activities of tannins, have been supported by the ESR spectral measurements. Other biological activities, i.e., inhibition of mutagenicity of carcinogens, inhibition of tumor promotion, etc., have been found for tannins including ellagitannins.