Oxidation products of caffeic acid as model substances for the antigonadotropic activity of plant extracts

Phenolic plant constituents exert antigonadotropic activity following an oxidation. The resulting complex mixture of mostly instable products impedes the elucidation of the various oxidation steps as well as the mode of antigonadotropic action. Thus caffeic acid was chosen as a single model phenolic to facilitate the interpretation. The oxidation of caffeic acid with KMnO4 as well as with polyphenoloxidase leads to the instable caffeic acid o-quinone as the first oxidation product. Following the initial oxidation, a number of products was indicated via HPLC. Two of them were isolated and characterized as oligomers of caffeic acid, one of them with phenolic, acid and, quinoic structural components and a relative molecular mass similar to caffeic acid tetramer. It was shown that caffeic acid quinone cannot be the antigonadotropically active principle. Correspondingly, the isolated oxidation products exhibit pronounced antigonadotropic activity. It could be proved that oxidation products of caffeic acid bind to PMSG, forming PMSG-inhibitor-complexes. In such complexes the gonadotropic activity of PMSG is completely abolished.