Oxidative activation of the propolis hapten isoprenyl caffeate

The reactions of isoprenyl caffeate, the strongest allergen known to be present in propolis, with plausible functional groups of proteins were studied. The thiols glutathione and cysteine, which add to isoprenyl caffeate after having been oxidised to the corresponding quinone, served as model substances. The structure of the thiol adducts was determined by spectroscopic methods, addition being found to occur at C-2. The results support the hypothesis that the catecholic propolis hapten is a pro-hapten that forms a complete antigen after oxidation to caffeate quinone and addition to nucleophilic groups of proteins.