Assessment of the Reactivity of Selected Isoflavones against Proteins in Comparison to Quercetin

Selected isoflavones (genistein, daidzein, formononetin, prunetin, biochanin A, and two synthetic isoflavones) were allowed to interact with soy and whey proteins. The reaction products were analyzed in terms of covalent binding at the nucleophilic side chains of proteins. Changes in molecular properties of the proteins derivatives were documented by SDS-PAGE, IEF, and SELDI-TOF-MS. The structural changes induced were studied using circular dichroism. The in vitro digestibility was assessed with trypsin. The results show that the occurrence of the catechol moiety, that is, the two adjacent (ortho) aromatic hydroxyl groups on ring B of the flavonoid structural skeleton appears to be perquisite condition for covalent binding to proteins. The catechol moiety on ring A was less reactive. Its absence lead to a slight or no significant reaction, although noncovalent interactions may still be possible, even when lacking this structural element. A comparison of the data is also made with quercetin representing the flavonols. Keywords: Isoflavones; quercetin; soy proteins; whey proteins; protein−isoflavone interactions; protein derivatization; structural changes; in vitro proteolytic degradation