Estrogenic Activity of Branched 4-Nonylphenol Isomers Examined by Yeast Two-Hybrid Assay

Various branched isomers of 4-nonylphenol (NP) and, in addition, the other 4-alkylphenols (APs) were synthesized and their estrogenic activities were assessed using the yeast two-hybrid system. We investigated the relationships between the structure of the nonyl group of NP isomers and their estrogenic activities based on the following five factors: the length of the main alkyl chain, the degree of branching on the α-carbon, the degree of bulkiness, the position of the branch, and the cyclic structure in the nonyl group. An appropriate length of the main alkyl chain was essential for the estrogenic activity. A small effect of the branching on the α-carbon was observed. The importance of the bulkiness and position of the branch in the nonyl group was suggested from the results of the synthesized NP isomers possessing the high estrogenic activities. The bulkiness on the β-carbon was the most important factor for the high estrogenic activity. The investigation of the cyclic structure also indicated the significance of the bulkiness around the β-carbon. The bulkiness on the γ-carbon was also suggested to be an important factor. The metabolic effect of the synthesized APs on the estrogenic activity was also examined using a liver S9. The estrogenic activities of the selected APs were either reduced or lost. In addition, GC-MS analyses of the commercial NP and synthesized NP isomers revealed that the nine synthesized NP isomers were included in the commercial NP.