Novel Metabolic Pathways of p-n-Nonylphenol Catalyzed by Cytochrome P450 and Estrogen Receptor Binding Activity of New Metabolites

Abstract

Nonylphenol, which is used industrially as a surfactant, is an endocrine-disrupting chemical (EDC) which has estrogenic activity. The novel biotransformation of nonylphenol was investigated, based on our previously reported ipso-metabolism of para-substituted phenols by cytochrome P450 (P450). Three novel metabolites of nonylphenol, i.e., nonylquinol, 4 -hydroxynonanophenone (CO-NP) as benzyl-oxidized nonylphenol, and hydroquinone, were detected in a rat liver microsome reaction mixture. On the other hand, production of 1-(4 -hydroxyphenyl)nonan-1-ol (OH-NP), namely benzyl-hydroxylated nonylphenol, was detected in a human liver microsome reaction mixture. The formation of all these metabolites was suppressed by the addition of P450 inhibitor. This showed that all nonylphenol metabolism was catalyzed by P450. To identify which P450 isoenzyme is involved in each reaction, fourteen human P450 (CYP) isozymes, CYP1A1, 1A2, 2A6, 2B6, 2C8, 2C9, 2C18, 2C19, 2D6, 2E1, 3A4, 3A5, 3A7, and CYP4A11, were examined. CYP1A1, 1A2, and CYP2B6 effectively catalyzed the production of nonylquinol. CYP2B6 also catalyzed the benzyl-hydroxylation to give OH-NP. Hydroquinone was formed mainly from OH-NP, not via CO-NP. We examined the estrogenic activity of these new metabolites by estrogen receptor (ER)-binding reporter gene assay. Nonylquinol, OH-NP and hydroquinone have no ER-binding activity. However, CO-NP showed the same level of estrogen receptor binding activity as nonylphenol. Moreover, the amount of CO-NP formed was small. Therefore, the novel metabolic pathways led overall to metabolic inactivation, as concerns the estrogenic activity of nonylphenol through the ER.

Evidence for the Bioactivation of 4-Nonylphenol to Quinone Methide andortho-Benzoquinone Metabolites in Human Liver Microsomes

“…Catalyzed by Individual CYP Enzymes 4-nonyl-2,5-cyclohexadien-1-one, 4′-hydroxynonanophenone, and hydroquinone were detected in rat liver microsomes, while 4-(1-hydroxynonyl) phenol was detected in HLMs (22). The present study was initiated with the intent of characterizing metabolic pathways of 4-NP in HLMs that might lead to bioactivation.…”

Molecular Mechanism of Alternative P450-Catalyzed Metabolism of Environmental Phenolic Endocrine-Disrupting Chemicals

“…In order to study the detailed reaction mechanism and to verify the initial rate-determining step for ipso-position metabolism, we studied several other widely-used phenolic EDCs distributed in the environment such as bisphenol analogues, alkylphenols and chlorophenols with available in vitro or in vivo assay data on the P450 metabolism. 3,14,15,21,23,71 As shown in Table 2 Table S9 in the Supporting Information), while the OH rebound on the LS pathway is essentially barrier-free. Therefore, we suggest that the P450-catalyzed ipso-position metabolism of these diverse phenolic EDCs follows the same reaction mode as displayed in Figure 1 of BPA, i.e.…”

A Fungal P450 (CYP5136A3) Capable of Oxidizing Polycyclic Aromatic Hydrocarbons and Endocrine Disrupting Alkylphenols: Role of Trp129 and Leu324

“…The resulting information could improve strategies using P450-based biocatalysts for biotechnological and bioremediation applications. Additionally, considering the overall low sequence homology (12–23%) of this fungal P450 to its orthologs in humans and animals [16] – [20] , it provides a unique experimental model to expand our knowledgebase on critical catalytic amino acid residues for oxidation of these xenobiotics by the eukaryotic P450s, a topic of significant current interest [21] – [23] . Indirectly, such basic information from a lower eukaryotic P450 may also provide insights into our understanding of the role of genetic polymorphisms at the active site in modulating functional diversity in the xenobiotic- and drug-metabolizing human or animal P450 enzymes.…”