AEM Accepts, published online ahead of print on 8 June 2007

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Appl. Environ. Microbiol. doi:10.1128/AEM.00329-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Degradation pathway of bisphenol A - Does ipso-substitution apply to phenols para-substituted by a quaternary C alpha structure?

B. Kolvenbach, N. Schlaich, Z. Raoui, J. Prell, S. Zühlke, A. Schäffer, F. P. Guengerich, and P. F. X. Corvini^*

Department of Environmental Research, Rheinisch-Westfälische Technische Hochschule (RWTH) -Aachen University, D-52074 Aachen, Germany; Department of Plant Physiology, RWTH Aachen University, D-52074 Aachen, Germany; School of Biological Sciences, University of Reading, UK, RG6 6AJ; Institute of Environmental Research (INFU), University of Dortmund, D-44221 Dortmund, Germany; Department of Biochemistry and Center in Molecular Toxicology, Vanderbilt University School of Medicine Nashville, Tennessee 37232-0146; Institute for Ecopreneurship, University of Applied Sciences Northwestern Switzerland, CH-4132 Muttenz, Switzerland

^* To whom correspondence should be addressed. Email: philippe.corvini{at}fhnw.de.

The degradation of bisphenol A and nonylphenol involves the unusual rearrangement of stable carbon-carbon bonds. Some nonylphenol isomers and bisphenol A possess a quaternary -carbon atom as common structural feature. The degradation of nonylphenol in Sphingomonas sp. strain TTNP3 occurs via a type II ipso-substitution with the presence of a quaternary -carbon as a prerequisite. We report here a new degradation pathway of bisphenol A. Consequent to the hydroxylation at position C₄ according to a type II ipso-substitution mechanism, the C-C bond between the phenolic moiety and the isopropyl group of bisphenol A is broken. Beside the formation of hydroquinone and 4-(2-hydroxypropan-2-yl)phenol as main metabolites, further compounds resulting from molecular rearrangements consistent with a carbocationic intermediate were identified. Assays with resting cells or cell extracts of Sphingomonas sp. strain TTNP3 under an ¹⁸O₂ atmosphere were performed. One atom of ¹⁸O₂ was present in hydroquinone resulting from the monooxygenation of bisphenol A and nonylphenol. The monooxygenase activity was dependent on both NADPH and FAD. Various cytochrome P450 inhibitors had identical inhibition effects on the conversion of both xenobiotics. Using a mutant of Sphingomonas sp. strain TTNP3, which is defective for growth on nonylphenol, we demonstrated that the reaction is catalyzed by the same enzymatic system. In conclusion, the degradation of bisphenol A and nonylphenol is initiated by the same monooxygenase, which may also lead to ipso-substitution in other xenobiotics containing phenol with a quaternary -carbon.

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