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Appl. Environ. Microbiol. doi:10.1128/AEM.02994-06
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Elucidation of the ipso-substitution mechanism for side chain cleavage of -quaternary 4-nonylphenols and 4-t-butoxyphenol in Sphingobium xenophagum Bayram

From the Swiss Federal Institute for Aquatic Science and Technology (Eawag), CH-8600 Dübendorf, Switzerland, and the Research Center Jülich, Institute of Chemistry and Dynamics of the Geosphere, Institute III: Phytosphere, D-52428 Jülich, Germany

^* To whom correspondence should be addressed. Email: kohler{at}eawag.ch.

	Abstract

Recently we showed that degradation of several nonylphenol isomers with -quaternary carbon atoms is initiated by ipso-hydroxylation in Sphingobium xenophagum Bayram (Gabriel F. L. P., Heidlberger A, Rentsch D, Giger W., Guenther K., and Kohler H.-P. E. (2005) J. Biol. Chem. 280: 15526-15533). Here, we demonstrate with ¹⁸O-labeling experiments that the ipso-hydroxy group was derived from molecular oxygen and that in the major pathway for cleavage of the alkyl moiety the resulting nonanol metabolite contained an oxygen atom originating from water, and not from the ipso-hydroxy group, as was previously assumed. Our results clearly show that the alkyl cation derived from the -quaternary nonylphenol 4-(1-ethyl-1,4-dimethyl-pentyl)-phenol through ipso-hydroxylation and subsequent dissociation of the 4-alkyl-4-hydroxy-cyclohexadienone intermediate preferentially combines with a molecule of water to yield the corresponding alcohol and hydroquinone. However, metabolism of certain ,-dimethyl-substituted nonylphenols appears to involve also a reaction of the cation with the ipso-hydroxy group to form the corresponding 4-alkoxyphenols. Growth, oxygen uptake, and ¹⁸O labeling experiments clearly indicate that strain Bayram metabolized 4-t-butoxyphenol by ipso-hydroxylation to a hemiketal followed by spontaneous dissociation to the corresponding alcohol and p-quinone. Hydroquinone effected high oxygen uptake in assays with induced resting cells as well as in assays with cell free extracts. This further corroborates the role of hydroquinone as the ring cleavage intermediate during degradation of 4-nonylphenols and 4-alkoxyphenols.

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