A novel metabolic pathway for degradation of 4-nonylphenol environmental contaminants by Sphingomonas xenophaga Bayram -: ipso-hydroxylation and intramolecular rearrangement*

Several nonylphenol isomers with alpha-quaternary carbon atoms serve as growth substrates for Sphingomonas xenophaga Bayram, whereas isomers containing hydrogen atoms at the alpha-carbon do not (Gabriel, F. L. P., Giger, W., Guenther, K., and Kohler, H.- P. E. ( 2005) Appl. Environ. Microbiol. 71, 1123 - 1129). Three metabolites of 4-( 1- methyloctyl)phenol were isolated in mg quantities from cultures of strain Bayram supplemented with the growth substrate isomer 4-( 1- ethyl- 1,4- dimethyl- pentyl)- phenol. They were unequivocally identified as 4- hydroxy- 4-( 1- methyl- octyl)cyclohexa- 2,5- dienone, 4- hydroxy- 4-( 1- methyl- octyl)- cyclohex2- enone, and 2-( 1- methyl- octyl)- benzene- 1,4- diol by high pressure liquid chromatography- mass spectrometry and nuclear magnetic resonance spectroscopy. Furthermore, two metabolites originating from 4-n-nonylphenol were identified as 4- hydroxy- 4-nonyl-cyclohexa-2,5-dienone and 4- hydroxy- 4- nonyl-cyclohex-2-enone by high pressure liquid chromatography- mass spectrometry. We conclude that nonylphenols were initially hydroxylated at the ipsoposition forming 4-alkyl-4-hydroxy-cyclohexa-2,5-dienones. Dienones originating from growth substrate nonylphenol isomers underwent a rearrangement that involved a 1,2-C, O shift of the alkyl moiety as a cation to the oxygen atom of the geminal hydroxy group yielding 4-alkoxyphenols, from which the alkyl moieties can be easily detached as alcohols by known mechanisms. Dienones originating from nongrowth substrates did not undergo such a rearrangement because the missing alkyl substituents at the alpha-carbon atom prevented stabilization of the putative alpha-carbocation. Instead they accumulated and subsequently underwent side reactions, such as 1,2- C, C shifts and dihydrogenations. The ipso-hydroxylation and the proposed 1,2- C, O shift constitute key steps in a novel pathway that enables bacteria to detach alpha-branched alkyl moieties of alkylphenols for utilization of the aromatic part as a carbon and energy source.