Degradation of a Nonylphenol Single Isomer by Sphingomonas sp. Strain TTNP3 Leads to a Hydroxylation-Induced Migration Product

Abstract

Sphingomonas sp. strain TTNP3 degrades 4(3,5-dimethyl-3-heptyl)-phenol and unidentified metabolites that were described previously. The chromatographic analyses of the synthesized reference compound and the metabolites led to their identification as 2(3,5-dimethyl-3-heptyl)-1,4-benzenediol. This finding indicates that the nonylphenol metabolism of this bacterium involves unconventional degradation pathways where an NIH shift mechanism occurs.Nonylphenol (NP) is a ubiquitous pollutant which mainly results from the biodegradation of widely used NP polyethoxylate surfactants, and it is well known for its endocrine systemdisrupting potential (5,8,11,13,20,21). NP is used as part of a technical mixture which consists of more than 20 branched isomers of the nonyl chain (23, 28). Although some studies report an easy degradation of the NP with a linear alkyl chain (4-n-NP) through oxidation of the side chain, the situation is different for branched isomers of NP (2,25,26). The main reason for the poor degradability of NP is a quaternary carbon atom (␣-carbon) of the branched alkyl chain, which is involved in the attachment of the nonyl group to the phenol moiety. More than 85% of the isomers of the technical mixture contain such quaternary ␣-carbon atoms (26, 28). Some strains belonging to the genera Sphingomonas, Sphingobium, and Pseudomonas have been reported to be able to degrade the branched isomers of NP, but the catabolic pathways in these bacteria remain poorly characterized (6,7,18,19,21,24).Previously, it was proven that the dead-end product of the degradation of 4(3Ј,5Ј-dimethyl-3Ј-heptyl)-phenol (p353NP) diastereomers is 3,5-dimethyl-3-heptanol (nonanol). This metabolite corresponds to the nonyl chain of p353NP, where the aromatic ring has been replaced by a hydroxyl group (3). It remains to be elucidated how the very stable quaternary C atom structure of NP can lead to the formation of nonanol. Nonanol has been found only in cultures of Sphingomonas sp. strain TTNP3 and Sphingomonas cloacae, so these bacteria seem to possess particular degradation pathways for NP (3,6,21). Recently, it was reported that neither 4-n-NP nor shortchain alkylphenols were degraded by Sphingomonas sp. strain TTNP3, whereas the diastereomers of the branched single isomer p353NP (Fig. 1) led to the formation of ring-oxidized metabolites (4). Ring-oxidized metabolites were detected only in the intracellular fraction and were shown to be different from 4(3Ј,5Ј-dimethyl-3Ј-heptyl)-catechol (Fig. 1b) and 4(3Ј,5Ј-dimethyl-3Ј-heptyl)-resorcinol (Fig. 1c), hydroxylated at positions C-2 and C-3 (Fig. 1), respectively. We hypothesize the formation of metabolites which undergo either hydroxylation with an alkyl chain internal rearrangement, as is the case for bisphenol A (14), or hydroxylation-induced migration of the alkyl chain immediately after hydroxylation at position C-4. The latter process would lead to the formation of 2(3Ј,5Ј-dimethyl-3Ј-heptyl)-1,4-benzenediol (o353NHQ) (Fig. 1d). In order to identify definitively these metabolites ...

Degradation Pathway of Bisphenol A: Does ipso Substitution Apply to Phenols Containing a Quaternary α-Carbon Structure in the para Position?

“…In fact, this compound could be rationalized by an NIH shift mechanism, namely the migration of the substituent to the adjacent C atom consecutively with hydroxylation, followed by hydrogenation reactions. By analogy with studies with NP (8,14), a carbanionic shift of the isopropylphenol moiety to the adjacent carbon could occur. Another possibility is that the result of this NIH shift does not involve an intramolecular migration mechanism but rather a condensation reaction between the leaving carbocationic intermediate and the newly formed HQ (6).…”

“…Investigations were carried out mainly with Sphingomonas sp. strain TTNP3 (6)(7)(8)(9)(10)(11) and S. xenophaga strain Bayram (13,14). In both strains, degradation pathways involve ipso-hydroxylation of NP as the first step (Fig.…”

“…1B, compound 4) and alkyl benzenediol (Fig. 1B, compound 5) as terminal metabolites (7,8). Both NP and BPA belong to the alkylphenol family and share a common structural feature, i.e., a quaternary ␣-carbon.…”

Two identical nonylphenol monooxygenase genes linked to IS6100 and some putative insertion sequence elements in Sphingomonas sp. NP5

“…Intensive studies on the NP degradation pathways of Sphingomonas sp. TTNP3 and Sphingobium xenophagum Bayram revealed that NP can be oxidized at the aromatic carbon atom substituted with the alkyl side chain (ipso-hydroxylation) (Corvini et al, 2004(Corvini et al, , 2006aGabriel et al, 2005aGabriel et al, , b, 2007. From the oxidized intermediate, the aromatic moiety and the side chain are separated as hydroquinone and a carbocation with nine carbon atoms (finally forming an alcohol or several aliphatic compounds), respectively.…”

Role of P450 Monooxygenases in the Degradation of the Endocrine-Disrupting Chemical Nonylphenol by the White Rot Fungus Phanerochaete chrysosporium

“…Certain microorganisms belonging to the bacterial and yeast groups have the ability to degrade NP (4,5,38,39). Recent studies have shown the abilities of selected fungi, including white rot fungi, to degrade this chemical, albeit to various extents (33).…”